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904 lines
53 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<title>GMP Itemized Development Tasks</title>
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<link rel="shortcut icon" href="favicon.ico">
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<link rel="stylesheet" href="gmp.css">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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</head>
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<center>
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<h1>
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GMP Itemized Development Tasks
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</h1>
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</center>
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<font size=-1>
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Copyright 2000, 2001, 2002, 2003, 2004, 2006 Free Software Foundation, Inc. <br><br>
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This file is part of the GNU MP Library. <br><br>
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published
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by the Free Software Foundation; either version 2.1 of the License, or (at
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your option) any later version. <br><br>
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details. <br><br>
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA.
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</font>
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<hr>
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<!-- NB. timestamp updated automatically by emacs -->
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This file current as of 21 Apr 2006. An up-to-date version is available at
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<a href="http://swox.com/gmp/tasks.html">http://swox.com/gmp/tasks.html</a>.
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Please send comments about this page to
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<a href="mailto:gmp-devel@swox.com">gmp-devel@swox.com</a>.
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<p> These are itemized GMP development tasks. Not all the tasks
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listed here are suitable for volunteers, but many of them are.
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Please see the <a href="projects.html">projects file</a> for more
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sizeable projects.
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<h4>Correctness and Completeness</h4>
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<ul>
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<li> <code>_LONG_LONG_LIMB</code> in gmp.h is not namespace clean. Reported
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by Patrick Pelissier.
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<br>
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We sort of mentioned <code>_LONG_LONG_LIMB</code> in past releases, so
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need to be careful about changing it. It used to be a define
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applications had to set for long long limb systems, but that in
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particular is no longer relevant now that it's established automatically.
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<li> The various reuse.c tests need to force reallocation by calling
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<code>_mpz_realloc</code> with a small (1 limb) size.
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<li> One reuse case is missing from mpX/tests/reuse.c:
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<code>mpz_XXX(a,a,a)</code>.
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<li> When printing <code>mpf_t</code> numbers with exponents >2^53 on
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machines with 64-bit <code>mp_exp_t</code>, the precision of
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<code>__mp_bases[base].chars_per_bit_exactly</code> is insufficient and
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<code>mpf_get_str</code> aborts. Detect and compensate. Alternately,
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think seriously about using some sort of fixed-point integer value.
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Avoiding unnecessary floating point is probably a good thing in general,
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and it might be faster on some CPUs.
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<li> Make the string reading functions allow the `0x' prefix when the base is
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explicitly 16. They currently only allow that prefix when the base is
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unspecified (zero).
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<li> <code>mpf_eq</code> is not always correct, when one operand is
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1000000000... and the other operand is 0111111111..., i.e., extremely
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close. There is a special case in <code>mpf_sub</code> for this
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situation; put similar code in <code>mpf_eq</code>. [In progress.]
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<li> <code>mpf_eq</code> doesn't implement what gmp.texi specifies. It should
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not use just whole limbs, but partial limbs. [In progress.]
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<li> <code>mpf_set_str</code> doesn't validate it's exponent, for instance
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garbage 123.456eX789X is accepted (and an exponent 0 used), and overflow
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of a <code>long</code> is not detected.
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<li> <code>mpf_add</code> doesn't check for a carry from truncated portions of
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the inputs, and in that respect doesn't implement the "infinite precision
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followed by truncate" specified in the manual.
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<li> Windows DLLs: tests/mpz/reuse.c and tests/mpf/reuse.c initialize global
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variables with pointers to <code>mpz_add</code> etc, which doesn't work
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when those routines are coming from a DLL (because they're effectively
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function pointer global variables themselves). Need to rearrange perhaps
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to a set of calls to a test function rather than iterating over an array.
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<li> <code>mpz_pow_ui</code>: Detect when the result would be more memory than
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a <code>size_t</code> can represent and raise some suitable exception,
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probably an alloc call asking for <code>SIZE_T_MAX</code>, and if that
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somehow succeeds then an <code>abort</code>. Various size overflows of
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this kind are not handled gracefully, probably resulting in segvs.
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<br>
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In <code>mpz_n_pow_ui</code>, detect when the count of low zero bits
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exceeds an <code>unsigned long</code>. There's a (small) chance of this
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happening but still having enough memory to represent the value.
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Reported by Winfried Dreckmann in for instance <code>mpz_ui_pow_ui (x,
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4UL, 1431655766UL)</code>.
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<li> <code>mpf</code>: Detect exponent overflow and raise some exception.
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It'd be nice to allow the full <code>mp_exp_t</code> range since that's
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how it's been in the past, but maybe dropping one bit would make it
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easier to test if e1+e2 goes out of bounds.
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</ul>
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<h4>Machine Independent Optimization</h4>
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<ul>
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<li> <code>mpf_cmp</code>: For better cache locality, don't test for low zero
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limbs until the high limbs fail to give an ordering. Reduce code size by
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turning the three <code>mpn_cmp</code>'s into a single loop stopping when
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the end of one operand is reached (and then looking for a non-zero in the
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rest of the other).
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<li> <code>mpf_mul_2exp</code>, <code>mpf_div_2exp</code>: The use of
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<code>mpn_lshift</code> for any size<=prec means repeated
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<code>mul_2exp</code> and <code>div_2exp</code> calls accumulate low zero
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limbs until size==prec+1 is reached. Those zeros will slow down
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subsequent operations, especially if the value is otherwise only small.
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If low bits of the low limb are zero, use <code>mpn_rshift</code> so as
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to not increase the size.
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<li> <code>mpn_dc_sqrtrem</code>: Don't use <code>mpn_addmul_1</code> with
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multiplier==2, instead either <code>mpn_addlsh1_n</code> when available,
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or <code>mpn_lshift</code>+<code>mpn_add_n</code> if not.
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<li> <code>mpn_dc_sqrtrem</code>, <code>mpn_sqrtrem2</code>: Don't use
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<code>mpn_add_1</code> and <code>mpn_sub_1</code> for 1 limb operations,
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instead <code>ADDC_LIMB</code> and <code>SUBC_LIMB</code>.
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<li> <code>mpn_sqrtrem2</code>: Use plain variables for <code>sp[0]</code> and
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<code>rp[0]</code> calculations, so the compiler needn't worry about
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aliasing between <code>sp</code> and <code>rp</code>.
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<li> <code>mpn_sqrtrem</code>: Some work can be saved in the last step when
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the remainder is not required, as noted in Paul's paper.
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<li> <code>mpq_add</code>, <code>mpq_add</code>: The division "op1.den / gcd"
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is done twice, where of course only once is necessary. Reported by Larry
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Lambe.
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<li> <code>mpq_add</code>, <code>mpq_sub</code>: The gcd fits a single limb
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with high probability and in this case <code>modlimb_invert</code> could
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be used to calculate the inverse just once for the two exact divisions
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"op1.den / gcd" and "op2.den / gcd", rather than letting
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<code>mpn_divexact_1</code> do it each time. This would require a new
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<code>mpn_preinv_divexact_1</code> interface. Not sure if it'd be worth
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the trouble.
