[PATCH v6 7/8] x86: Shrink code size of memchr-avx2.S
Noah Goldstein
goldstein.w.n@gmail.com
Thu Jul 14 02:41:07 GMT 2022
On Wed, Jul 13, 2022 at 7:32 PM Sunil Pandey <skpgkp2@gmail.com> wrote:
>
> On Tue, Jun 7, 2022 at 11:19 AM H.J. Lu via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > On Mon, Jun 6, 2022 at 9:11 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > This is not meant as a performance optimization. The previous code was
> > > far to liberal in aligning targets and wasted code size unnecissarily.
> > >
> > > The total code size saving is: 59 bytes
> > >
> > > There are no major changes in the benchmarks.
> > > Geometric Mean of all benchmarks New / Old: 0.967
> > >
> > > Full xcheck passes on x86_64.
> > > ---
> > > sysdeps/x86_64/multiarch/memchr-avx2-rtm.S | 1 +
> > > sysdeps/x86_64/multiarch/memchr-avx2.S | 109 +++++++++++----------
> > > 2 files changed, 60 insertions(+), 50 deletions(-)
> > >
> > > diff --git a/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S b/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
> > > index 87b076c7c4..c4d71938c5 100644
> > > --- a/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
> > > +++ b/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
> > > @@ -2,6 +2,7 @@
> > > # define MEMCHR __memchr_avx2_rtm
> > > #endif
> > >
> > > +#define COND_VZEROUPPER COND_VZEROUPPER_XTEST
> > > #define ZERO_UPPER_VEC_REGISTERS_RETURN \
> > > ZERO_UPPER_VEC_REGISTERS_RETURN_XTEST
> > >
> > > diff --git a/sysdeps/x86_64/multiarch/memchr-avx2.S b/sysdeps/x86_64/multiarch/memchr-avx2.S
> > > index 75bd7262e0..28a01280ec 100644
> > > --- a/sysdeps/x86_64/multiarch/memchr-avx2.S
> > > +++ b/sysdeps/x86_64/multiarch/memchr-avx2.S
> > > @@ -57,7 +57,7 @@
> > > # define CHAR_PER_VEC (VEC_SIZE / CHAR_SIZE)
> > >
> > > .section SECTION(.text),"ax",@progbits
> > > -ENTRY (MEMCHR)
> > > +ENTRY_P2ALIGN (MEMCHR, 5)
> > > # ifndef USE_AS_RAWMEMCHR
> > > /* Check for zero length. */
> > > # ifdef __ILP32__
> > > @@ -87,12 +87,14 @@ ENTRY (MEMCHR)
> > > # endif
> > > testl %eax, %eax
> > > jz L(aligned_more)
> > > - tzcntl %eax, %eax
> > > + bsfl %eax, %eax
> > > addq %rdi, %rax
> > > - VZEROUPPER_RETURN
> > > +L(return_vzeroupper):
> > > + ZERO_UPPER_VEC_REGISTERS_RETURN
> > > +
> > >
> > > # ifndef USE_AS_RAWMEMCHR
> > > - .p2align 5
> > > + .p2align 4
> > > L(first_vec_x0):
> > > /* Check if first match was before length. */
> > > tzcntl %eax, %eax
> > > @@ -100,58 +102,31 @@ L(first_vec_x0):
> > > /* NB: Multiply length by 4 to get byte count. */
> > > sall $2, %edx
> > > # endif
> > > - xorl %ecx, %ecx
> > > + COND_VZEROUPPER
> > > + /* Use branch instead of cmovcc so L(first_vec_x0) fits in one fetch
> > > + block. branch here as opposed to cmovcc is not that costly. Common
> > > + usage of memchr is to check if the return was NULL (if string was
> > > + known to contain CHAR user would use rawmemchr). This branch will be
> > > + highly correlated with the user branch and can be used by most
> > > + modern branch predictors to predict the user branch. */
> > > cmpl %eax, %edx
> > > - leaq (%rdi, %rax), %rax
> > > - cmovle %rcx, %rax
> > > - VZEROUPPER_RETURN
> > > -
> > > -L(null):
> > > - xorl %eax, %eax
> > > - ret
> > > -# endif
> > > - .p2align 4
> > > -L(cross_page_boundary):
> > > - /* Save pointer before aligning as its original value is
> > > - necessary for computer return address if byte is found or
