Re: [PATCH v3 4/7] x86: Optimize memrchr-evex.S

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Oct 18, 2022 at 5:44 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> Optimizations are:
> 1. Use the fact that lzcnt(0) -> VEC_SIZE for memchr to save a branch
>    in short string case.
> 2. Save several instructions in len = [VEC_SIZE, 4 * VEC_SIZE] case.
> 3. Use more code-size efficient instructions.
>         - tzcnt ...     -> bsf ...
>         - vpcmpb $0 ... -> vpcmpeq ...
>
> Code Size Changes:
> memrchr-evex.S      :  -29 bytes
>
> Net perf changes:
>
> Reported as geometric mean of all improvements / regressions from N=10
> runs of the benchtests. Value as New Time / Old Time so < 1.0 is
> improvement and 1.0 is regression.
>
> memrchr-evex.S      : 0.949 (Mostly from improvements in small strings)
>
> Full results attached in email.
>
> Full check passes on x86-64.
> ---
>  sysdeps/x86_64/multiarch/memrchr-evex.S | 538 ++++++++++++++----------
>  1 file changed, 324 insertions(+), 214 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/memrchr-evex.S b/sysdeps/x86_64/multiarch/memrchr-evex.S
> index 550b328c5a..dbcf52808f 100644
> --- a/sysdeps/x86_64/multiarch/memrchr-evex.S
> +++ b/sysdeps/x86_64/multiarch/memrchr-evex.S
> @@ -21,17 +21,19 @@
>  #if ISA_SHOULD_BUILD (4)
>
>  # include <sysdep.h>
> -# include "x86-evex256-vecs.h"
> -# if VEC_SIZE != 32
> -#  error "VEC_SIZE != 32 unimplemented"
> +
> +# ifndef VEC_SIZE
> +#  include "x86-evex256-vecs.h"
>  # endif
>
> +# include "reg-macros.h"
> +
>  # ifndef MEMRCHR
> -#  define MEMRCHR                              __memrchr_evex
> +#  define MEMRCHR      __memrchr_evex
>  # endif
>
> -# define PAGE_SIZE                     4096
> -# define VMMMATCH                      VMM(0)
> +# define PAGE_SIZE     4096
> +# define VMATCH        VMM(0)
>
>         .section SECTION(.text), "ax", @progbits
>  ENTRY_P2ALIGN(MEMRCHR, 6)
> @@ -43,294 +45,402 @@ ENTRY_P2ALIGN(MEMRCHR, 6)
>  # endif
>         jz      L(zero_0)
>
> -       /* Get end pointer. Minus one for two reasons. 1) It is necessary for a
> -          correct page cross check and 2) it correctly sets up end ptr to be
> -          subtract by lzcnt aligned.  */
> +       /* Get end pointer. Minus one for three reasons. 1) It is
> +          necessary for a correct page cross check and 2) it correctly
> +          sets up end ptr to be subtract by lzcnt aligned. 3) it is a
> +          necessary step in aligning ptr.  */
>         leaq    -1(%rdi, %rdx), %rax
> -       vpbroadcastb %esi, %VMMMATCH
> +       vpbroadcastb %esi, %VMATCH
>
>         /* Check if we can load 1x VEC without cross a page.  */
>         testl   $(PAGE_SIZE - VEC_SIZE), %eax
>         jz      L(page_cross)
>
> -       /* Don't use rax for pointer here because EVEX has better encoding with
> -          offset % VEC_SIZE == 0.  */
> -       vpcmpb  $0, -(VEC_SIZE)(%rdi, %rdx), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> -
> -       /* Fall through for rdx (len) <= VEC_SIZE (expect small sizes).  */
> -       cmpq    $VEC_SIZE, %rdx
> -       ja      L(more_1x_vec)
> -L(ret_vec_x0_test):
> -
> -       /* If ecx is zero (no matches) lzcnt will set it 32 (VEC_SIZE) which
> -          will guarantee edx (len) is less than it.  */
> -       lzcntl  %ecx, %ecx
> -       cmpl    %ecx, %edx
> -       jle     L(zero_0)
> -       subq    %rcx, %rax
> +       /* Don't use rax for pointer here because EVEX has better
> +          encoding with offset % VEC_SIZE == 0.  */
> +       vpcmpeqb (VEC_SIZE * -1)(%rdi, %rdx), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
> +
> +       /* If rcx is zero then lzcnt -> VEC_SIZE.  NB: there is a
> +          already a dependency between rcx and rsi so no worries about
> +          false-dep here.  */
> +       lzcnt   %VRCX, %VRSI
> +       /* If rdx <= rsi then either 1) rcx was non-zero (there was a
> +          match) but it was out of bounds or 2) rcx was zero and rdx
> +          was <= VEC_SIZE so we are done scanning.  */
> +       cmpq    %rsi, %rdx
> +       /* NB: Use branch to return zero/non-zero.  Common usage will
> +          branch on result of function (if return is null/non-null).
