https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502
--- Comment #40 from Alexander Cherepanov <ch3root at openwall dot com> ---
Example with a flexible array member:
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#include <string.h>
#include <stdio.h>
__attribute__((noipa)) // imagine it in a separate TU
static void *opaque(void *p) { return p; }
int main()
{
int x[5];
struct { int i, j, a[]; } y;
printf("eq1: %d\n", x == y.a);
int *p = x;
int *q = y.a;
printf("eq2: %d\n", p == q);
printf("diff: %d\n", memcmp(&p, &q, sizeof(int *)));
opaque(q); // escaped
opaque(&p); // move the next comparison to run-time
printf("eq3: %d\n", p == q);
}
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$ gcc -std=c11 -pedantic -Wall -Wextra test.c && ./a.out
eq1: 0
eq2: 1
diff: 0
eq3: 1
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
eq1: 0
eq2: 0
diff: 0
eq3: 1
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gcc x86-64 version: gcc (GCC) 10.0.0 20200110 (experimental)
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This example should be standard-compliant.
eq1 is wrong even without optimization, eq2 folded by fre1.
