glibc 源码分析

获取source

git clone https://sourceware.org/git/glibc.git
cd glibc
git checkout release/2.17/master
git checkout release/2.20/master

libc-2.20 分析

./sysdeps/x86_64/multiarch/memset.S

/* Define multiple versions only for the definition in lib.  */
# ifndef NOT_IN_libc
ENTRY(memset)
        .type   memset, @gnu_indirect_function
        cmpl    $0, __cpu_features+KIND_OFFSET(%rip)
        jne     1f
        call    __init_cpu_features
1:      leaq    __memset_sse2(%rip), %rax
        testl   $bit_AVX2_Usable, __cpu_features+FEATURE_OFFSET+index_AVX2_Usable(%rip)
        jz      2f
        leaq    __memset_avx2(%rip), %rax
2:      ret
END(memset)
# endif

./sysdeps/x86_64/multiarch/memcpy.S

/* Define multiple versions only for the definition in lib and for
   DSO.  In static binaries we need memcpy before the initialization
   happened.  */
#if defined SHARED && !defined NOT_IN_libc
        .text
ENTRY(__new_memcpy)
        .type   __new_memcpy, @gnu_indirect_function
        cmpl    $0, KIND_OFFSET+__cpu_features(%rip)
        jne     1f
        call    __init_cpu_features
1:      leaq    __memcpy_avx_unaligned(%rip), %rax
        testl   $bit_AVX_Fast_Unaligned_Load, __cpu_features+FEATURE_OFFSET+index_AVX_Fast_Unaligned_Load(%rip)
        jz 1f
        ret
1:      leaq    __memcpy_sse2(%rip), %rax
        testl   $bit_Slow_BSF, __cpu_features+FEATURE_OFFSET+index_Slow_BSF(%rip)
        jnz     2f
        leaq    __memcpy_sse2_unaligned(%rip), %rax
        ret
2:      testl   $bit_SSSE3, __cpu_features+CPUID_OFFSET+index_SSSE3(%rip)
        jz 3f
        leaq    __memcpy_ssse3(%rip), %rax
3:      ret
END(__new_memcpy)