> +          This branch can be used to predict the ensuing one so there
> +          is no reason to extend the data-dependency with cmovcc.  */
> +       jbe     L(zero_0)
> +
> +       /* If rcx is zero then len must be > RDX, otherwise since we
> +          already tested len vs lzcnt(rcx) (in rsi) we are good to
> +          return this match.  */
> +       test    %VRCX, %VRCX
> +       jz      L(more_1x_vec)
> +       subq    %rsi, %rax
>         ret
>
> -       /* Fits in aligning bytes of first cache line.  */
> +       /* Fits in aligning bytes of first cache line for VEC_SIZE ==
> +          32.  */
> +# if VEC_SIZE == 32
> +       .p2align 4,, 2
>  L(zero_0):
>         xorl    %eax, %eax
>         ret
> -
> -       .p2align 4,, 9
> -L(ret_vec_x0_dec):
> -       decq    %rax
> -L(ret_vec_x0):
> -       lzcntl  %ecx, %ecx
> -       subq    %rcx, %rax
> -       ret
> +# endif
>
>         .p2align 4,, 10
>  L(more_1x_vec):
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x0)
> -
>         /* Align rax (pointer to string).  */
>         andq    $-VEC_SIZE, %rax
> -
> +L(page_cross_continue):
>         /* Recompute length after aligning.  */
> -       movq    %rax, %rdx
> +       subq    %rdi, %rax
>
> -       /* Need no matter what.  */
> -       vpcmpb  $0, -(VEC_SIZE)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> -
> -       subq    %rdi, %rdx
> -
> -       cmpq    $(VEC_SIZE * 2), %rdx
> +       cmpq    $(VEC_SIZE * 2), %rax
>         ja      L(more_2x_vec)
> +
>  L(last_2x_vec):
> +       vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
>
> -       /* Must dec rax because L(ret_vec_x0_test) expects it.  */
> -       decq    %rax
> -       cmpl    $VEC_SIZE, %edx
> -       jbe     L(ret_vec_x0_test)
> +       test    %VRCX, %VRCX
> +       jnz     L(ret_vec_x0_test)
>
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x0)
> +       /* If VEC_SIZE == 64 need to subtract because lzcntq won't
> +          implicitly add VEC_SIZE to match position.  */
> +# if VEC_SIZE == 64
> +       subl    $VEC_SIZE, %eax
> +# else
> +       cmpb    $VEC_SIZE, %al
> +# endif
> +       jle     L(zero_2)
>
> -       /* Don't use rax for pointer here because EVEX has better encoding with
> -          offset % VEC_SIZE == 0.  */
> -       vpcmpb  $0, -(VEC_SIZE * 2)(%rdi, %rdx), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> -       /* NB: 64-bit lzcnt. This will naturally add 32 to position.  */
> +       /* We adjusted rax (length) for VEC_SIZE == 64 so need seperate
> +          offsets.  */
> +# if VEC_SIZE == 64
> +       vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
> +# else
> +       vpcmpeqb (VEC_SIZE * -2)(%rdi, %rax), %VMATCH, %k0
> +# endif
> +       KMOV    %k0, %VRCX
> +       /* NB: 64-bit lzcnt. This will naturally add 32 to position for
> +          VEC_SIZE == 32.  */
>         lzcntq  %rcx, %rcx
> -       cmpl    %ecx, %edx
> -       jle     L(zero_0)
> -       subq    %rcx, %rax
> -       ret
> -
> -       /* Inexpensive place to put this regarding code size / target alignments
> -          / ICache NLP. Necessary for 2-byte encoding of jump to page cross
> -          case which in turn is necessary for hot path (len <= VEC_SIZE) to fit
> -          in first cache line.  */
> -L(page_cross):
> -       movq    %rax, %rsi
> -       andq    $-VEC_SIZE, %rsi
> -       vpcmpb  $0, (%rsi), %VMMMATCH, %k0
> -       kmovd   %k0, %r8d
> -       /* Shift out negative alignment (because we are starting from endptr and
