Commit 20f3337d authored by Linus Torvalds's avatar Linus Torvalds
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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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 68674f94

arch/x86/lib/memset_64.S

+11 −36
Original line number Diff line number Diff line
@@ -18,27 +18,22 @@ 
 * rdx   count (bytes)

 *

 * rax   original destination

 */

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.

 * 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.

 */

	ALTERNATIVE_2 "jmp memset_orig", "", X86_FEATURE_REP_GOOD, \

		      "jmp memset_erms", X86_FEATURE_ERMS

SYM_FUNC_START(__memset)

	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