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* [glibc/release/2.28/master] x86-64: Avoid rep movsb with short distance [BZ #27130]
@ 2021-01-12 15:44 H.J. Lu
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From: H.J. Lu @ 2021-01-12 15:44 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=56fbd0ba218c59a1cc8c29464f153d45a4c887c0
commit 56fbd0ba218c59a1cc8c29464f153d45a4c887c0
Author: H.J. Lu <hjl.tools@gmail.com>
Date: Tue Jan 12 05:15:49 2021 -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.
(cherry picked from commit 3ec5d83d2a237d39e7fd6ef7a0bc8ac4c171a4a5)
Diff:
---
NEWS | 1 +
.../x86_64/multiarch/memmove-vec-unaligned-erms.S | 21 +++++++++++++++++++++
2 files changed, 22 insertions(+)
diff --git a/NEWS b/NEWS
index 1884f741e6..271f033314 100644
--- a/NEWS
+++ b/NEWS
@@ -76,6 +76,7 @@ The following bugs are resolved with this release:
[25414] 'glob' use-after-free bug (CVE-2020-1752)
[25423] Array overflow in backtrace on powerpc
[25933] Off by one error in __strncmp_avx2
+ [27130] "rep movsb" performance issue
Security related changes:
diff --git a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
index 9bab1147d5..5aaadc233f 100644
--- a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
+++ b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
@@ -67,6 +67,13 @@
# define REP_MOVSB_THRESHOLD (2048 * (VEC_SIZE / 16))
#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
@@ -257,7 +264,21 @@ L(movsb):
# error Unsupported REP_MOVSB_THRESHOLD and VEC_SIZE!
# endif
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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