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<li> <code>mpq_add</code>, <code>mpq_sub</code>: The use of
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<code>mpz_mul(x,y,x)</code> causes temp allocation or copying in
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<code>mpz_mul</code> which can probably be avoided. A rewrite using
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<code>mpn</code> might be best.
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<li> <code>mpn_gcdext</code>: Don't test <code>count_leading_zeros</code> for
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zero, instead check the high bit of the operand and avoid invoking
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<code>count_leading_zeros</code>. This is an optimization on all
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machines, and significant on machines with slow
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<code>count_leading_zeros</code>, though it's possible an already
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normalized operand might not be encountered very often.
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<li> Rewrite <code>umul_ppmm</code> to use floating-point for generating the
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most significant limb (if <code>BITS_PER_MP_LIMB</code> <= 52 bits).
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(Peter Montgomery has some ideas on this subject.)
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<li> Improve the default <code>umul_ppmm</code> code in longlong.h: Add partial
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products with fewer operations.
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<li> Consider inlining <code>mpz_set_ui</code>. This would be both small and
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fast, especially for compile-time constants, but would make application
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binaries depend on having 1 limb allocated to an <code>mpz_t</code>,
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preventing the "lazy" allocation scheme below.
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<li> Consider inlining <code>mpz_[cft]div_ui</code> and maybe
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<code>mpz_[cft]div_r_ui</code>. A <code>__gmp_divide_by_zero</code>
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would be needed for the divide by zero test, unless that could be left to
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<code>mpn_mod_1</code> (not sure currently whether all the risc chips
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provoke the right exception there if using mul-by-inverse).
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<li> Consider inlining: <code>mpz_fits_s*_p</code>. The setups for
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<code>LONG_MAX</code> etc would need to go into gmp.h, and on Cray it
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might, unfortunately, be necessary to forcibly include <limits.h>
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since there's no apparent way to get <code>SHRT_MAX</code> with an
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expression (since <code>short</code> and <code>unsigned short</code> can
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be different sizes).
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<li> <code>mpz_powm</code> and <code>mpz_powm_ui</code> aren't very
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fast on one or two limb moduli, due to a lot of function call
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overheads. These could perhaps be handled as special cases.
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<li> <code>mpz_powm</code> and <code>mpz_powm_ui</code> want better
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algorithm selection, and the latter should use REDC. Both could
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change to use an <code>mpn_powm</code> and <code>mpn_redc</code>.
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<li> <code>mpz_powm</code> REDC should do multiplications by <code>g[]</code>
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using the division method when they're small, since the REDC form of a
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small multiplier is normally a full size product. Probably would need a
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new tuned parameter to say what size multiplier is "small", as a function
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of the size of the modulus.
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<li> <code>mpz_powm</code> REDC should handle even moduli if possible. Maybe
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this would mean for m=n*2^k doing mod n using REDC and an auxiliary
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calculation mod 2^k, then putting them together at the end.
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<li> <code>mpn_gcd</code> might be able to be sped up on small to
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moderate sizes by improving <code>find_a</code>, possibly just by
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providing an alternate implementation for CPUs with slowish
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<code>count_leading_zeros</code>.
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<li> Toom3 could use a low to high cache localized evaluate and interpolate.
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The necessary <code>mpn_divexact_by3c</code> exists.
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<li> <code>mpf_set_str</code> produces low zero limbs when a string has a
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fraction but is exactly representable, eg. 0.5 in decimal. These could be
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stripped to save work in later operations.
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<li> <code>mpz_and</code>, <code>mpz_ior</code> and <code>mpz_xor</code> should
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use <code>mpn_and_n</code> etc for the benefit of the small number of
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targets with native versions of those routines. Need to be careful not to
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pass size==0. Is some code sharing possible between the <code>mpz</code>
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routines?
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<li> <code>mpf_add</code>: Don't do a copy to avoid overlapping operands
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unless it's really necessary (currently only sizes are tested, not
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whether r really is u or v).
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<li> <code>mpf_add</code>: Under the check for v having no effect on the
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result, perhaps test for r==u and do nothing in that case, rather than
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currently it looks like an <code>MPN_COPY_INCR</code> will be done to
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reduce prec+1 limbs to prec.
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<li> <code>mpf_div_ui</code>: Instead of padding with low zeros, call
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<code>mpn_divrem_1</code> asking for fractional quotient limbs.
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<li> <code>mpf_div_ui</code>: Eliminate <code>TMP_ALLOC</code>. When r!=u
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there's no overlap and the division can be called on those operands.
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When r==u and is prec+1 limbs, then it's an in-place division. If r==u
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and not prec+1 limbs, then move the available limbs up to prec+1 and do
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an in-place there.
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<li> <code>mpf_div_ui</code>: Whether the high quotient limb is zero can be
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determined by testing the dividend for high<divisor. When non-zero, the
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divison can be done on prec dividend limbs instead of prec+1. The result
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size is also known before the division, so that can be a tail call (once
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the <code>TMP_ALLOC</code> is eliminated).
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<li> <code>mpn_divrem_2</code> could usefully accept unnormalized divisors and
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shift the dividend on-the-fly, since this should cost nothing on
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superscalar processors and avoid the need for temporary copying in
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<code>mpn_tdiv_qr</code>.
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<li> <code>mpf_sqrt</code>: If r!=u, and if u doesn't need to be padded with
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zeros, then there's no need for the tp temporary.
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<li> <code>mpq_cmp_ui</code> could form the <code>num1*den2</code> and
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<code>num2*den1</code> products limb-by-limb from high to low and look at
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each step for values differing by more than the possible carry bit from
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the uncalculated portion.
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<li> <code>mpq_cmp</code> could do the same high-to-low progressive multiply
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and compare. The benefits of karatsuba and higher multiplication
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algorithms are lost, but if it's assumed only a few high limbs will be
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needed to determine an order then that's fine.
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<li> <code>mpn_add_1</code>, <code>mpn_sub_1</code>, <code>mpn_add</code>,
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<code>mpn_sub</code>: Internally use <code>__GMPN_ADD_1</code> etc
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instead of the functions, so they get inlined on all compilers, not just
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gcc and others with <code>inline</code> recognised in gmp.h.
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<code>__GMPN_ADD_1</code> etc are meant mostly to support application
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inline <code>mpn_add_1</code> etc and if they don't come out good for
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internal uses then special forms can be introduced, for instance many
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internal uses are in-place. Sometimes a block of code is executed based
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on the carry-out, rather than using it arithmetically, and those places
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might want to do their own loops entirely.
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<li> <code>__gmp_extract_double</code> on 64-bit systems could use just one
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bitfield for the mantissa extraction, not two, when endianness permits.
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Might depend on the compiler allowing <code>long long</code> bit fields
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when that's the only actual 64-bit type.
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<li> tal-notreent.c could keep a block of memory permanently allocated.
