dnl Intel P5 mpn_rshift -- mpn right shift.
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 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 P5: 1.75 cycles/limb.
C mp_limb_t mpn_rshift (mp_ptr dst, mp_srcptr src, mp_size_t size,
C unsigned shift);
C
C Shift src,size right by shift many bits and store the result in dst,size.
C Zeros are shifted in at the left. Return the bits shifted out at the
C right.
C
C It takes 6 mmx instructions to process 2 limbs, making 1.5 cycles/limb,
C and with a 4 limb loop and 1 cycle of loop overhead the total is 1.75 c/l.
C
C Full speed depends on source and destination being aligned. Unaligned mmx
C loads and stores on P5 don't pair and have a 2 cycle penalty. Some hairy
C setups and finish-ups are done to ensure alignment for the loop.
C
C MMX shifts work out a bit faster even for the simple loop.
defframe(PARAM_SHIFT,16)
defframe(PARAM_SIZE, 12)
defframe(PARAM_SRC, 8)
defframe(PARAM_DST, 4)
deflit(`FRAME',0)
dnl Minimum 5, because the unrolled loop can't handle less.
deflit(UNROLL_THRESHOLD, 5)
TEXT
ALIGN(8)
PROLOGUE(mpn_rshift)
pushl %ebx
pushl %edi
deflit(`FRAME',8)
movl PARAM_SIZE, %eax
movl PARAM_DST, %edx
movl PARAM_SRC, %ebx
movl PARAM_SHIFT, %ecx
cmp $UNROLL_THRESHOLD, %eax
jae L(unroll)
decl %eax
movl (%ebx), %edi C src low limb
jnz L(simple)
shrdl( %cl, %edi, %eax) C eax was decremented to zero
shrl %cl, %edi
movl %edi, (%edx) C dst low limb
popl %edi C risk of data cache bank clash
popl %ebx
ret
C -----------------------------------------------------------------------------
ALIGN(8)
L(simple):
C eax size-1
C ebx src
C ecx shift
C edx dst
C esi
C edi
C ebp
deflit(`FRAME',8)
movd (%ebx), %mm5 C src[0]
leal (%ebx,%eax,4), %ebx C &src[size-1]
movd %ecx, %mm6 C rshift
leal -4(%edx,%eax,4), %edx C &dst[size-2]
psllq $32, %mm5
negl %eax
C This loop is 5 or 8 cycles, with every second load unaligned and a wasted
C cycle waiting for the mm0 result to be ready. For comparison a shrdl is 4
C cycles and would be 8 in a simple loop. Using mmx helps the return value
C and last limb calculations too.
L(simple_top):
C eax counter, limbs, negative
C ebx &src[size-1]
C ecx return value
C edx &dst[size-2]
C
C mm0 scratch
C mm5 return value
C mm6 shift
movq (%ebx,%eax,4), %mm0
incl %eax
psrlq %mm6, %mm0
movd %mm0, (%edx,%eax,4)
jnz L(simple_top)
movd (%ebx), %mm0
psrlq %mm6, %mm5 C return value
psrlq %mm6, %mm0
popl %edi
movd %mm5, %eax
popl %ebx
movd %mm0, 4(%edx)
emms
ret
C -----------------------------------------------------------------------------
ALIGN(8)
L(unroll):
C eax size
C ebx src
C ecx shift
C edx dst
C esi
C edi
C ebp
deflit(`FRAME',8)
movd (%ebx), %mm5 C src[0]
movl $4, %edi
movd %ecx, %mm6 C rshift
testl %edi, %ebx
psllq $32, %mm5
jz L(start_src_aligned)
C src isn't aligned, process low limb separately (marked xxx) and
C step src and dst by one limb, making src aligned.
C
C source ebx
C --+-------+-------+-------+
C | xxx |
C --+-------+-------+-------+
C 4mod8 0mod8 4mod8
C
C dest edx
C --+-------+-------+
C | | xxx |
C --+-------+-------+
movq (%ebx), %mm0 C unaligned load
psrlq %mm6, %mm0
addl $4, %ebx
decl %eax
movd %mm0, (%edx)
addl $4, %edx
L(start_src_aligned):
movq (%ebx), %mm1
testl %edi, %edx
psrlq %mm6, %mm5 C retval
jz L(start_dst_aligned)
C dst isn't aligned, add 4 to make it so, and pretend the shift is
C 32 bits extra. Low limb of dst (marked xxx) handled here
C separately.
