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* [glibc] x86-64: Avoid rep movsb with short distance [BZ #27130]
@ 2021-01-04 16:04 H.J. Lu
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From: H.J. Lu @ 2021-01-04 16:04 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=3ec5d83d2a237d39e7fd6ef7a0bc8ac4c171a4a5
commit 3ec5d83d2a237d39e7fd6ef7a0bc8ac4c171a4a5
Author: H.J. Lu <hjl.tools@gmail.com>
Date: Sat Jan 25 14:19:40 2020 -0800
x86-64: Avoid rep movsb with short distance [BZ #27130]
When copying with "rep movsb", if the distance between source and
destination is N*4GB + [1..63] with N >= 0, performance may be very
slow. This patch updates memmove-vec-unaligned-erms.S for AVX and
AVX512 versions with the distance in RCX:
cmpl $63, %ecx
// Don't use "rep movsb" if ECX <= 63
jbe L(Don't use rep movsb")
Use "rep movsb"
Benchtests data with bench-memcpy, bench-memcpy-large, bench-memcpy-random
and bench-memcpy-walk on Skylake, Ice Lake and Tiger Lake show that its
performance impact is within noise range as "rep movsb" is only used for
data size >= 4KB.
Diff:
---
.../x86_64/multiarch/memmove-vec-unaligned-erms.S | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
index 7d54095f04..0980c95378 100644
--- a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
+++ b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
@@ -56,6 +56,13 @@
# endif
#endif
+/* Avoid short distance rep movsb only with non-SSE vector. */
+#ifndef AVOID_SHORT_DISTANCE_REP_MOVSB
+# define AVOID_SHORT_DISTANCE_REP_MOVSB (VEC_SIZE > 16)
+#else
+# define AVOID_SHORT_DISTANCE_REP_MOVSB 0
+#endif
+
#ifndef PREFETCH
# define PREFETCH(addr) prefetcht0 addr
#endif
@@ -243,7 +250,21 @@ L(movsb):
cmpq %r9, %rdi
/* Avoid slow backward REP MOVSB. */
jb L(more_8x_vec_backward)
+# if AVOID_SHORT_DISTANCE_REP_MOVSB
+ movq %rdi, %rcx
+ subq %rsi, %rcx
+ jmp 2f
+# endif
1:
+# if AVOID_SHORT_DISTANCE_REP_MOVSB
+ movq %rsi, %rcx
+ subq %rdi, %rcx
+2:
+/* Avoid "rep movsb" if RCX, the distance between source and destination,
+ is N*4GB + [1..63] with N >= 0. */
+ cmpl $63, %ecx
+ jbe L(more_2x_vec) /* Avoid "rep movsb" if ECX <= 63. */
+# endif
mov %RDX_LP, %RCX_LP
rep movsb
L(nop):
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2021-01-04 16:04 [glibc] x86-64: Avoid rep movsb with short distance [BZ #27130] H.J. Lu
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