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Currently the last nested <code>TMP_FREE</code> releases all memory, so
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there's an allocate and free every time a top-level function using
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<code>TMP</code> is called. Would need
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<code>mp_set_memory_functions</code> to tell tal-notreent.c to release
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any cached memory when changing allocation functions though.
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<li> <code>__gmp_tmp_alloc</code> from tal-notreent.c could be partially
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inlined. If the current chunk has enough room then a couple of pointers
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can be updated. Only if more space is required then a call to some sort
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of <code>__gmp_tmp_increase</code> would be needed. The requirement that
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<code>TMP_ALLOC</code> is an expression might make the implementation a
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bit ugly and/or a bit sub-optimal.
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<pre>
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#define TMP_ALLOC(n)
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((ROUND_UP(n) > current->end - current->point ?
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__gmp_tmp_increase (ROUND_UP (n)) : 0),
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current->point += ROUND_UP (n),
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current->point - ROUND_UP (n))
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</pre>
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<li> <code>__mp_bases</code> has a lot of data for bases which are pretty much
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never used. Perhaps the table should just go up to base 16, and have
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code to generate data above that, if and when required. Naturally this
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assumes the code would be smaller than the data saved.
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<li> <code>__mp_bases</code> field <code>big_base_inverted</code> is only used
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if <code>USE_PREINV_DIVREM_1</code> is true, and could be omitted
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otherwise, to save space.
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<li> <code>mpz_get_str</code>, <code>mtox</code>: For power-of-2 bases, which
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are of course fast, it seems a little silly to make a second pass over
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the <code>mpn_get_str</code> output to convert to ASCII. Perhaps combine
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that with the bit extractions.
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<li> <code>mpz_gcdext</code>: If the caller requests only the S cofactor (of
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A), and A<B, then the code ends up generating the cofactor T (of B) and
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deriving S from that. Perhaps it'd be possible to arrange to get S in
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the first place by calling <code>mpn_gcdext</code> with A+B,B. This
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might only be an advantage if A and B are about the same size.
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<li> <code>mpz_n_pow_ui</code> does a good job with small bases and stripping
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powers of 2, but it's perhaps a bit too complicated for what it gains.
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The simpler <code>mpn_pow_1</code> is a little faster on small exponents.
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(Note some of the ugliness in <code>mpz_n_pow_ui</code> is due to
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||
supporting <code>mpn_mul_2</code>.)
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||
<br>
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Perhaps the stripping of 2s in <code>mpz_n_pow_ui</code> should be
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confined to single limb operands for simplicity and since that's where
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the greatest gain would be.
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<br>
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Ideally <code>mpn_pow_1</code> and <code>mpz_n_pow_ui</code> would be
|
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merged. The reason <code>mpz_n_pow_ui</code> writes to an
|
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<code>mpz_t</code> is that its callers leave it to make a good estimate
|
||
of the result size. Callers of <code>mpn_pow_1</code> already know the
|
||
size by separate means (<code>mp_bases</code>).
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<li> <code>mpz_invert</code> should call <code>mpn_gcdext</code> directly.
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</ul>
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<h4>Machine Dependent Optimization</h4>
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<ul>
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<li> <code>invert_limb</code> on various processors might benefit from the
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little Newton iteration done for alpha and ia64.
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||
<li> Alpha 21264: <code>mpn_addlsh1_n</code> could be implemented with
|
||
<code>mpn_addmul_1</code>, since that code at 3.5 is a touch faster than
|
||
a separate <code>lshift</code> and <code>add_n</code> at
|
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1.75+2.125=3.875. Or very likely some specific <code>addlsh1_n</code>
|
||
code could beat both.
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<li> Alpha 21264: Improve feed-in code for <code>mpn_mul_1</code>,
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<code>mpn_addmul_1</code>, and <code>mpn_submul_1</code>.
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<li> Alpha 21164: Rewrite <code>mpn_mul_1</code>, <code>mpn_addmul_1</code>,
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and <code>mpn_submul_1</code> for the 21164. This should use both integer
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||
multiplies and floating-point multiplies. For the floating-point
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operations, the single-limb multiplier should be split into three 21-bit
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chunks, or perhaps even better in four 16-bit chunks. Probably possible
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||
to reach 9 cycles/limb.
|
||
<li> Alpha: GCC 3.4 will introduce <code>__builtin_ctzl</code>,
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||
<code>__builtin_clzl</code> and <code>__builtin_popcountl</code> using
|
||
the corresponding CIX <code>ct</code> instructions, and
|
||
<code>__builtin_alpha_cmpbge</code>. These should give GCC more
|
||
information about sheduling etc than the <code>asm</code> blocks
|
||
currently used in longlong.h and gmp-impl.h.
|
||
<li> Alpha Unicos: Apparently there's no <code>alloca</code> on this system,
|
||
making <code>configure</code> choose the slower
|
||
<code>malloc-reentrant</code> allocation method. Is there a better way?
|
||
Maybe variable-length arrays per notes below.
|
||
<li> Alpha Unicos 21164, 21264: <code>.align</code> is not used since it pads
|
||
with garbage. Does the code get the intended slotting required for the
|
||
claimed speeds? <code>.align</code> at the start of a function would
|
||
presumably be safe no matter how it pads.
|
||
<li> ARM V5: <code>count_leading_zeros</code> can use the <code>clz</code>
|
||
instruction. For GCC 3.4 and up, do this via <code>__builtin_clzl</code>
|
||
since then gcc knows it's "predicable".
|
||
<li> Itanium: GCC 3.4 introduces <code>__builtin_popcount</code> which can be
|
||
used instead of an <code>asm</code> block. The builtin should give gcc
|
||
more opportunities for scheduling, bundling and predication.
|
||
<code>__builtin_ctz</code> similarly (it just uses popcount as per
|
||
current longlong.h).
|
||
<li> UltraSPARC/64: Optimize <code>mpn_mul_1</code>, <code>mpn_addmul_1</code>,
|
||
for s2 < 2^32 (or perhaps for any zero 16-bit s2 chunk). Not sure how
|
||
much this can improve the speed, though, since the symmetry that we rely
|
||
on is lost. Perhaps we can just gain cycles when s2 < 2^16, or more
|
||
accurately, when two 16-bit s2 chunks which are 16 bits apart are zero.
|
||
<li> UltraSPARC/64: Write native <code>mpn_submul_1</code>, analogous to
|
||
<code>mpn_addmul_1</code>.
|
||
<li> UltraSPARC/64: Write <code>umul_ppmm</code>. Using four
|
||
"<code>mulx</code>"s either with an asm block or via the generic C code is
|
||
about 90 cycles. Try using fp operations, and also try using karatsuba
|
||
for just three "<code>mulx</code>"s.
|
||
<li> UltraSPARC/32: Rewrite <code>mpn_lshift</code>, <code>mpn_rshift</code>.
|
||
Will give 2 cycles/limb. Trivial modifications of mpn/sparc64 should do.
|
||
<li> UltraSPARC/32: Write special mpn_Xmul_1 loops for s2 < 2^16.
