dnl Intel P5 mpn_rshift -- mpn right shift.
dnl Copyright 2000, 2002 Free Software Foundation,
Inc.
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 modify
dnl it under the terms of either:
dnl
dnl * the GNU Lesser General
Public License as published by the Free
dnl Software Foundation
; either version 3 of the License, or (at your
dnl
option) any later
version.
dnl
dnl
or
dnl
dnl * the GNU General
Public License as published by the Free Software
dnl Foundation
; either version 2 of the License, or (at your option) any
dnl later
version.
dnl
dnl
or both in parallel, as here.
dnl
dnl The GNU MP Library is distributed in the hope that it will be useful, but
dnl WITHOUT ANY WARRANTY
; without even the implied warranty of MERCHANTABILITY
dnl
or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General
Public License
dnl
for more details.
dnl
dnl You should have received copies of the GNU General
Public License
and the
dnl GNU Lesser General
Public License along with the GNU MP Library.
If not,
dnl see
https://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()
