On Sat, May 20, 2017 at 12:58 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sat, May 20, 2017 at 7:59 AM, Zack Weinberg <zackw@panix.com> wrote:
>> On Sat, May 20, 2017 at 10:50 AM, H.J. Lu <hongjiu.lu@intel.com> wrote:
>>> There is no need to use 64-bit registers when only the lower 32 bits
>>> are non-zero.
>>
>> This code is used generically for x86-64, not for a specific
>> microarchitecture. Is there a reason why this will never cause partial
>> register stalls, now or in the future?
>
> By x86-64 specification, 32-bit destination registers in these instructions
> are zero-extended to 64 bits and there is register stall at all.
>
Here is the updated patch with one more REX byte removed.
Any other comments?
From 2db36b54338f65a803080ce8085977299a4f52a0 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 18 May 2017 12:22:31 -0700
Subject: [PATCH] x86_64: Remove redundant REX bytes from memchr.S
By x86-64 specification, 32-bit destination registers are zero-extended
to 64 bits. There is no need to use 64-bit registers when only the lower
32 bits are non-zero.
* sysdeps/x86_64/memchr.S (MEMCHR): Use 32-bit registers for
the lower 32 bits.
---
sysdeps/x86_64/memchr.S | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)
@@ -31,7 +31,7 @@
.text
ENTRY(MEMCHR)
movd %esi, %xmm1
- mov %rdi, %rcx
+ mov %edi, %ecx
#ifdef USE_AS_WMEMCHR
test %rdx, %rdx
@@ -44,10 +44,10 @@ ENTRY(MEMCHR)
punpcklbw %xmm1, %xmm1
#endif
- and $63, %rcx
+ and $63, %ecx
pshufd $0, %xmm1, %xmm1
- cmp $48, %rcx
+ cmp $48, %ecx
ja L(crosscache)
movdqu (%rdi), %xmm0
@@ -59,7 +59,7 @@ ENTRY(MEMCHR)
sub $16, %rdx
jbe L(return_null)
add $16, %rdi
- and $15, %rcx
+ and $15, %ecx
and $-16, %rdi
add %rcx, %rdx
sub $64, %rdx
@@ -68,7 +68,7 @@ ENTRY(MEMCHR)
.p2align 4
L(crosscache):
- and $15, %rcx
+ and $15, %ecx
and $-16, %rdi
movdqa (%rdi), %xmm0
@@ -162,7 +162,7 @@ L(loop_prolog):
mov %rdi, %rcx
and $-64, %rdi
- and $63, %rcx
+ and $63, %ecx
add %rcx, %rdx
.p2align 4
@@ -214,7 +214,7 @@ L(align64_loop):
.p2align 4
L(exit_loop):
- add $32, %rdx
+ add $32, %edx
jle L(exit_loop_32)
movdqa (%rdi), %xmm0
@@ -234,7 +234,7 @@ L(exit_loop):
pmovmskb %xmm3, %eax
test %eax, %eax
jnz L(matches32_1)
- sub $16, %rdx
+ sub $16, %edx
jle L(return_null)
PCMPEQ 48(%rdi), %xmm1
@@ -246,13 +246,13 @@ L(exit_loop):
.p2align 4
L(exit_loop_32):
- add $32, %rdx
+ add $32, %edx
movdqa (%rdi), %xmm0
PCMPEQ %xmm1, %xmm0
pmovmskb %xmm0, %eax
test %eax, %eax
jnz L(matches_1)
- sub $16, %rdx
+ sub $16, %edx
jbe L(return_null)
PCMPEQ 16(%rdi), %xmm1
--
2.9.4