> -          working backwards).  */
> -       movl    %eax, %ecx
> -       /* notl because eax already has endptr - 1.  (-x = ~(x - 1)).  */
> -       notl    %ecx
> -       shlxl   %ecx, %r8d, %ecx
> -       cmpq    %rdi, %rsi
> -       ja      L(more_1x_vec)
> -       lzcntl  %ecx, %ecx
> -       cmpl    %ecx, %edx
> -       jle     L(zero_1)
> -       subq    %rcx, %rax
> +       subl    %ecx, %eax
> +       ja      L(first_vec_x1_ret)
> +       /* If VEC_SIZE == 64 put L(zero_0) here as we can't fit in the
> +          first cache line (this is the second cache line).  */
> +# if VEC_SIZE == 64
> +L(zero_0):
> +# endif
> +L(zero_2):
> +       xorl    %eax, %eax
>         ret
>
> -       /* Continue creating zero labels that fit in aligning bytes and get
> -          2-byte encoding / are in the same cache line as condition.  */
> -L(zero_1):
> -       xorl    %eax, %eax
> +       /* NB: Fits in aligning bytes before next cache line for
> +          VEC_SIZE == 32.  For VEC_SIZE == 64 this is attached to
> +          L(first_vec_x0_test).  */
> +# if VEC_SIZE == 32
> +L(first_vec_x1_ret):
> +       leaq    -1(%rdi, %rax), %rax
>         ret
> +# endif
>
> -       .p2align 4,, 8
> -L(ret_vec_x1):
> -       /* This will naturally add 32 to position.  */
> -       bsrl    %ecx, %ecx
> -       leaq    -(VEC_SIZE * 2)(%rcx, %rax), %rax
> +       .p2align 4,, 6
> +L(ret_vec_x0_test):
> +       lzcnt   %VRCX, %VRCX
> +       subl    %ecx, %eax
> +       jle     L(zero_2)
> +# if VEC_SIZE == 64
> +       /* Reuse code at the end of L(ret_vec_x0_test) as we can't fit
> +          L(first_vec_x1_ret) in the same cache line as its jmp base
> +          so we might as well save code size.  */
> +L(first_vec_x1_ret):
> +# endif
> +       leaq    -1(%rdi, %rax), %rax
>         ret
>
> -       .p2align 4,, 8
> +       .p2align 4,, 6
> +L(loop_last_4x_vec):
> +       /* Compute remaining length.  */
> +       subl    %edi, %eax
> +L(last_4x_vec):
> +       cmpl    $(VEC_SIZE * 2), %eax
> +       jle     L(last_2x_vec)
> +# if VEC_SIZE == 32
> +       /* Only align for VEC_SIZE == 32.  For VEC_SIZE == 64 we need
> +          the spare bytes to align the loop properly.  */
> +       .p2align 4,, 10
> +# endif
>  L(more_2x_vec):
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x0_dec)
>
> -       vpcmpb  $0, -(VEC_SIZE * 2)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x1)
> +       /* Length > VEC_SIZE * 2 so check the first 2x VEC for match and
> +          return if either hit.  */
> +       vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
> +
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x0)
> +
> +       vpcmpeqb (VEC_SIZE * -2)(%rdi, %rax), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x1)
>
>         /* Need no matter what.  */
> -       vpcmpb  $0, -(VEC_SIZE * 3)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> +       vpcmpeqb (VEC_SIZE * -3)(%rdi, %rax), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
>
> -       subq    $(VEC_SIZE * 4), %rdx
> +       /* Check if we are near the end.  */
> +       subq    $(VEC_SIZE * 4), %rax
>         ja      L(more_4x_vec)
>
> -       cmpl    $(VEC_SIZE * -1), %edx
> -       jle     L(ret_vec_x2_test)
> -L(last_vec):
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x2)
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x2_test)
>
> +       /* Adjust length for final check and check if we are at the end.
