【发布时间】：2021-06-29 13:05:37
【问题描述】：
我在下面编译了一个示例代码：
#cat array_addition.c
#define MAX 1000000
#define S 1024
#include <string.h>
int a[S], b[S], c[S];
__attribute__((target_clones("avx512f", "avx2","arch=atom","default")))
void foo(int argc){
int i,x;
for (x=0; x<1024; x++){
for (i=0; i<S; i++){
a[i] = b[i] + c[i];
}
}
b[0] = argc;
memcpy(&a[0], &b[0], argc *sizeof(int));
}
int main(int argc, char** argv) {
foo(argc);
return 0;
}
调用memcpy；
从 objdump 中，我们可以发现它会调用 GLIBC memcpy：
#readelf -r a.out
Relocation section '.rela.dyn' at offset 0x418 contains 1 entry:
Offset Info Type Sym. Value Sym. Name + Addend
000000403ff8 000200000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
Relocation section '.rela.plt' at offset 0x430 contains 4 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000404018 000100000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main@GLIBC_2.2.5 + 0
000000404020 000200000007 R_X86_64_JUMP_SLO 0000000000000000 __gmon_start__ + 0
000000404028 000300000007 R_X86_64_JUMP_SLO 0000000000000000 memcpy@GLIBC_2.14 + 0
000000404030 000000000025 R_X86_64_IRELATIV 4018f0
然后，我使用 gdb 来跟踪它使用了哪个 glibc 实现；
(gdb) b memcpy@plt
Breakpoint 1 at 0x401050
(gdb) s
The program is not being run.
(gdb) r
Starting program: /root/a.out
Breakpoint 1, 0x0000000000401050 in memcpy@plt ()
(gdb) s
Single stepping until exit from function memcpy@plt,
which has no line number information.
0x00007ffff7b623a0 in __memcpy_ssse3_back () from /lib64/libc.so.6
(gdb) info function __memcpy_*
All functions matching regular expression "__memcpy_*":
Non-debugging symbols:
0x00007ffff7aa2840 __memcpy_chk_sse2
0x00007ffff7aa2850 __memcpy_sse2
0x00007ffff7ab1b40 __memcpy_chk_avx512_no_vzeroupper
0x00007ffff7ab1b50 __memcpy_avx512_no_vzeroupper
0x00007ffff7b23360 __memcpy_chk
0x00007ffff7b5a470 __memcpy_chk_ssse3
0x00007ffff7b5a480 __memcpy_ssse3
0x00007ffff7b62390 __memcpy_chk_ssse3_back
0x00007ffff7b623a0 __memcpy_ssse3_back
(gdb)
有__memcpy_avx512_no_zeroupper，但没有被选中；
而且我的 cpu 支持它的功能：
标志：fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe 系统调用 nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 显示器 ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti intel_ppin ssbd mba ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts pku ospke flush_l1d
gcc 版本：
使用内置规范。 COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/root/china-gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper 目标：x86_64-pc-linux-gnu 配置：./configure --prefix=/root/china-gcc-10.2.0 --disable-multilib 线程模型：posix 支持的LTO压缩算法：zlib gcc version 10.2.0 (GCC)
【问题讨论】：
-
Glibc 是免费软件。您可以下载它的源代码，研究它并改进它
标签： c compiler-optimization glibc avx512