tools/arch/x86/lib/memset_64.S

Source file repositories/reference/linux-study-clean/tools/arch/x86/lib/memset_64.S

File Facts

System
Linux kernel
Corpus path
tools/arch/x86/lib/memset_64.S
Extension
.S
Size
2521 bytes
Lines
120
Domain
Support Tooling And Documentation
Bucket
tools
Inferred role
Support Tooling And Documentation: exported/initcall integration point
Status
integration implementation candidate

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

#include <linux/export.h>
#include <linux/linkage.h>
#include <linux/cfi_types.h>
#include <asm/cpufeatures.h>
#include <asm/alternative.h>

.section .noinstr.text, "ax"

/*
 * ISO C memset - set a memory block to a byte value. This function uses fast
 * string to get better performance than the original function. The code is
 * simpler and shorter than the original function as well.
 *
 * rdi   destination
 * rsi   value (char)
 * 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_TYPED_FUNC_START(__memset)
	ALTERNATIVE "jmp memset_orig", "", X86_FEATURE_FSRS

	movq %rdi,%r9
	movb %sil,%al
	movq %rdx,%rcx
	rep stosb
	movq %r9,%rax
	RET
SYM_FUNC_END(__memset)
EXPORT_SYMBOL(__memset)

SYM_FUNC_ALIAS_MEMFUNC(memset, __memset)
SYM_PIC_ALIAS(memset)
EXPORT_SYMBOL(memset)

SYM_FUNC_START_LOCAL(memset_orig)
	movq %rdi,%r10

	/* expand byte value  */
	movzbl %sil,%ecx
	movabs $0x0101010101010101,%rax
	imulq  %rcx,%rax

	/* align dst */
	movl  %edi,%r9d
	andl  $7,%r9d
	jnz  .Lbad_alignment
.Lafter_bad_alignment:

	movq  %rdx,%rcx
	shrq  $6,%rcx
	jz	 .Lhandle_tail

	.p2align 4
.Lloop_64:
	decq  %rcx
	movq  %rax,(%rdi)
	movq  %rax,8(%rdi)
	movq  %rax,16(%rdi)
	movq  %rax,24(%rdi)
	movq  %rax,32(%rdi)

Annotation

Implementation Notes