|
||
<li> UltraSPARC/32: Use <code>mulx</code> for <code>umul_ppmm</code> if
|
||
possible (see commented out code in longlong.h). This is unlikely to
|
||
save more than a couple of cycles, so perhaps isn't worth bothering with.
|
||
<li> UltraSPARC/32: On Solaris gcc doesn't give us <code>__sparc_v9__</code>
|
||
or anything to indicate V9 support when -mcpu=v9 is selected. See
|
||
gcc/config/sol2-sld-64.h. Will need to pass something through from
|
||
./configure to select the right code in longlong.h. (Currently nothing
|
||
is lost because <code>mulx</code> for multiplying is commented out.)
|
||
<li> UltraSPARC/32: <code>mpn_divexact_1</code> and
|
||
<code>mpn_modexact_1c_odd</code> can use a 64-bit inverse and take
|
||
64-bits at a time from the dividend, as per the 32-bit divisor case in
|
||
mpn/sparc64/mode1o.c. This must be done in assembler, since the full
|
||
64-bit registers (<code>%gN</code>) are not available from C.
|
||
<li> UltraSPARC/32: <code>mpn_divexact_by3c</code> can work 64-bits at a time
|
||
using <code>mulx</code>, in assembler. This would be the same as for
|
||
sparc64.
|
||
<li> UltraSPARC: <code>modlimb_invert</code> might save a few cycles from
|
||
masking down to just the useful bits at each point in the calculation,
|
||
since <code>mulx</code> speed depends on the highest bit set. Either
|
||
explicit masks or small types like <code>short</code> and
|
||
<code>int</code> ought to work.
|
||
<li> Sparc64 HAL R1 <code>popc</code>: This chip reputedly implements
|
||
<code>popc</code> properly (see gcc sparc.md). Would need to recognise
|
||
it as <code>sparchalr1</code> or something in configure / config.sub /
|
||
config.guess. <code>popc_limb</code> in gmp-impl.h could use this (per
|
||
commented out code). <code>count_trailing_zeros</code> could use it too.
|
||
<li> PA64: Improve <code>mpn_addmul_1</code>, <code>mpn_submul_1</code>, and
|
||
<code>mpn_mul_1</code>. The current code runs at 11 cycles/limb. It
|
||
should be possible to saturate the cache, which will happen at 8
|
||
cycles/limb (7.5 for mpn_mul_1). Write special loops for s2 < 2^32;
|
||
it should be possible to make them run at about 5 cycles/limb.
|
||
<li> PPC601: See which of the power or powerpc32 code runs better. Currently
|
||
the powerpc32 is used, but only because it's the default for
|
||
<code>powerpc*</code>.
|
||
<li> PPC630: Rewrite <code>mpn_addmul_1</code>, <code>mpn_submul_1</code>, and
|
||
<code>mpn_mul_1</code>. Use both integer and floating-point operations,
|
||
possibly two floating-point and one integer limb per loop. Split operands
|
||
into four 16-bit chunks for fast fp operations. Should easily reach 9
|
||
cycles/limb (using one int + one fp), but perhaps even 7 cycles/limb
|
||
(using one int + two fp).
|
||
<li> PPC630: <code>mpn_rshift</code> could do the same sort of unrolled loop
|
||
as <code>mpn_lshift</code>. Some judicious use of m4 might let the two
|
||
share source code, or with a register to control the loop direction
|
||
perhaps even share object code.
|
||
<li> Implement <code>mpn_mul_basecase</code> and <code>mpn_sqr_basecase</code>
|
||
for important machines. Helping the generic sqr_basecase.c with an
|
||
<code>mpn_sqr_diagonal</code> might be enough for some of the RISCs.
|
||
<li> POWER2/POWER2SC: Schedule <code>mpn_lshift</code>/<code>mpn_rshift</code>.
|
||
Will bring time from 1.75 to 1.25 cycles/limb.
|
||
<li> X86: Optimize non-MMX <code>mpn_lshift</code> for shifts by 1. (See
|
||
Pentium code.)
|
||
<li> X86: Good authority has it that in the past an inline <code>rep
|
||
movs</code> would upset GCC register allocation for the whole function.
|
||
Is this still true in GCC 3? It uses <code>rep movs</code> itself for
|
||
<code>__builtin_memcpy</code>. Examine the code for some simple and
|
||
complex functions to find out. Inlining <code>rep movs</code> would be
|
||
desirable, it'd be both smaller and faster.
|
||
<li> Pentium P54: <code>mpn_lshift</code> and <code>mpn_rshift</code> can come
|
||
down from 6.0 c/l to 5.5 or 5.375 by paying attention to pairing after
|
||
<code>shrdl</code> and <code>shldl</code>, see mpn/x86/pentium/README.
|
||
<li> Pentium P55 MMX: <code>mpn_lshift</code> and <code>mpn_rshift</code>
|
||
might benefit from some destination prefetching.
|
||
<li> PentiumPro: <code>mpn_divrem_1</code> might be able to use a
|
||
mul-by-inverse, hoping for maybe 30 c/l.
|
||
<li> K7: <code>mpn_lshift</code> and <code>mpn_rshift</code> might be able to
|
||
do something branch-free for unaligned startups, and shaving one insn
|
||
from the loop with alternative indexing might save a cycle.
|
||
<li> PPC32: Try using fewer registers in the current <code>mpn_lshift</code>.
|
||
The pipeline is now extremely deep, perhaps unnecessarily deep.
|
||
<li> Fujitsu VPP: Vectorize main functions, perhaps in assembly language.
|
||
<li> Fujitsu VPP: Write <code>mpn_mul_basecase</code> and
|
||
<code>mpn_sqr_basecase</code>. This should use a "vertical multiplication
|
||
method", to avoid carry propagation. splitting one of the operands in
|
||
11-bit chunks.
|
||
<li> Pentium: <code>mpn_lshift</code> by 31 should use the special rshift
|
||
by 1 code, and vice versa <code>mpn_rshift</code> by 31 should use the
|
||
special lshift by 1. This would be best as a jump across to the other
|
||
routine, could let both live in lshift.asm and omit rshift.asm on finding
|
||
<code>mpn_rshift</code> already provided.
|
||
<li> Cray T3E: Experiment with optimization options. In particular,
|
||
-hpipeline3 seems promising. We should at least up -O to -O2 or -O3.
|
||
<li> Cray: <code>mpn_com_n</code> and <code>mpn_and_n</code> etc very probably
|
||
wants a pragma like <code>MPN_COPY_INCR</code>.
|
||
<li> Cray vector systems: <code>mpn_lshift</code>, <code>mpn_rshift</code>,
|
||
<code>mpn_popcount</code> and <code>mpn_hamdist</code> are nice and small
|
||
and could be inlined to avoid function calls.
|
||
<li> Cray: Variable length arrays seem to be faster than the tal-notreent.c
|
||
scheme. Not sure why, maybe they merely give the compiler more
|
||
information about aliasing (or the lack thereof). Would like to modify
|
||
<code>TMP_ALLOC</code> to use them, or introduce a new scheme. Memory
|
||
blocks wanted unconditionally are easy enough, those wanted only
|
||
sometimes are a problem. Perhaps a special size calculation to ask for a
|
||
dummy length 1 when unwanted, or perhaps an inlined subroutine
|
||
duplicating code under each conditional. Don't really want to turn
|
||
everything into a dog's dinner just because Cray don't offer an
|
||
<code>alloca</code>.
