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Message-Id: <1423761423-30050-2-git-send-email-vda.linux@googlemail.com>
Date: Thu, 12 Feb 2015 18:17:03 +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 2/2] x86_64/memset: align destination to 8 byte boundary
8-byte alignment gives ~25% speedup on "rep stosq" memsets
to L1 cache, compared to intentionally misaligned ones.
It is a smaller win of ~15% on larger memsets to L2 too.
Measured on Intel Sandy Bridge CPU (i7-2620M, 2.70GHz)
Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com>
---
src/string/x86_64/memset.s | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/src/string/x86_64/memset.s b/src/string/x86_64/memset.s
index 523caa0..5c9e333 100644
--- a/src/string/x86_64/memset.s
+++ b/src/string/x86_64/memset.s
@@ -4,16 +4,23 @@ memset:
movzbq %sil,%rax
cmp $16,%rdx
jb .Less_than_16
+
test %esi,%esi
jnz .L_widen_rax # unlikely
.L_widened:
- lea -1(%rdx),%rcx
mov %rdi,%r8
+
+ test $7,%dil
+ jnz .L_align # unlikely
+.L_aligned:
+
+ lea -1(%rdx),%rcx
shr $3,%rcx
mov %rax,-8(%rdi,%rdx)
rep
stosq
+
mov %r8,%rax
ret
@@ -23,6 +30,19 @@ memset:
imul %rsi,%rax
jmp .L_widened
+# 8-byte alignment gives ~25% speedup on "rep stosq" memsets
+# to L1 cache, compared to intentionally misaligned ones.
+# It is a smaller win of ~15% on larger memsets to L2 too.
+# Measured on Intel Sandy Bridge CPU (i7-2620M, 2.70GHz)
+.L_align:
+ mov %rax,(%rdi)
+1: inc %rdi
+ dec %rdx
+ test $7,%dil
+ jnz 1b
+ jmp .L_aligned
+
+
.Less_than_16:
test %edx,%edx
jz .L_ret
--
1.8.1.4
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