> > > - adjusting length if it is not and this is memchr. */
> > > - movq %rdi, %rcx
> > > - /* Align data to VEC_SIZE - 1. ALGN_PTR_REG is rcx for memchr
> > > - and rdi for rawmemchr. */
> > > - orq $(VEC_SIZE - 1), %ALGN_PTR_REG
> > > - VPCMPEQ -(VEC_SIZE - 1)(%ALGN_PTR_REG), %ymm0, %ymm1
> > > - vpmovmskb %ymm1, %eax
> > > -# ifndef USE_AS_RAWMEMCHR
> > > - /* Calculate length until end of page (length checked for a
> > > - match). */
> > > - leaq 1(%ALGN_PTR_REG), %rsi
> > > - subq %RRAW_PTR_REG, %rsi
> > > -# ifdef USE_AS_WMEMCHR
> > > - /* NB: Divide bytes by 4 to get wchar_t count. */
> > > - shrl $2, %esi
> > > -# endif
> > > -# endif
> > > - /* Remove the leading bytes. */
> > > - sarxl %ERAW_PTR_REG, %eax, %eax
> > > -# ifndef USE_AS_RAWMEMCHR
> > > - /* Check the end of data. */
> > > - cmpq %rsi, %rdx
> > > - jbe L(first_vec_x0)
> > > + jle L(null)
> > > + addq %rdi, %rax
> > > + ret
> > > # endif
> > > - testl %eax, %eax
> > > - jz L(cross_page_continue)
> > > - tzcntl %eax, %eax
> > > - addq %RRAW_PTR_REG, %rax
> > > -L(return_vzeroupper):
> > > - ZERO_UPPER_VEC_REGISTERS_RETURN
> > >
> > > - .p2align 4
> > > + .p2align 4,, 10
> > > L(first_vec_x1):
> > > - tzcntl %eax, %eax
> > > + bsfl %eax, %eax
> > > incq %rdi
> > > addq %rdi, %rax
> > > VZEROUPPER_RETURN
> > > -
> > > +# ifndef USE_AS_RAWMEMCHR
> > > + /* First in aligning bytes here. */
> > > +L(null):
> > > + xorl %eax, %eax
> > > + ret
> > > +# endif
> > > .p2align 4
> > > L(first_vec_x2):
> > > tzcntl %eax, %eax
> > > @@ -340,7 +315,7 @@ L(first_vec_x1_check):
> > > incq %rdi
> > > addq %rdi, %rax
> > > VZEROUPPER_RETURN
> > > - .p2align 4
> > > + .p2align 4,, 6
> > > L(set_zero_end):
> > > xorl %eax, %eax
> > > VZEROUPPER_RETURN
> > > @@ -428,5 +403,39 @@ L(last_vec_x3):
> > > VZEROUPPER_RETURN
> > > # endif
> > >
> > > + .p2align 4
> > > +L(cross_page_boundary):
> > > + /* Save pointer before aligning as its original value is necessary for
> > > + computer return address if byte is found or adjusting length if it
> > > + is not and this is memchr. */
> > > + movq %rdi, %rcx
> > > + /* Align data to VEC_SIZE. ALGN_PTR_REG is rcx for memchr and rdi for
> > > + rawmemchr. */
> > > + andq $-VEC_SIZE, %ALGN_PTR_REG
> > > + VPCMPEQ (%ALGN_PTR_REG), %ymm0, %ymm1
> > > + vpmovmskb %ymm1, %eax
> > > +# ifndef USE_AS_RAWMEMCHR
> > > + /* Calculate length until end of page (length checked for a match). */
> > > + leal VEC_SIZE(%ALGN_PTR_REG), %esi
> > > + subl %ERAW_PTR_REG, %esi
> > > +# ifdef USE_AS_WMEMCHR
> > > + /* NB: Divide bytes by 4 to get wchar_t count. */
> > > + shrl $2, %esi
> > > +# endif
> > > +# endif
> > > + /* Remove the leading bytes. */
> > > + sarxl %ERAW_PTR_REG, %eax, %eax
> > > +# ifndef USE_AS_RAWMEMCHR
> > > + /* Check the end of data. */
> > > + cmpq %rsi, %rdx
> > > + jbe L(first_vec_x0)
> > > +# endif
> > > + testl %eax, %eax
> > > + jz L(cross_page_continue)
> > > + bsfl %eax, %eax
> > > + addq %RRAW_PTR_REG, %rax
> > > + VZEROUPPER_RETURN
> > > +
> > > +
> > > END (MEMCHR)
> > > #endif
> > > --
> > > 2.34.1
> > >
> >
> > LGTM.
> >
> > Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
> >
> > Thanks.
> >
> > --
> > H.J.
>
> I would like to backport this patch to release branches.
> Any comments or objections?
Probably best to squash with:
https://sourceware.org/git/?p=glibc.git;a=commit;h=2c9af8421d2b4a7fcce163e7bc81a118d22fd346
>
> --Sunil
More information about the Libc-alpha
mailing list