diff options
| author | Linus Torvalds <torvalds@linux-foundation.org> | 2023-04-15 21:01:14 +0200 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2023-04-19 02:05:28 +0200 |
| commit | 20f3337d350c4e1b4ac66d731fd4e98565bf6cc0 (patch) | |
| tree | 8367e98527a5dc7ba4e3ae67ea7f2bf3b6b41267 | |
| parent | x86: don't use REP_GOOD or ERMS for small memory copies (diff) | |
| download | linux-20f3337d350c4e1b4ac66d731fd4e98565bf6cc0.tar.xz linux-20f3337d350c4e1b4ac66d731fd4e98565bf6cc0.zip | |
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>
| -rw-r--r-- | arch/x86/lib/memset_64.S | 47 |
1 files changed, 11 insertions, 36 deletions
diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S index 6143b1a6fa2c..7c59a704c458 100644 --- a/arch/x86/lib/memset_64.S +++ b/arch/x86/lib/memset_64.S @@ -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 |