C
C source ebx
C --+-------+-------+
C | mm1 |
C --+-------+-------+
C 4mod8 0mod8
C
C dest edx
C --+-------+-------+-------+
C | xxx |
C --+-------+-------+-------+
C 4mod8 0mod8 4mod8
movq %mm1, %mm0
addl $32, %ecx C new shift
psrlq %mm6, %mm0
movd %ecx, %mm6
movd %mm0, (%edx)
addl $4, %edx
L(start_dst_aligned):
movq 8(%ebx), %mm3
negl %ecx
movq %mm3, %mm2 C mm2 src qword
addl $64, %ecx
movd %ecx, %mm7
psrlq %mm6, %mm1
leal -12(%ebx,%eax,4), %ebx
leal -20(%edx,%eax,4), %edx
psllq %mm7, %mm3
subl $7, %eax C size-7
por %mm1, %mm3 C mm3 ready to store
negl %eax C -(size-7)
jns L(finish)
C This loop is the important bit, the rest is just support. Careful
C instruction scheduling achieves the claimed 1.75 c/l. The
C relevant parts of the pairing rules are:
C
C - mmx loads and stores execute only in the U pipe
C - only one mmx shift in a pair
C - wait one cycle before storing an mmx register result
C - the usual address generation interlock
C
C Two qword calculations are slightly interleaved. The instructions
C marked "C" belong to the second qword, and the "C prev" one is for
C the second qword from the previous iteration.
ALIGN(8)
L(unroll_loop):
C eax counter, limbs, negative
C ebx &src[size-12]
C ecx
C edx &dst[size-12]
C esi
C edi
C
C mm0
C mm1
C mm2 src qword from -8(%ebx,%eax,4)
C mm3 dst qword ready to store to -8(%edx,%eax,4)
C
C mm5 return value
C mm6 rshift
C mm7 lshift
movq (%ebx,%eax,4), %mm0
psrlq %mm6, %mm2
movq %mm0, %mm1
psllq %mm7, %mm0
movq %mm3, -8(%edx,%eax,4) C prev
por %mm2, %mm0
movq 8(%ebx,%eax,4), %mm3 C
psrlq %mm6, %mm1 C
movq %mm0, (%edx,%eax,4)
movq %mm3, %mm2 C
psllq %mm7, %mm3 C
addl $4, %eax
por %mm1, %mm3 C
js L(unroll_loop)
L(finish):
C eax 0 to 3 representing respectively 3 to 0 limbs remaining
testb $2, %al
jnz L(finish_no_two)
movq (%ebx,%eax,4), %mm0
psrlq %mm6, %mm2
movq %mm0, %mm1
psllq %mm7, %mm0
movq %mm3, -8(%edx,%eax,4) C prev
por %mm2, %mm0
movq %mm1, %mm2
movq %mm0, %mm3
addl $2, %eax
L(finish_no_two):
C eax 2 or 3 representing respectively 1 or 0 limbs remaining
C
C mm2 src prev qword, from -8(%ebx,%eax,4)
C mm3 dst qword, for -8(%edx,%eax,4)
testb $1, %al
popl %edi
movd %mm5, %eax C retval
jnz L(finish_zero)
C One extra limb, destination was aligned.
C
C source ebx
C +-------+---------------+--
C | | mm2 |
C +-------+---------------+--
C
C dest edx
C +-------+---------------+---------------+--
C | | | mm3 |
C +-------+---------------+---------------+--
C
C mm6 = shift
C mm7 = ecx = 64-shift
C One extra limb, destination was unaligned.
C
C source ebx
C +-------+---------------+--
C | | mm2 |
C +-------+---------------+--
C
C dest edx
C +---------------+---------------+--
C | | mm3 |
C +---------------+---------------+--
C
C mm6 = shift+32
C mm7 = ecx = 64-(shift+32)
C In both cases there's one extra limb of src to fetch and combine
C with mm2 to make a qword at 8(%edx), and in the aligned case
C there's a further extra limb of dst to be formed.
movd 8(%ebx), %mm0
psrlq %mm6, %mm2
movq %mm0, %mm1
psllq %mm7, %mm0
movq %mm3, (%edx)
por %mm2, %mm0
psrlq %mm6, %mm1
andl $32, %ecx
popl %ebx
jz L(finish_one_unaligned)
C dst was aligned, must store one extra limb
movd %mm1, 16(%edx)
L(finish_one_unaligned):
movq %mm0, 8(%edx)
emms
ret
L(finish_zero):
C No extra limbs, destination was aligned.
C
C source ebx
C +---------------+--
C | mm2 |
C +---------------+--
C
C dest edx+4
C +---------------+---------------+--
C | | mm3 |
C +---------------+---------------+--
C
C mm6 = shift
C mm7 = ecx = 64-shift
C No extra limbs, destination was unaligned.
C
C source ebx
C +---------------+--
C | mm2 |
C +---------------+--
C
C dest edx+4
C +-------+---------------+--
C | | mm3 |
C +-------+---------------+--
C
C mm6 = shift+32
C mm7 = 64-(shift+32)
C The movd for the unaligned case is clearly the same data as the
C movq for the aligned case, it's just a choice between whether one
C or two limbs should be written.
movq %mm3, 4(%edx)
psrlq %mm6, %mm2
movd %mm2, 12(%edx)
andl $32, %ecx
popl %ebx
jz L(finish_zero_unaligned)
movq %mm2, 12(%edx)
L(finish_zero_unaligned):
emms
ret
EPILOGUE()
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