> +        */
> +       addl    $(VEC_SIZE * 1), %eax
> +       jle     L(zero_1)
>
> -       /* Need no matter what.  */
> -       vpcmpb  $0, -(VEC_SIZE * 4)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> -       lzcntl  %ecx, %ecx
> -       subq    $(VEC_SIZE * 3 + 1), %rax
> -       subq    %rcx, %rax
> -       cmpq    %rax, %rdi
> -       ja      L(zero_1)
> +       vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
> +
> +       lzcnt   %VRCX, %VRCX
> +       subl    %ecx, %eax
> +       ja      L(first_vec_x3_ret)
> +L(zero_1):
> +       xorl    %eax, %eax
> +       ret
> +L(first_vec_x3_ret):
> +       leaq    -1(%rdi, %rax), %rax
>         ret
>
> -       .p2align 4,, 8
> -L(ret_vec_x2_test):
> -       lzcntl  %ecx, %ecx
> -       subq    $(VEC_SIZE * 2 + 1), %rax
> -       subq    %rcx, %rax
> -       cmpq    %rax, %rdi
> -       ja      L(zero_1)
> +       .p2align 4,, 6
> +L(first_vec_x2_test):
> +       /* Must adjust length before check.  */
> +       subl    $-(VEC_SIZE * 2 - 1), %eax
> +       lzcnt   %VRCX, %VRCX
> +       subl    %ecx, %eax
> +       jl      L(zero_4)
> +       addq    %rdi, %rax
>         ret
>
> -       .p2align 4,, 8
> -L(ret_vec_x2):
> -       bsrl    %ecx, %ecx
> -       leaq    -(VEC_SIZE * 3)(%rcx, %rax), %rax
> +
> +       .p2align 4,, 10
> +L(first_vec_x0):
> +       bsr     %VRCX, %VRCX
> +       leaq    (VEC_SIZE * -1)(%rdi, %rax), %rax
> +       addq    %rcx, %rax
>         ret
>
> -       .p2align 4,, 8
> -L(ret_vec_x3):
> -       bsrl    %ecx, %ecx
> -       leaq    -(VEC_SIZE * 4)(%rcx, %rax), %rax
> +       /* Fits unobtrusively here.  */
> +L(zero_4):
> +       xorl    %eax, %eax
> +       ret
> +
> +       .p2align 4,, 10
> +L(first_vec_x1):
> +       bsr     %VRCX, %VRCX
> +       leaq    (VEC_SIZE * -2)(%rdi, %rax), %rax
> +       addq    %rcx, %rax
>         ret
>
>         .p2align 4,, 8
> +L(first_vec_x3):
> +       bsr     %VRCX, %VRCX
> +       addq    %rdi, %rax
> +       addq    %rcx, %rax
> +       ret
> +
> +       .p2align 4,, 6
> +L(first_vec_x2):
> +       bsr     %VRCX, %VRCX
> +       leaq    (VEC_SIZE * 1)(%rdi, %rax), %rax
> +       addq    %rcx, %rax
> +       ret
> +
> +       .p2align 4,, 2
>  L(more_4x_vec):
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x2)
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x2)
>
> -       vpcmpb  $0, -(VEC_SIZE * 4)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> +       vpcmpeqb (%rdi, %rax), %VMATCH, %k0
> +       KMOV    %k0, %VRCX
>
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x3)
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x3)
>
>         /* Check if near end before re-aligning (otherwise might do an
>            unnecessary loop iteration).  */
> -       addq    $-(VEC_SIZE * 4), %rax
> -       cmpq    $(VEC_SIZE * 4), %rdx
> +       cmpq    $(VEC_SIZE * 4), %rax
>         jbe     L(last_4x_vec)
>
> -       decq    %rax
> -       andq    $-(VEC_SIZE * 4), %rax
> -       movq    %rdi, %rdx
> -       /* Get endptr for loop in rdx. NB: Can't just do while rax > rdi because
> -          lengths that overflow can be valid and break the comparison.  */