|
||
<li> Cray: <code>mpn_get_str</code> on power-of-2 bases ought to vectorize.
|
||
Does it? <code>bits_per_digit</code> and the inner loop over bits in a
|
||
limb might prevent it. Perhaps special cases for binary, octal and hex
|
||
would be worthwhile (very possibly for all processors too).
|
||
<li> S390: <code>BSWAP_LIMB_FETCH</code> looks like it could be done with
|
||
<code>lrvg</code>, as per glibc sysdeps/s390/s390-64/bits/byteswap.h.
|
||
This is only for 64-bit mode or something is it, since 32-bit mode has
|
||
other code? Also, is it worth using for <code>BSWAP_LIMB</code> too, or
|
||
would that mean a store and re-fetch? Presumably that's what comes out
|
||
in glibc.
|
||
<li> Improve <code>count_leading_zeros</code> for 64-bit machines:
|
||
<pre>
|
||
if ((x >> 32) == 0) { x <<= 32; cnt += 32; }
|
||
if ((x >> 48) == 0) { x <<= 16; cnt += 16; }
|
||
... </pre>
|
||
<li> IRIX 6 MIPSpro compiler has an <code>__inline</code> which could perhaps
|
||
be used in <code>__GMP_EXTERN_INLINE</code>. What would be the right way
|
||
to identify suitable versions of that compiler?
|
||
<li> IRIX <code>cc</code> is rumoured to have an <code>_int_mult_upper</code>
|
||
(in <code><intrinsics.h></code> like Cray), but it didn't seem to
|
||
exist on some IRIX 6.5 systems tried. If it does actually exist
|
||
somewhere it would very likely be an improvement over a function call to
|
||
umul.asm.
|
||
<li> <code>mpn_get_str</code> final divisions by the base with
|
||
<code>udiv_qrnd_unnorm</code> could use some sort of multiply-by-inverse
|
||
on suitable machines. This ends up happening for decimal by presenting
|
||
the compiler with a run-time constant, but the same for other bases would
|
||
be good. Perhaps use could be made of the fact base<256.
|
||
<li> <code>mpn_umul_ppmm</code>, <code>mpn_udiv_qrnnd</code>: Return a
|
||
structure like <code>div_t</code> to avoid going through memory, in
|
||
particular helping RISCs that don't do store-to-load forwarding. Clearly
|
||
this is only possible if the ABI returns a structure of two
|
||
<code>mp_limb_t</code>s in registers.
|
||
<br>
|
||
On PowerPC, structures are returned in memory on AIX and Darwin. In SVR4
|
||
they're returned in registers, except that draft SVR4 had said memory, so
|
||
it'd be prudent to check which is done. We can jam the compiler into the
|
||
right mode if we know how, since all this is purely internal to libgmp.
|
||
(gcc has an option, though of course gcc doesn't matter since we use
|
||
inline asm there.)
|
||
</ul>
|
||
|
||
<h4>New Functionality</h4>
|
||
<ul>
|
||
<li> Maybe add <code>mpz_crr</code> (Chinese Remainder Reconstruction).
|
||
<li> Let `0b' and `0B' mean binary input everywhere.
|
||
<li> <code>mpz_init</code> and <code>mpq_init</code> could do lazy allocation.
|
||
Set <code>ALLOC(var)</code> to 0 to indicate nothing allocated, and let
|
||
<code>_mpz_realloc</code> do the initial alloc. Set
|
||
<code>z->_mp_d</code> to a dummy that <code>mpz_get_ui</code> and
|
||
similar can unconditionally fetch from. Niels M<>ller has had a go at
|
||
this.
|
||
<br>
|
||
The advantages of the lazy scheme would be:
|
||
<ul>
|
||
<li> Initial allocate would be the size required for the first value
|
||
stored, rather than getting 1 limb in <code>mpz_init</code> and then
|
||
more or less immediately reallocating.
|
||
<li> <code>mpz_init</code> would only store magic values in the
|
||
<code>mpz_t</code> fields, and could be inlined.
|
||
<li> A fixed initializer could even be used by applications, like
|
||
<code>mpz_t z = MPZ_INITIALIZER;</code>, which might be convenient
|
||
for globals.
|
||
</ul>
|
||
The advantages of the current scheme are:
|
||
<ul>
|
||
<li> <code>mpz_set_ui</code> and other similar routines needn't check the
|
||
size allocated and can just store unconditionally.
|
||
<li> <code>mpz_set_ui</code> and perhaps others like
|
||
<code>mpz_tdiv_r_ui</code> and a prospective
|
||
<code>mpz_set_ull</code> could be inlined.
|
||
</ul>
|
||
<li> Add <code>mpf_out_raw</code> and <code>mpf_inp_raw</code>. Make sure
|
||
format is portable between 32-bit and 64-bit machines, and between
|
||
little-endian and big-endian machines. A format which MPFR can use too
|
||
would be good.
|
||
<li> <code>mpn_and_n</code> ... <code>mpn_copyd</code>: Perhaps make the mpn
|
||
logops and copys available in gmp.h, either as library functions or
|
||
inlines, with the availability of library functions instantiated in the
|
||
generated gmp.h at build time.
|
||
<li> <code>mpz_set_str</code> etc variants taking string lengths rather than
|
||
null-terminators.
|
||
<li> <code>mpz_andn</code>, <code>mpz_iorn</code>, <code>mpz_nand</code>,
|
||
<code>mpz_nior</code>, <code>mpz_xnor</code> might be useful additions,
|
||
if they could share code with the current such functions (which should be
|
||
possible).
|
||
<li> <code>mpz_and_ui</code> etc might be of use sometimes. Suggested by
|
||
Niels M<>ller.
|
||
<li> <code>mpf_set_str</code> and <code>mpf_inp_str</code> could usefully
|
||
accept 0x, 0b etc when base==0. Perhaps the exponent could default to
|
||
decimal in this case, with a further 0x, 0b etc allowed there.
|
||
Eg. 0xFFAA@0x5A. A leading "0" for octal would match the integers, but
|
||
probably something like "0.123" ought not mean octal.
|
||
<li> <code>GMP_LONG_LONG_LIMB</code> or some such could become a documented
|
||
feature of gmp.h, so applications could know whether to
|
||
<code>printf</code> a limb using <code>%lu</code> or <code>%Lu</code>.
|
||
<li> <code>GMP_PRIdMP_LIMB</code> and similar defines following C99
|
||
<inttypes.h> might be of use to applications printing limbs. But
|
||
if <code>GMP_LONG_LONG_LIMB</code> or whatever is added then perhaps this
|
||
can easily enough be left to applications.
|
||
<li> <code>gmp_printf</code> could accept <code>%b</code> for binary output.
