--- /dev/null
+dnl Intel Pentium-4 mpn_divexact_1 -- mpn by limb exact division.
+
+dnl Copyright 2001, 2002, 2007 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 3 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 License
+dnl along with the GNU MP Library. If not, see http://www.gnu.org/licenses/.
+
+include(`../config.m4')
+
+
+C P4: 19.0 cycles/limb
+
+
+C void mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size,
+C mp_limb_t divisor);
+C
+C Pairs of movd's are used to avoid unaligned loads. Despite the loads not
+C being on the dependent chain and there being plenty of cycles available,
+C using an unaligned movq on every second iteration measured about 23 c/l.
+C
+C Using divl for size==1 seems a touch quicker than mul-by-inverse. The mul
+C will be about 9+2*4+2*2+10*4+19+12 = 92 cycles latency, though some of
+C that might be hidden by out-of-order execution, whereas divl is around 60.
+C At size==2 an extra 19 for the mul versus 60 for the divl will see the mul
+C faster.
+
+defframe(PARAM_DIVISOR,16)
+defframe(PARAM_SIZE, 12)
+defframe(PARAM_SRC, 8)
+defframe(PARAM_DST, 4)
+
+ TEXT
+
+ ALIGN(16)
+PROLOGUE(mpn_divexact_1)
+deflit(`FRAME',0)
+
+ movl PARAM_SIZE, %edx
+
+ movl PARAM_SRC, %eax
+
+ movl PARAM_DIVISOR, %ecx
+ subl $1, %edx
+ jnz L(two_or_more)
+
+ movl (%eax), %eax
+ xorl %edx, %edx
+
+ divl %ecx
+ movl PARAM_DST, %ecx
+
+ movl %eax, (%ecx)
+ ret
+
+
+L(two_or_more):
+ C eax src
+ C ebx
+ C ecx divisor
+ C edx size-1
+
+ movl %ecx, %eax
+ bsfl %ecx, %ecx C trailing twos
+
+ shrl %cl, %eax C d = divisor without twos
+ movd %eax, %mm6
+ movd %ecx, %mm7 C shift
+
+ shrl %eax C d/2
+
+ andl $127, %eax C d/2, 7 bits
+
+ifdef(`PIC',`
+ LEA( binvert_limb_table, %ecx)
+ movzbl (%eax,%ecx), %eax C inv 8 bits
+',`
+ movzbl binvert_limb_table(%eax), %eax C inv 8 bits
+')
+
+ C
+
+ movd %eax, %mm5 C inv
+
+ movd %eax, %mm0 C inv
+
+ pmuludq %mm5, %mm5 C inv*inv
+
+ C
+
+ pmuludq %mm6, %mm5 C inv*inv*d
+ paddd %mm0, %mm0 C 2*inv
+
+ C
+
+ psubd %mm5, %mm0 C inv = 2*inv - inv*inv*d
+ pxor %mm5, %mm5
+
+ paddd %mm0, %mm5
+ pmuludq %mm0, %mm0 C inv*inv
+
+ pcmpeqd %mm4, %mm4
+ psrlq $32, %mm4 C 0x00000000FFFFFFFF
+
+ C
+
+ pmuludq %mm6, %mm0 C inv*inv*d
+ paddd %mm5, %mm5 C 2*inv
+
+ movl PARAM_SRC, %eax
+ movl PARAM_DST, %ecx
+ pxor %mm1, %mm1 C initial carry limb
+
+ C
+
+ psubd %mm0, %mm5 C inv = 2*inv - inv*inv*d
+
+ ASSERT(e,` C expect d*inv == 1 mod 2^BITS_PER_MP_LIMB
+ pushl %eax FRAME_pushl()
+ movq %mm6, %mm0
+ pmuludq %mm5, %mm0
+ movd %mm0, %eax
+ cmpl $1, %eax
+ popl %eax FRAME_popl()')
+
+ pxor %mm0, %mm0 C initial carry bit
+
+
+C The dependent chain here is as follows.
+C
+C latency
+C psubq s = (src-cbit) - climb 2
+C pmuludq q = s*inverse 8
+C pmuludq prod = q*divisor 8
+C psrlq climb = high(prod) 2
+C --
+C 20
+C
+C Yet the loop measures 19.0 c/l, so obviously there's something gained
+C there over a straight reading of the chip documentation.
+
+L(top):
+ C eax src, incrementing
+ C ebx
+ C ecx dst, incrementing
+ C edx counter, size-1 iterations
+ C
+ C mm0 carry bit
+ C mm1 carry limb
+ C mm4 0x00000000FFFFFFFF
+ C mm5 inverse
+ C mm6 divisor
+ C mm7 shift
+
+ movd (%eax), %mm2
+ movd 4(%eax), %mm3
+ addl $4, %eax
+ punpckldq %mm3, %mm2
+
+ psrlq %mm7, %mm2
+ pand %mm4, %mm2 C src
+ psubq %mm0, %mm2 C src - cbit
+
+ psubq %mm1, %mm2 C src - cbit - climb
+ movq %mm2, %mm0
+ psrlq $63, %mm0 C new cbit
+
+ pmuludq %mm5, %mm2 C s*inverse
+ movd %mm2, (%ecx) C q
+ addl $4, %ecx
+
+ movq %mm6, %mm1
+ pmuludq %mm2, %mm1 C q*divisor
+ psrlq $32, %mm1 C new climb
+
+ subl $1, %edx
+ jnz L(top)
+
+
+L(done):
+ movd (%eax), %mm2
+ psrlq %mm7, %mm2 C src
+ psubq %mm0, %mm2 C src - cbit
+
+ psubq %mm1, %mm2 C src - cbit - climb
+
+ pmuludq %mm5, %mm2 C s*inverse
+ movd %mm2, (%ecx) C q
+
+ emms
+ ret
+
+EPILOGUE()
projects / msp430-gcc.git / blobdiff