> -       andq    $-(VEC_SIZE * 4), %rdx
> +
> +       /* NB: We setup the loop to NOT use index-address-mode for the
> +          buffer.  This costs some instructions & code size but avoids
> +          stalls due to unlaminated micro-fused instructions (as used
> +          in the loop) from being forced to issue in the same group
> +          (essentially narrowing the backend width).  */
> +
> +       /* Get endptr for loop in rdx. NB: Can't just do while rax > rdi
> +          because lengths that overflow can be valid and break the
> +          comparison.  */
> +# if VEC_SIZE == 64
> +       /* Use rdx as intermediate to compute rax, this gets us imm8
> +          encoding which just allows the L(more_4x_vec) block to fit
> +          in 1 cache-line.  */
> +       leaq    (VEC_SIZE * 4)(%rdi), %rdx
> +       leaq    (VEC_SIZE * -1)(%rdx, %rax), %rax
> +
> +       /* No evex machine has partial register stalls. This can be
> +          replaced with: `andq $(VEC_SIZE * -4), %rax/%rdx` if that
> +          changes.  */
> +       xorb    %al, %al
> +       xorb    %dl, %dl
> +# else
> +       leaq    (VEC_SIZE * 3)(%rdi, %rax), %rax
> +       andq    $(VEC_SIZE * -4), %rax
> +       leaq    (VEC_SIZE * 4)(%rdi), %rdx
> +       andq    $(VEC_SIZE * -4), %rdx
> +# endif
> +
>
>         .p2align 4
>  L(loop_4x_vec):
> -       /* Store 1 were not-equals and 0 where equals in k1 (used to mask later
> -          on).  */
> -       vpcmpb  $4, (VEC_SIZE * 3)(%rax), %VMMMATCH, %k1
> +       /* NB: We could do the same optimization here as we do for
> +          memchr/rawmemchr by using VEX encoding in the loop for access
> +          to VEX vpcmpeqb + vpternlogd.  Since memrchr is not as hot as
> +          memchr it may not be worth the extra code size, but if the
> +          need arises it an easy ~15% perf improvement to the loop.  */
> +
> +       cmpq    %rdx, %rax
> +       je      L(loop_last_4x_vec)
> +       /* Store 1 were not-equals and 0 where equals in k1 (used to
> +          mask later on).  */
> +       vpcmpb  $4, (VEC_SIZE * -1)(%rax), %VMATCH, %k1
>
>         /* VEC(2/3) will have zero-byte where we found a CHAR.  */
> -       vpxorq  (VEC_SIZE * 2)(%rax), %VMMMATCH, %VMM(2)
> -       vpxorq  (VEC_SIZE * 1)(%rax), %VMMMATCH, %VMM(3)
> -       vpcmpb  $0, (VEC_SIZE * 0)(%rax), %VMMMATCH, %k4
> +       vpxorq  (VEC_SIZE * -2)(%rax), %VMATCH, %VMM(2)
> +       vpxorq  (VEC_SIZE * -3)(%rax), %VMATCH, %VMM(3)
> +       vpcmpeqb (VEC_SIZE * -4)(%rax), %VMATCH, %k4
>
> -       /* Combine VEC(2/3) with min and maskz with k1 (k1 has zero bit where
> -          CHAR is found and VEC(2/3) have zero-byte where CHAR is found.  */
> +       /* Combine VEC(2/3) with min and maskz with k1 (k1 has zero bit
> +          where CHAR is found and VEC(2/3) have zero-byte where CHAR
> +          is found.  */
>         vpminub %VMM(2), %VMM(3), %VMM(3){%k1}{z}
>         vptestnmb %VMM(3), %VMM(3), %k2
>
> -       /* Any 1s and we found CHAR.  */
> -       kortestd %k2, %k4
> -       jnz     L(loop_end)
> -
>         addq    $-(VEC_SIZE * 4), %rax
