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Subject[PATCH] x86/cpufeatures: Add support for fast short rep mov
Date
From the Intel Optimization Reference Manual:

3.7.6.1 Fast Short REP MOVSB
Beginning with processors based on Ice Lake Client microarchitecture,
REP MOVSB performance of short operations is enhanced. The enhancement
applies to string lengths between 1 and 128 bytes long. Support for
fast-short REP MOVSB is enumerated by the CPUID feature flag: CPUID
[EAX=7H, ECX=0H).EDX.FAST_SHORT_REP_MOVSB[bit 4] = 1. There is no change
in the REP STOS performance.

Add an X86_FEATURE_FSRM flag for this.

memmove() avoids REP MOVSB for short (< 32 byte) copies. Fix it
to check FSRM and use REP MOVSB for short copies on systems that
support it.

Signed-off-by: Tony Luck <tony.luck@intel.com>

---

Time (cycles) for memmove() sizes 1..31 with neither source nor
destination in cache.

1800 +-+-------+--------+---------+---------+---------+--------+-------+-+
+ + + + + + + +
1600 +-+ 'memmove-fsrm' *******-+
| ###### 'memmove-orig' ####### |
1400 +-+ # ##################### +-+
| # ############ |
1200 +-+# ################## +-+
| # |
1000 +-+# +-+
| # |
| # |
800 +-# +-+
| # |
600 +-*********************** +-+
| ***************************** |
400 +-+ ******* +-+
| |
200 +-+ +-+
+ + + + + + + +
0 +-+-------+--------+---------+---------+---------+--------+-------+-+
0 5 10 15 20 25 30 35
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/lib/memmove_64.S | 6 +++---
2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index e9b62498fe75..98c60fa31ced 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -357,6 +357,7 @@
/* Intel-defined CPU features, CPUID level 0x00000007:0 (EDX), word 18 */
#define X86_FEATURE_AVX512_4VNNIW (18*32+ 2) /* AVX-512 Neural Network Instructions */
#define X86_FEATURE_AVX512_4FMAPS (18*32+ 3) /* AVX-512 Multiply Accumulation Single precision */
+#define X86_FEATURE_FSRM (18*32+ 4) /* Fast Short Rep Mov */
#define X86_FEATURE_AVX512_VP2INTERSECT (18*32+ 8) /* AVX-512 Intersect for D/Q */
#define X86_FEATURE_MD_CLEAR (18*32+10) /* VERW clears CPU buffers */
#define X86_FEATURE_TSX_FORCE_ABORT (18*32+13) /* "" TSX_FORCE_ABORT */
diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S
index 337830d7a59c..4a23086806e6 100644
--- a/arch/x86/lib/memmove_64.S
+++ b/arch/x86/lib/memmove_64.S
@@ -29,10 +29,7 @@
SYM_FUNC_START_ALIAS(memmove)
SYM_FUNC_START(__memmove)

- /* Handle more 32 bytes in loop */
mov %rdi, %rax
- cmp $0x20, %rdx
- jb 1f

/* Decide forward/backward copy mode */
cmp %rdi, %rsi
@@ -43,6 +40,7 @@ SYM_FUNC_START(__memmove)
jg 2f

.Lmemmove_begin_forward:
+ ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "", X86_FEATURE_FSRM
ALTERNATIVE "", "movq %rdx, %rcx; rep movsb; retq", X86_FEATURE_ERMS

/*
@@ -114,6 +112,8 @@ SYM_FUNC_START(__memmove)
*/
.p2align 4
2:
+ cmp $0x20, %rdx
+ jb 1f
cmp $680, %rdx
jb 6f
cmp %dil, %sil
--
2.20.1
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