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Comment by dmitrygr

2 days ago

FYI, at least in C/C++, the compiler is free to throw away assignments to any memory pointed to by a pointer if said pointer is about to be passed to free(), so depending on how you did this, no perf impact could have been because your compiler removed the assignment. This will even affect a call to memset()

see here: https://godbolt.org/z/rMa8MbYox

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dmitrygr

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kentonv  2 days ago

I patched the free() implementation itself, not the code that calls free().

I did, of course, test it, and anyway we now run into the "freed memory" pattern regularly when debugging (yes including optimized builds), so it's definitely working.

shakna  2 days ago

However, if you recast to volatile, the compiler will keep it:

    #include <stdlib.h>
    #include <string.h>

    void free(void* ptr);
    void not_free(void* ptr);


    void test_with_free(char* ptr) {
        ptr[5] = 6;
        void *(* volatile memset_v)(void *s, int c, size_t n) = memset;
        memset_v(ptr + 2, 3, 4);
        free(ptr);
    }

    void test_with_other_func(char* ptr) {
        ptr[5] = 6;
        void *(* volatile memset_v)(void *s, int c, size_t n) = memset;
        memset_v(ptr + 2, 3, 4);
        not_free(ptr);
    }