> -       cmpq    %rdx, %rax
> -       jne     L(loop_4x_vec)
>
> -       /* Need to re-adjust rdx / rax for L(last_4x_vec).  */
> -       subq    $-(VEC_SIZE * 4), %rdx
> -       movq    %rdx, %rax
> -       subl    %edi, %edx
> -L(last_4x_vec):
> +       /* Any 1s and we found CHAR.  */
> +       KORTEST %k2, %k4
> +       jz      L(loop_4x_vec)
> +
>
> -       /* Used no matter what.  */
> -       vpcmpb  $0, (VEC_SIZE * -1)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> +       /* K1 has non-matches for first VEC. inc; jz will overflow rcx
> +          iff all bytes where non-matches.  */
> +       KMOV    %k1, %VRCX
> +       inc     %VRCX
> +       jnz     L(first_vec_x0_end)
>
> -       cmpl    $(VEC_SIZE * 2), %edx
> -       jbe     L(last_2x_vec)
> +       vptestnmb %VMM(2), %VMM(2), %k0
> +       KMOV    %k0, %VRCX
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x1_end)
> +       KMOV    %k2, %VRCX
> +
> +       /* Seperate logic for VEC_SIZE == 64 and VEC_SIZE == 32 for
> +          returning last 2x VEC. For VEC_SIZE == 64 we test each VEC
> +          individually, for VEC_SIZE == 32 we combine them in a single
> +          64-bit GPR.  */
> +# if VEC_SIZE == 64
> +       test    %VRCX, %VRCX
> +       jnz     L(first_vec_x2_end)
> +       KMOV    %k4, %VRCX
> +# else
> +       /* Combine last 2 VEC matches for VEC_SIZE == 32. If rcx (from
> +          VEC(3)) is zero (no CHAR in VEC(3)) then it won't affect the
> +          result in rsi (from VEC(4)). If rcx is non-zero then CHAR in
> +          VEC(3) and bsrq will use that position.  */
> +       KMOV    %k4, %VRSI
> +       salq    $32, %rcx
> +       orq     %rsi, %rcx
> +# endif
> +       bsrq    %rcx, %rcx
> +       addq    %rcx, %rax
> +       ret
>
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x0_dec)
> +       .p2align 4,, 4
> +L(first_vec_x0_end):
> +       /* rcx has 1s at non-matches so we need to `not` it. We used
> +          `inc` to test if zero so use `neg` to complete the `not` so
> +          the last 1 bit represent a match.  NB: (-x + 1 == ~x).  */
> +       neg     %VRCX
> +       bsr     %VRCX, %VRCX
> +       leaq    (VEC_SIZE * 3)(%rcx, %rax), %rax
> +       ret
>
> +       .p2align 4,, 10
> +L(first_vec_x1_end):
> +       bsr     %VRCX, %VRCX
> +       leaq    (VEC_SIZE * 2)(%rcx, %rax), %rax
> +       ret
>
> -       vpcmpb  $0, (VEC_SIZE * -2)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> +# if VEC_SIZE == 64
> +       /* Since we can't combine the last 2x VEC for VEC_SIZE == 64
> +          need return label for it.  */
> +       .p2align 4,, 4
> +L(first_vec_x2_end):
> +       bsr     %VRCX, %VRCX
> +       leaq    (VEC_SIZE * 1)(%rcx, %rax), %rax
> +       ret
> +# endif
>
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x1)
>
> -       /* Used no matter what.  */
> -       vpcmpb  $0, (VEC_SIZE * -3)(%rax), %VMMMATCH, %k0
> -       kmovd   %k0, %ecx
> +       .p2align 4,, 4
> +L(page_cross):
> +       /* only lower bits of eax[log2(VEC_SIZE):0] are set so we can
> +          use movzbl to get the amount of bytes we are checking here.
