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Message-Id: <1423761423-30050-1-git-send-email-vda.linux@googlemail.com>
Date: Thu, 12 Feb 2015 18:17:02 +0100
From: Denys Vlasenko <vda.linux@...glemail.com>
To: musl@...ts.openwall.com, Rich Felker <dalias@...c.org>
Cc: Denys Vlasenko <vda.linux@...glemail.com>
Subject: [PATCH 1/2] x86_64/memset: avoid multiply insn if possible
memset is very, very often called with fill=0,
and 64-bit imul is expensive on many CPUs.
Avoid it if fill=0.
Also avoid multiply on "short memset" codepath if possible,
and when we do need it, use 32-bit one, which is cheaper on many CPUs.
Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com>
---
src/string/x86_64/memset.s | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)
diff --git a/src/string/x86_64/memset.s b/src/string/x86_64/memset.s
index 3cc8fcf..523caa0 100644
--- a/src/string/x86_64/memset.s
+++ b/src/string/x86_64/memset.s
@@ -1,13 +1,12 @@
.global memset
.type memset,@function
memset:
- movzbl %sil,%esi
- mov $0x101010101010101,%rax
- # 64-bit imul has 3-7 cycles latency, launch early
- imul %rsi,%rax
-
+ movzbq %sil,%rax
cmp $16,%rdx
- jb 1f
+ jb .Less_than_16
+ test %esi,%esi
+ jnz .L_widen_rax # unlikely
+.L_widened:
lea -1(%rdx),%rcx
mov %rdi,%r8
@@ -18,26 +17,35 @@ memset:
mov %r8,%rax
ret
-1: test %edx,%edx
- jz 1f
+.L_widen_rax:
+ # 64-bit imul has 3-7 cycles latency
+ mov $0x101010101010101,%rsi
+ imul %rsi,%rax
+ jmp .L_widened
+
+.Less_than_16:
+ test %edx,%edx
+ jz .L_ret
mov %al,(%rdi)
mov %al,-1(%rdi,%rdx)
cmp $2,%edx
- jbe 1f
+ jbe .L_ret
mov %al,1(%rdi)
mov %al,-2(%rdi,%rdx)
+ # 32-bit imul has 3-4 cycles latency
+ imul $0x1010101,%eax
cmp $4,%edx
- jbe 1f
+ jbe .L_ret
mov %eax,(%rdi)
mov %eax,-4(%rdi,%rdx)
cmp $8,%edx
- jbe 1f
+ jbe .L_ret
mov %eax,4(%rdi)
mov %eax,-8(%rdi,%rdx)
-
-1: mov %rdi,%rax
+.L_ret:
+ mov %rdi,%rax
ret
--
1.8.1.4
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