|
||
It'd be nice if it worked for plain <code>int</code> etc too, not just
|
||
<code>mpz_t</code> etc.
|
||
<li> <code>gmp_printf</code> in fact could usefully accept an arbitrary base,
|
||
for both integer and float conversions. A base either in the format
|
||
string or as a parameter with <code>*</code> should be allowed. Maybe
|
||
<code>&13b</code> (b for base) or something like that.
|
||
<li> <code>gmp_printf</code> could perhaps accept <code>mpq_t</code> for float
|
||
conversions, eg. <code>"%.4Qf"</code>. This would be merely for
|
||
convenience, but still might be useful. Rounding would be the same as
|
||
for an <code>mpf_t</code> (ie. currently round-to-nearest, but not
|
||
actually documented). Alternately, perhaps a separate
|
||
<code>mpq_get_str_point</code> or some such might be more use. Suggested
|
||
by Pedro Gimeno.
|
||
<li> <code>mpz_rscan0</code> or <code>mpz_revscan0</code> or some such
|
||
searching towards the low end of an integer might match
|
||
<code>mpz_scan0</code> nicely. Likewise for <code>scan1</code>.
|
||
Suggested by Roberto Bagnara.
|
||
<li> <code>mpz_bit_subset</code> or some such to test whether one integer is a
|
||
bitwise subset of another might be of use. Some sort of return value
|
||
indicating whether it's a proper or non-proper subset would be good and
|
||
wouldn't cost anything in the implementation. Suggested by Roberto
|
||
Bagnara.
|
||
<li> <code>mpf_get_ld</code>, <code>mpf_set_ld</code>: Conversions between
|
||
<code>mpf_t</code> and <code>long double</code>, suggested by Dan
|
||
Christensen. Other <code>long double</code> routines might be desirable
|
||
too, but <code>mpf</code> would be a start.
|
||
<br>
|
||
<code>long double</code> is an ANSI-ism, so everything involving it would
|
||
need to be suppressed on a K&R compiler.
|
||
<br>
|
||
There'd be some work to be done by <code>configure</code> to recognise
|
||
the format in use, MPFR has a start on this. Often <code>long
|
||
double</code> is the same as <code>double</code>, which is easy but
|
||
pretty pointless. A single float format detector macro could look at
|
||
<code>double</code> then <code>long double</code>
|
||
<br>
|
||
Sometimes there's a compiler option for the size of a <code>long
|
||
double</code>, eg. xlc on AIX can use either 64-bit or 128-bit. It's
|
||
probably simplest to regard this as a compiler compatibility issue, and
|
||
leave it to users or sysadmins to ensure application and library code is
|
||
built the same.
|
||
<li> <code>mpz_sqrt_if_perfect_square</code>: When
|
||
<code>mpz_perfect_square_p</code> does its tests it calculates a square
|
||
root and then discards it. For some applications it might be useful to
|
||
return that root. Suggested by Jason Moxham.
|
||
<li> <code>mpz_get_ull</code>, <code>mpz_set_ull</code>,
|
||
<code>mpz_get_sll</code>, <code>mpz_get_sll</code>: Conversions for
|
||
<code>long long</code>. These would aid interoperability, though a
|
||
mixture of GMP and <code>long long</code> would probably not be too
|
||
common. Since <code>long long</code> is not always available (it's in
|
||
C99 and GCC though), disadvantages of using <code>long long</code> in
|
||
libgmp.a would be
|
||
<ul>
|
||
<li> Library contents vary according to the build compiler.
|
||
<li> gmp.h would need an ugly <code>#ifdef</code> block to decide if the
|
||
application compiler could take the <code>long long</code>
|
||
prototypes.
|
||
<li> Some sort of <code>LIBGMP_HAS_LONGLONG</code> might be wanted to
|
||
indicate whether the functions are available. (Applications using
|
||
autoconf could probe the library too.)
|
||
</ul>
|
||
It'd be possible to defer the need for <code>long long</code> to
|
||
application compile time, by having something like
|
||
<code>mpz_set_2ui</code> called with two halves of a <code>long
|
||
long</code>. Disadvantages of this would be,
|
||
<ul>
|
||
<li> Bigger code in the application, though perhaps not if a <code>long
|
||
long</code> is normally passed as two halves anyway.
|
||
<li> <code>mpz_get_ull</code> would be a rather big inline, or would have
|
||
to be two function calls.
|
||
<li> <code>mpz_get_sll</code> would be a worse inline, and would put the
|
||
treatment of <code>-0x10..00</code> into applications (see
|
||
<code>mpz_get_si</code> correctness above).
|
||
<li> Although having libgmp.a independent of the build compiler is nice,
|
||
it sort of sacrifices the capabilities of a good compiler to
|
||
uniformity with inferior ones.
|
||
</ul>
|
||
Plain use of <code>long long</code> is probably the lesser evil, if only
|
||
because it makes best use of gcc. In fact perhaps it would suffice to
|
||
guarantee <code>long long</code> conversions only when using GCC for both
|
||
application and library. That would cover free software, and we can
|
||
worry about selected vendor compilers later.
|
||
<br>
|
||
In C++ the situation is probably clearer, we demand fairly recent C++ so
|
||
<code>long long</code> should be available always. We'd probably prefer
|
||
to have the C and C++ the same in respect of <code>long long</code>
|
||
support, but it would be possible to have it unconditionally in gmpxx.h,
|
||
by some means or another.
|
||
<li> <code>mpz_strtoz</code> parsing the same as <code>strtol</code>.
|
||
Suggested by Alexander Kruppa.
|
||
</ul>
|
||
|
||
|
||
<h4>Configuration</h4>
|
||
|
||
<ul>
|
||
<li> Alpha ev7, ev79: Add code to config.guess to detect these. Believe ev7
|
||
will be "3-1307" in the current switch, but need to verify that. (On
|
||
OSF, current configfsf.guess identifies ev7 using psrinfo, we need to do
|
||
it ourselves for other systems.)
|
||
<li> Alpha OSF: Libtool (version 1.5) doesn't seem to recognise this system is
|
||
"pic always" and ends up running gcc twice with the same options. This
|
||
is wasteful, but harmless. Perhaps a newer libtool will be better.
|
||
<li> ARM: <code>umul_ppmm</code> in longlong.h always uses <code>umull</code>,
|
||
but is that available only for M series chips or some such? Perhaps it
|
||
should be configured in some way.
|
||
<li> HPPA: config.guess should recognize 7000, 7100, 7200, and 8x00.
|
||
<li> HPPA: gcc 3.2 introduces a <code>-mschedule=7200</code> etc parameter,
|
||
which could be driven by an exact hppa cpu type.
|
||
<li> Mips: config.guess should say mipsr3000, mipsr4000, mipsr10000, etc.
|
||
"hinv -c processor" gives lots of information on Irix. Standard
|
||
config.guess appends "el" to indicate endianness, but
|
||
<code>AC_C_BIGENDIAN</code> seems the best way to handle that for GMP.
