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Date: Tue, 10 Feb 2015 18:30:56 +0100
From: Denys Vlasenko <vda.linux@...glemail.com>
To: musl@...ts.openwall.com
Cc: Denys Vlasenko <vda.linux@...glemail.com>
Subject: [PATCH 1/2] x86_64/memset: simple optimizations
"and $0xff,%esi" is a six-byte insn (81 e6 ff 00 00 00), can use
4-byte "movzbl %sil,%esi" (40 0f b6 f6) instead.
64-bit imul is slow, move it as far up as possible so that the result
(rax) has more time to be ready by the time we start using it
in mem stores.
There is no need to shuffle registers in preparation to "rep movs"
if we are not going to take that code path. Thus, patch moves
"jump if len < 16" instructions up, and changes alternate code path
to use rdx and rdi instead of rcx and r8.
Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com>
---
src/string/x86_64/memset.s | 30 ++++++++++++++++--------------
1 file changed, 16 insertions(+), 14 deletions(-)
diff --git a/src/string/x86_64/memset.s b/src/string/x86_64/memset.s
index fc06eef..263336b 100644
--- a/src/string/x86_64/memset.s
+++ b/src/string/x86_64/memset.s
@@ -1,41 +1,43 @@
.global memset
.type memset,@function
memset:
- and $0xff,%esi
+ movzbl %sil,%esi
mov $0x101010101010101,%rax
- mov %rdx,%rcx
- mov %rdi,%r8
+ # 64-bit imul has 3-7 cycles latency, launch early
imul %rsi,%rax
- cmp $16,%rcx
+
+ cmp $16,%rdx
jb 1f
- mov %rax,-8(%rdi,%rcx)
+ mov %rdx,%rcx
+ mov %rdi,%r8
shr $3,%rcx
+ mov %rax,-8(%rdi,%rdx)
rep
stosq
mov %r8,%rax
ret
-1: test %ecx,%ecx
+1: test %edx,%edx
jz 1f
mov %al,(%rdi)
- mov %al,-1(%rdi,%rcx)
- cmp $2,%ecx
+ mov %al,-1(%rdi,%rdx)
+ cmp $2,%edx
jbe 1f
mov %al,1(%rdi)
- mov %al,-2(%rdi,%rcx)
- cmp $4,%ecx
+ mov %al,-2(%rdi,%rdx)
+ cmp $4,%edx
jbe 1f
mov %eax,(%rdi)
- mov %eax,-4(%rdi,%rcx)
- cmp $8,%ecx
+ mov %eax,-4(%rdi,%rdx)
+ cmp $8,%edx
jbe 1f
mov %eax,4(%rdi)
- mov %eax,-8(%rdi,%rcx)
+ mov %eax,-8(%rdi,%rdx)
-1: mov %r8,%rax
+1: mov %rdi,%rax
ret
--
1.8.1.4
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