dnl Intel P5 mpn_divexact_by3 -- mpn division by 3, expecting no remainder. dnl Copyright 2000, 2002 Free Software Foundation, Inc. dnl dnl This file is part of the GNU MP Library. dnl dnl The GNU MP Library is free software; you can redistribute it and/or dnl modify it under the terms of the GNU Lesser General Public License as dnl published by the Free Software Foundation; either version 2.1 of the dnl License, or (at your option) any later version. dnl dnl The GNU MP Library is distributed in the hope that it will be useful, dnl but WITHOUT ANY WARRANTY; without even the implied warranty of dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU dnl Lesser General Public License for more details. dnl dnl You should have received a copy of the GNU Lesser General Public dnl License along with the GNU MP Library; see the file COPYING.LIB. If dnl not, write to the Free Software Foundation, Inc., 51 Franklin Street, dnl Fifth Floor, Boston, MA 02110-1301, USA. include(`../config.m4') C P5: 15.0 cycles/limb C mp_limb_t mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, C mp_limb_t carry); defframe(PARAM_CARRY,16) defframe(PARAM_SIZE, 12) defframe(PARAM_SRC, 8) defframe(PARAM_DST, 4) dnl multiplicative inverse of 3, modulo 2^32 deflit(INVERSE_3, 0xAAAAAAAB) dnl ceil(b/3), ceil(b*2/3) and floor(b*2/3) where b=2^32 deflit(ONE_THIRD_CEIL, 0x55555556) deflit(TWO_THIRDS_CEIL, 0xAAAAAAAB) deflit(TWO_THIRDS_FLOOR, 0xAAAAAAAA) TEXT ALIGN(8) PROLOGUE(mpn_divexact_by3c) deflit(`FRAME',0) movl PARAM_SRC, %ecx movl PARAM_SIZE, %edx decl %edx jnz L(two_or_more) movl (%ecx), %edx movl PARAM_CARRY, %eax C risk of cache bank clash here movl PARAM_DST, %ecx subl %eax, %edx sbbl %eax, %eax C 0 or -1 imull $INVERSE_3, %edx, %edx negl %eax C 0 or 1 cmpl $ONE_THIRD_CEIL, %edx sbbl $-1, %eax C +1 if edx>=ceil(b/3) cmpl $TWO_THIRDS_CEIL, %edx sbbl $-1, %eax C +1 if edx>=ceil(b*2/3) movl %edx, (%ecx) ret L(two_or_more): C eax C ebx C ecx src C edx size-1 C esi C edi C ebp pushl %ebx FRAME_pushl() pushl %esi FRAME_pushl() pushl %edi FRAME_pushl() pushl %ebp FRAME_pushl() movl PARAM_DST, %edi movl PARAM_CARRY, %esi movl (%ecx), %eax C src low limb xorl %ebx, %ebx sub %esi, %eax movl $TWO_THIRDS_FLOOR, %esi leal (%ecx,%edx,4), %ecx C &src[size-1] leal (%edi,%edx,4), %edi C &dst[size-1] adcl $0, %ebx C carry, 0 or 1 negl %edx C -(size-1) C The loop needs a source limb ready at the top, which leads to one limb C handled separately at the end, and the special case above for size==1. C There doesn't seem to be any scheduling that would keep the speed but move C the source load and carry subtract up to the top. C C The destination cache line prefetching adds 1 cycle to the loop but is C considered worthwhile. The slowdown is a factor of 1.07, but will prevent C repeated write-throughs if the destination isn't in L1. A version using C an outer loop to prefetch only every 8 limbs (a cache line) proved to be C no faster, due to unavoidable branch mispreditions in the inner loop. C C setc is 2 cycles on P54, so an adcl is used instead. If the movl $0,%ebx C could be avoided then the src limb fetch could pair up and save a cycle. C This would probably mean going to a two limb loop with the carry limb C alternately positive or negative, since an sbbl %ebx,%ebx will leave a C value which is in the opposite sense to the preceding sbbl/adcl %ebx,%eax. C C A register is used for TWO_THIRDS_FLOOR because a cmp can't be done as C "cmpl %edx, $n" with the immediate as the second operand. C C The "4" source displacement is in the loop rather than the setup because C this gets L(top) aligned to 8 bytes at no cost. ALIGN(8) L(top): C eax source limb, carry subtracted C ebx carry (0 or 1) C ecx &src[size-1] C edx counter, limbs, negative C esi TWO_THIRDS_FLOOR C edi &dst[size-1] C ebp scratch (result limb) imull $INVERSE_3, %eax, %ebp cmpl $ONE_THIRD_CEIL, %ebp movl (%edi,%edx,4), %eax C dst cache line prefetch sbbl $-1, %ebx C +1 if ebp>=ceil(b/3) cmpl %ebp, %esi movl 4(%ecx,%edx,4), %eax C next src limb sbbl %ebx, %eax C and further -1 if ebp>=ceil(b*2/3) movl $0, %ebx adcl $0, %ebx C new carry movl %ebp, (%edi,%edx,4) incl %edx jnz L(top) imull $INVERSE_3, %eax, %edx cmpl $ONE_THIRD_CEIL, %edx movl %edx, (%edi) sbbl $-1, %ebx C +1 if edx>=ceil(b/3) cmpl $TWO_THIRDS_CEIL, %edx sbbl $-1, %ebx C +1 if edx>=ceil(b*2/3) popl %ebp movl %ebx, %eax popl %edi popl %esi popl %ebx ret EPILOGUE()