Stack variable alloc alignment

陈云星 chen.yunxing@me.com
Mon Jun 21 08:51:25 GMT 2021


unaligned variable may cause split cache line load/store;

Which can be fetch by perf event: mem_inst_retired.split_loads,mem_inst_retired.split_stores

Is gcc has some compile time option can control variable allocated on stack will aligned?

I use this code snippet tested that both default gcc and clang can’t promise this:

#include <stdio.h>
unsigned long rbp = 0;
int main() {
    asm("movq %%rbp,%0": "=r"(rbp):);
    char a='c';
    int  b=1;
    char e='b';
    long c=20;
    printf("%lu\n%lu, %d\n%lu, %d \n%lu, %d\n%lu, %d", rbp, &a, sizeof(a), &b, sizeof(b), &e, sizeof(e), &c, sizeof(c));
}


$./a.out 
140733249436400
140733249436159, 1
140733249436152, 4 
140733249436151, 1
140733249436136, 8


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