165 lines
3.8 KiB
NASM
165 lines
3.8 KiB
NASM
dnl x86 mpn_mod_1 -- mpn by limb remainder.
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dnl Copyright 1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
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dnl
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dnl This file is part of the GNU MP Library.
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dnl
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dnl The GNU MP Library is free software; you can redistribute it and/or
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dnl modify it under the terms of the GNU Lesser General Public License as
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dnl published by the Free Software Foundation; either version 2.1 of the
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dnl License, or (at your option) any later version.
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dnl
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dnl The GNU MP Library is distributed in the hope that it will be useful,
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dnl but WITHOUT ANY WARRANTY; without even the implied warranty of
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dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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dnl Lesser General Public License for more details.
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dnl
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dnl You should have received a copy of the GNU Lesser General Public
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dnl License along with the GNU MP Library; see the file COPYING.LIB. If
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dnl not, write to the Free Software Foundation, Inc., 51 Franklin Street,
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dnl Fifth Floor, Boston, MA 02110-1301, USA.
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include(`../config.m4')
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C cycles/limb
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C 486 42 approx, maybe
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C P5 44
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C P6 39
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C K6 20
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C K7 41
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C P4 58
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C mp_limb_t mpn_mod_1 (mp_srcptr src, mp_size_t size, mp_limb_t divisor);
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C mp_limb_t mpn_mod_1c (mp_srcptr src, mp_size_t size, mp_limb_t divisor,
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C mp_limb_t carry);
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C
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C Essentially this code is the same as the division based part of
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C mpn/generic/mod_1.c, but has the advantage that we get the desired divl
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C instruction even when gcc is not being used (where longlong.h only has the
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C rather slow generic C udiv_qrnnd().
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C
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C A test is done to see if the high limb is less the the divisor, and if so
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C one less div is done. A div is between 20 and 40 cycles on the various
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C x86s, so assuming high<divisor about half the time, then this test saves
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C half that amount. The branch misprediction penalty on each chip is less
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C than half a div.
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C
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C
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C Notes for K6:
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C
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C Back-to-back div instructions take 20 cycles, the same as the loop here,
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C so it seems there's nothing to gain by rearranging. Pairing the mov and
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C loop instructions was found to gain nothing. Normally we use a loop
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C instruction rather than decl/jnz, but it gains nothing here.
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C
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C A multiply-by-inverse is used in mpn/x86/k6/pre_mod_1.asm, but it saves
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C only 2 c/l so currently we haven't bothered with the same for mpn_mod_1.
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C If an inverse takes about 40 cycles for normalized or perhaps 60 for
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C unnormalized (due to bsfl being slow on k6) then the threshold would be at
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C least 20 or 30 limbs.
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C
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defframe(PARAM_CARRY, 16)
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defframe(PARAM_DIVISOR,12)
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defframe(PARAM_SIZE, 8)
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defframe(PARAM_SRC, 4)
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TEXT
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ALIGN(16)
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PROLOGUE(mpn_mod_1)
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deflit(`FRAME',0)
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movl PARAM_SIZE, %ecx
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pushl %ebx FRAME_pushl()
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movl PARAM_SRC, %ebx
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pushl %esi FRAME_pushl()
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orl %ecx, %ecx
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jz L(done_zero)
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movl PARAM_DIVISOR, %esi
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movl -4(%ebx,%ecx,4), %eax C src high limb
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cmpl %esi, %eax
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sbbl %edx, %edx C -1 if high<divisor
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addl %edx, %ecx C skip one division if high<divisor
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jz L(done_eax)
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andl %eax, %edx C carry if high<divisor
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L(top):
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C eax scratch (quotient)
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C ebx src
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C ecx counter
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C edx carry (remainder)
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C esi divisor
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C edi
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C ebp
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movl -4(%ebx,%ecx,4), %eax
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divl %esi
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decl %ecx
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jnz L(top)
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movl %edx, %eax
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L(done_eax):
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popl %esi
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popl %ebx
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ret
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EPILOGUE()
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C This code located after mpn_mod_1, so the jump to L(top) here is
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C back and hence will be predicted as taken. (size==0 is considered
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C unlikely.)
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ALIGN(16)
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PROLOGUE(mpn_mod_1c)
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deflit(`FRAME',0)
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movl PARAM_SIZE, %ecx
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pushl %ebx FRAME_pushl()
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movl PARAM_SRC, %ebx
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pushl %esi FRAME_pushl()
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movl PARAM_DIVISOR, %esi
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orl %ecx, %ecx
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movl PARAM_CARRY, %edx
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jnz L(top)
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popl %esi
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movl %edx, %eax
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popl %ebx
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ret
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C This code is for mpn_mod_1, but is positioned here to save some
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C space in the alignment padding.
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C
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L(done_zero):
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popl %esi
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xorl %eax, %eax
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popl %ebx
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ret
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EPILOGUE()
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