|
||
<li> PowerPC: The function descriptor nonsense for AIX is currently driven by
|
||
<code>*-*-aix*</code>. It might be more reliable to do some sort of
|
||
feature test, examining the compiler output perhaps. It might also be
|
||
nice to merge the aix.m4 files into powerpc-defs.m4.
|
||
<li> config.m4 is generated only by the configure script, it won't be
|
||
regenerated by config.status. Creating it as an <code>AC_OUTPUT</code>
|
||
would work, but it might upset "make" to have things like <code>L$</code>
|
||
get into the Makefiles through <code>AC_SUBST</code>.
|
||
<code>AC_CONFIG_COMMANDS</code> would be the alternative. With some
|
||
careful m4 quoting the <code>changequote</code> calls might not be
|
||
needed, which might free up the order in which things had to be output.
|
||
<li> Automake: Latest automake has a <code>CCAS</code>, <code>CCASFLAGS</code>
|
||
scheme. Though we probably wouldn't be using its assembler support we
|
||
could try to use those variables in compatible ways.
|
||
<li> <code>GMP_LDFLAGS</code> could probably be done with plain
|
||
<code>LDFLAGS</code> already used by automake for all linking. But with
|
||
a bit of luck the next libtool will pass pretty much all
|
||
<code>CFLAGS</code> through to the compiler when linking, making
|
||
<code>GMP_LDFLAGS</code> unnecessary.
|
||
<li> mpn/Makeasm.am uses <code>-c</code> and <code>-o</code> together in the
|
||
.S and .asm rules, but apparently that isn't completely portable (there's
|
||
an autoconf <code>AC_PROG_CC_C_O</code> test for it). So far we've not
|
||
had problems, but perhaps the rules could be rewritten to use "foo.s" as
|
||
the temporary, or to do a suitable "mv" of the result. The only danger
|
||
from using foo.s would be if a compile failed and the temporary foo.s
|
||
then looked like the primary source. Hopefully if the
|
||
<code>SUFFIXES</code> are ordered to have .S and .asm ahead of .s that
|
||
wouldn't happen. Might need to check.
|
||
</ul>
|
||
|
||
|
||
<h4>Random Numbers</h4>
|
||
<ul>
|
||
<li> <code>_gmp_rand</code> is not particularly fast on the linear
|
||
congruential algorithm and could stand various improvements.
|
||
<ul>
|
||
<li> Make a second seed area within <code>gmp_randstate_t</code> (or
|
||
<code>_mp_algdata</code> rather) to save some copying.
|
||
<li> Make a special case for a single limb <code>2exp</code> modulus, to
|
||
avoid <code>mpn_mul</code> calls. Perhaps the same for two limbs.
|
||
<li> Inline the <code>lc</code> code, to avoid a function call and
|
||
<code>TMP_ALLOC</code> for every chunk.
|
||
<li> Perhaps the <code>2exp</code> and general LC cases should be split,
|
||
for clarity (if the general case is retained).
|
||
</ul>
|
||
<li> <code>gmp_randstate_t</code> used for parameters perhaps should become
|
||
<code>gmp_randstate_ptr</code> the same as other types.
|
||
<li> Some of the empirical randomness tests could be included in a "make
|
||
check". They ought to work everywhere, for a given seed at least.
|
||
</ul>
|
||
|
||
|
||
<h4>C++</h4>
|
||
<ul>
|
||
<li> <code>mpz_class(string)</code>, etc: Use the C++ global locale to
|
||
identify whitespace.
|
||
<br>
|
||
<code>mpf_class(string)</code>: Use the C++ global locale decimal point,
|
||
rather than the C one.
|
||
<br>
|
||
Consider making these variant <code>mpz_set_str</code> etc forms
|
||
available for <code>mpz_t</code> too, not just <code>mpz_class</code>
|
||
etc.
|
||
<li> <code>mpq_class operator+=</code>: Don't emit an unnecssary
|
||
<code>mpq_set(q,q)</code> before <code>mpz_addmul</code> etc.
|
||
<li> Put various bits of gmpxx.h into libgmpxx, to avoid excessive inlining.
|
||
Candidates for this would be,
|
||
<ul>
|
||
<li> <code>mpz_class(const char *)</code>, etc: since they're normally
|
||
not fast anyway, and we can hide the exception <code>throw</code>.
|
||
<li> <code>mpz_class(string)</code>, etc: to hide the <code>cstr</code>
|
||
needed to get to the C conversion function.
|
||
<li> <code>mpz_class string, char*</code> etc constructors: likewise to
|
||
hide the throws and conversions.
|
||
<li> <code>mpz_class::get_str</code>, etc: to hide the <code>char*</code>
|
||
to <code>string</code> conversion and free. Perhaps
|
||
<code>mpz_get_str</code> can write directly into a
|
||
<code>string</code>, to avoid copying.
|
||
<br>
|
||
Consider making such <code>string</code> returning variants
|
||
available for use with plain <code>mpz_t</code> etc too.
|
||
</ul>
|
||
</ul>
|
||
|
||
<h4>Miscellaneous</h4>
|
||
<ul>
|
||
<li> <code>mpz_gcdext</code> and <code>mpn_gcdext</code> ought to document
|
||
what range of values the generated cofactors can take, and preferably
|
||
ensure the definition uniquely specifies the cofactors for given inputs.
|
||
A basic extended Euclidean algorithm or multi-step variant leads to
|
||
|x|<|b| and |y|<|a| or something like that, but there's probably
|
||
two solutions under just those restrictions.
|
||
<li> demos/factorize.c: use <code>mpz_divisible_ui_p</code> rather than
|
||
<code>mpz_tdiv_qr_ui</code>. (Of course dividing multiple primes at a
|
||
time would be better still.)
|
||
<li> The various test programs use quite a bit of the main
|
||
<code>libgmp</code>. This establishes good cross-checks, but it might be
|
||
better to use simple reference routines where possible. Where it's not
|
||
possible some attention could be paid to the order of the tests, so a
|
||
<code>libgmp</code> routine is only used for tests once it seems to be
|
||
good.
|
||
<li> <code>MUL_FFT_THRESHOLD</code> etc: the FFT thresholds should allow a
|
||
return to a previous k at certain sizes. This arises basically due to
|
||
the step effect caused by size multiples effectively used for each k.
|
||
Looking at a graph makes it fairly clear.
|
||
<li> <code>__gmp_doprnt_mpf</code> does a rather unattractive round-to-nearest
|
||
on the string returned by <code>mpf_get_str</code>. Perhaps some variant
|
||
of <code>mpf_get_str</code> could be made which would better suit.
|
||
</ul>
|
||
|
||
|
||
<h4>Aids to Development</h4>
|
||
<ul>
|
||
<li> Add <code>ASSERT</code>s at the start of each user-visible mpz/mpq/mpf
|
||
function to check the validity of each <code>mp?_t</code> parameter, in
|
||
particular to check they've been <code>mp?_init</code>ed. This might
|
||
catch elementary mistakes in user programs. Care would need to be taken
|
||
over <code>MPZ_TMP_INIT</code>ed variables used internally. If nothing
|
||
else then consistency checks like size<=alloc, ptr not
|
||
<code>NULL</code> and ptr+size not wrapping around the address space,
|
||
would be possible. A more sophisticated scheme could track
|
||
<code>_mp_d</code> pointers and ensure only a valid one is used. Such a
|
||
scheme probably wouldn't be reentrant, not without some help from the
|
||
system.
