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x86: don't use REP_GOOD or ERMS for small memory clearing 

The modern target to use is FSRS (Fast Short REP STOS), and the other
cases should only be used for bigger areas (ie mainly things like page
clearing).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2023-04-15 12:01:14 -07:00
parent 68674f94ff
commit 20f3337d35
1 changed files with 11 additions and 36 deletions
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47
arch/x86/lib/memset_64.S
View File

@ -18,27 +18,22 @@
* rdx count (bytes)
*
* rax original destination
*
* The FSRS alternative should be done inline (avoiding the call and
* the disgusting return handling), but that would require some help
* from the compiler for better calling conventions.
*
* The 'rep stosb' itself is small enough to replace the call, but all
* the register moves blow up the code. And two of them are "needed"
* only for the return value that is the same as the source input,
* which the compiler could/should do much better anyway.
*/
SYM_FUNC_START(__memset)
/*
* Some CPUs support enhanced REP MOVSB/STOSB feature. It is recommended
* to use it when possible. If not available, use fast string instructions.
*
* Otherwise, use original memset function.
*/
ALTERNATIVE_2 "jmp memset_orig", "", X86_FEATURE_REP_GOOD, \
"jmp memset_erms", X86_FEATURE_ERMS
ALTERNATIVE "jmp memset_orig", "", X86_FEATURE_FSRS
movq %rdi,%r9
movb %sil,%al
movq %rdx,%rcx
andl $7,%edx
shrq $3,%rcx
/* expand byte value */
movzbl %sil,%esi
movabs $0x0101010101010101,%rax
imulq %rsi,%rax
rep stosq
movl %edx,%ecx
rep stosb
movq %r9,%rax
RET
@ -48,26 +43,6 @@ EXPORT_SYMBOL(__memset)
SYM_FUNC_ALIAS(memset, __memset)
EXPORT_SYMBOL(memset)
/*
* ISO C memset - set a memory block to a byte value. This function uses
* enhanced rep stosb to override the fast string function.
* The code is simpler and shorter than the fast string function as well.
*
* rdi destination
* rsi value (char)
* rdx count (bytes)
*
* rax original destination
*/
SYM_FUNC_START_LOCAL(memset_erms)
movq %rdi,%r9
movb %sil,%al
movq %rdx,%rcx
rep stosb
movq %r9,%rax
RET
SYM_FUNC_END(memset_erms)
SYM_FUNC_START_LOCAL(memset_orig)
movq %rdi,%r10