> +        */
> +       movzbl  %al, %ecx
> +       andq    $-VEC_SIZE, %rax
> +       vpcmpeqb (%rax), %VMATCH, %k0
> +       KMOV    %k0, %VRSI
>
> -       cmpl    $(VEC_SIZE * 3), %edx
> -       ja      L(last_vec)
> +       /* eax was comptued as %rdi + %rdx - 1 so need to add back 1
> +          here.  */
> +       leal    1(%rcx), %r8d
>
> -       lzcntl  %ecx, %ecx
> -       subq    $(VEC_SIZE * 2 + 1), %rax
> -       subq    %rcx, %rax
> -       cmpq    %rax, %rdi
> -       jbe     L(ret_1)
> +       /* Invert ecx to get shift count for byte matches out of range.
> +        */
> +       notl    %ecx
> +       shlx    %VRCX, %VRSI, %VRSI
> +
> +       /* if r8 < rdx then the entire [buf, buf + len] is handled in
> +          the page cross case.  NB: we can't use the trick here we use
> +          in the non page-cross case because we aren't checking full
> +          VEC_SIZE.  */
> +       cmpq    %r8, %rdx
> +       ja      L(page_cross_check)
> +       lzcnt   %VRSI, %VRSI
> +       subl    %esi, %edx
> +       ja      L(page_cross_ret)
>         xorl    %eax, %eax
> -L(ret_1):
>         ret
>
> -       .p2align 4,, 6
> -L(loop_end):
> -       kmovd   %k1, %ecx
> -       notl    %ecx
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x0_end)
> +L(page_cross_check):
> +       test    %VRSI, %VRSI
> +       jz      L(page_cross_continue)
>
> -       vptestnmb %VMM(2), %VMM(2), %k0
> -       kmovd   %k0, %ecx
> -       testl   %ecx, %ecx
> -       jnz     L(ret_vec_x1_end)
> -
> -       kmovd   %k2, %ecx
> -       kmovd   %k4, %esi
> -       /* Combine last 2 VEC matches. If ecx (VEC3) is zero (no CHAR in VEC3)
> -          then it won't affect the result in esi (VEC4). If ecx is non-zero
> -          then CHAR in VEC3 and bsrq will use that position.  */
> -       salq    $32, %rcx
> -       orq     %rsi, %rcx
> -       bsrq    %rcx, %rcx
> -       addq    %rcx, %rax
> -       ret
> -       .p2align 4,, 4
> -L(ret_vec_x0_end):
> -       addq    $(VEC_SIZE), %rax
> -L(ret_vec_x1_end):
> -       bsrl    %ecx, %ecx
> -       leaq    (VEC_SIZE * 2)(%rax, %rcx), %rax
> +       lzcnt   %VRSI, %VRSI
> +       subl    %esi, %edx
> +L(page_cross_ret):
> +       leaq    -1(%rdi, %rdx), %rax
>         ret
> -
>  END(MEMRCHR)
>  #endif
> --
> 2.34.1
>

LGTM.

Thanks.

-- 
H.J.