|
||
<li> tune/time.c could try to determine at runtime whether
|
||
<code>getrusage</code> and <code>gettimeofday</code> are reliable.
|
||
Currently we pretend in configure that the dodgy m68k netbsd 1.4.1
|
||
<code>getrusage</code> doesn't exist. If a test might take a long time
|
||
to run then perhaps cache the result in a file somewhere.
|
||
<li> tune/time.c could choose the default precision based on the
|
||
<code>speed_unittime</code> determined, independent of the method in use.
|
||
<li> Cray vector systems: CPU frequency could be determined from
|
||
<code>sysconf(_SC_CLK_TCK)</code>, since it seems to be clock cycle
|
||
based. Is this true for all Cray systems? Would like some documentation
|
||
or something to confirm.
|
||
</ul>
|
||
|
||
|
||
<h4>Documentation</h4>
|
||
<ul>
|
||
<li> <code>mpz_inp_str</code> (etc) doesn't say when it stops reading digits.
|
||
<li> <code>mpn_get_str</code> isn't terribly clear about how many digits it
|
||
produces. It'd probably be possible to say at most one leading zero,
|
||
which is what both it and <code>mpz_get_str</code> currently do. But
|
||
want to be careful not to bind ourselves to something that might not suit
|
||
another implementation.
|
||
<li> <code>va_arg</code> doesn't do the right thing with <code>mpz_t</code>
|
||
etc directly, but instead needs a pointer type like <code>MP_INT*</code>.
|
||
It'd be good to show how to do this, but we'd either need to document
|
||
<code>mpz_ptr</code> and friends, or perhaps fallback on something
|
||
slightly nasty with <code>void*</code>.
|
||
</ul>
|
||
|
||
|
||
<h4>Bright Ideas</h4>
|
||
|
||
<p> The following may or may not be feasible, and aren't likely to get done in the
|
||
near future, but are at least worth thinking about.
|
||
|
||
<ul>
|
||
<li> Reorganize longlong.h so that we can inline the operations even for the
|
||
system compiler. When there is no such compiler feature, make calls to
|
||
stub functions. Write such stub functions for as many machines as
|
||
possible.
|
||
<li> longlong.h could declare when it's using, or would like to use,
|
||
<code>mpn_umul_ppmm</code>, and the corresponding umul.asm file could be
|
||
included in libgmp only in that case, the same as is effectively done for
|
||
<code>__clz_tab</code>. Likewise udiv.asm and perhaps cntlz.asm. This
|
||
would only be a very small space saving, so perhaps not worth the
|
||
complexity.
|
||
<li> longlong.h could be built at configure time by concatenating or
|
||
#including fragments from each directory in the mpn path. This would
|
||
select CPU specific macros the same way as CPU specific assembler code.
|
||
Code used would no longer depend on cpp predefines, and the current
|
||
nested conditionals could be flattened out.
|
||
<li> <code>mpz_get_si</code> returns 0x80000000 for -0x100000000, whereas it's
|
||
sort of supposed to return the low 31 (or 63) bits. But this is
|
||
undocumented, and perhaps not too important.
|
||
<li> <code>mpz_init_set*</code> and <code>mpz_realloc</code> could allocate
|
||
say an extra 16 limbs over what's needed, so as to reduce the chance of
|
||
having to do a reallocate if the <code>mpz_t</code> grows a bit more.
|
||
This could only be an option, since it'd badly bloat memory usage in
|
||
applications using many small values.
|
||
<li> <code>mpq</code> functions could perhaps check for numerator or
|
||
denominator equal to 1, on the assumption that integers or
|
||
denominator-only values might be expected to occur reasonably often.
|
||
<li> <code>count_trailing_zeros</code> is used on more or less uniformly
|
||
distributed numbers in a couple of places. For some CPUs
|
||
<code>count_trailing_zeros</code> is slow and it's probably worth handling
|
||
the frequently occurring 0 to 2 trailing zeros cases specially.
|
||
<li> <code>mpf_t</code> might like to let the exponent be undefined when
|
||
size==0, instead of requiring it 0 as now. It should be possible to do
|
||
size==0 tests before paying attention to the exponent. The advantage is
|
||
not needing to set exp in the various places a zero result can arise,
|
||
which avoids some tedium but is otherwise perhaps not too important.
|
||
Currently <code>mpz_set_f</code> and <code>mpf_cmp_ui</code> depend on
|
||
exp==0, maybe elsewhere too.
|
||
<li> <code>__gmp_allocate_func</code>: Could use GCC <code>__attribute__
|
||
((malloc))</code> on this, though don't know if it'd do much. GCC 3.0
|
||
allows that attribute on functions, but not function pointers (see info
|
||
node "Attribute Syntax"), so would need a new autoconf test. This can
|
||
wait until there's a GCC that supports it.
|
||
<li> <code>mpz_add_ui</code> contains two <code>__GMPN_COPY</code>s, one from
|
||
<code>mpn_add_1</code> and one from <code>mpn_sub_1</code>. If those two
|
||
routines were opened up a bit maybe that code could be shared. When a
|
||
copy needs to be done there's no carry to append for the add, and if the
|
||
copy is non-empty no high zero for the sub.
|
||
</ul>
|
||
|
||
|
||
<h4>Old and Obsolete Stuff</h4>
|
||
|
||
<p> The following tasks apply to chips or systems that are old and/or obsolete.
|
||
It's unlikely anything will be done about them unless anyone is actively using
|
||
them.
|
||
|
||
<ul>
|
||
<li> Sparc32: The integer based udiv_nfp.asm used to be selected by
|
||
<code>configure --nfp</code> but that option is gone now that autoconf is
|
||
used. The file could go somewhere suitable in the mpn search if any
|
||
chips might benefit from it, though it's possible we don't currently
|
||
differentiate enough exact cpu types to do this properly.
|
||
<li> VAX D and G format <code>double</code> floats are straightforward and
|
||
could perhaps be handled directly in <code>__gmp_extract_double</code>
|
||
and maybe in <code>mpn_get_d</code>, rather than falling back on the
|
||
generic code. (Both formats are detected by <code>configure</code>.)
|
||
</ul>
|
||
|
||
|
||
<hr>
|
||
|
||
</body>
|
||
</html>
|
||
|
||
<!--
|
||
Local variables:
|
||
eval: (add-hook 'write-file-hooks 'time-stamp)
|
||
time-stamp-start: "This file current as of "
|
||
time-stamp-format: "%:d %3b %:y"
|
||
time-stamp-end: "\\."
|
||
time-stamp-line-limit: 50
|
||
End:
|
||
-->
|