[Bug ipa/122610] [13/14/15/16 regression] Load after store incorrectly deleted with -O2 -fno-strict-aliasing

[lzy20000419 at outlook dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122610

liu zhenyu <lzy20000419 at outlook dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lzy20000419 at outlook dot com

--- Comment #8 from liu zhenyu <lzy20000419 at outlook dot com> ---
I have synthesized a minimal, dependency-free C reproducer for this regression.

Crucially, my testcase proves two key points:

It does NOT rely on Flexible Array Members (FAM) or C23 features.

The miscompilation disappears when compiled with -fno-strict-aliasing,
suggesting an overly aggressive TBAA query within ipa-modref.

Results: -O1: 800 (Correct) -O3: 1920 (Wrong Code)
// Complete GCC Bug - 2025 + Regression Analysis
// Generation: 0
// Results: {'-O1': 800, '-O2': 1280, '-O3': 1920}
// Compile with: gcc -O1/-O2/-O3 -fstrict-aliasing


#include <stdio.h>
#include <stdint.h>
#include <string.h>

struct RString_0 {
    char *ptr;
    long len;
};

char *ptr_global_0;

__attribute__((noinline))
char * get_ptr_0(struct RString_0 *str) {
    return str->ptr;
}

// Trick GCC's IPA into thinking this store is dead code
__attribute__((noinline))
int ipa_modref_bug(struct RString_0 *str) {
    str->ptr = ptr_global_0;
    char *val = get_ptr_0(str);

    if (val == NULL) {
        return -1;
    }

    int result = 0;
    for (int i = 0; i < 10; i++) {
        result += (int)(val - (char*)0) & 0xFF;
    }

    return result;
}

__attribute__((noinline))
int test_ipa_modref_dse() {
    struct RString_0 str;
    char buffer[116];

    ptr_global_0 = buffer;

    for (int i = 0; i < 116; i++) {
        buffer[i] = (char)((i * 827312095) & 0xFF);
    }

    int result = ipa_modref_bug(&str);

    if (str.ptr != buffer) {
        result += 1000;
    }

    return result;
}

int main() {
    int result = test_ipa_modref_dse();
    printf("%d\n", result);
    return 0;
}