c - Are compilers allowed to optimize out realloc?

Question

I came across a situation where it would be useful to have unnecessary calls to realloc being optimized out. However, it seems like neither Clang nor GCC do such a thing (Compiler Explorer (godbolt.org)) - although I see optimizations being made with multiple calls to malloc.

The example:

void *myfunc() {
    void *data;
    data = malloc(100);
    data = realloc(data, 200);
    return data;
}

I expected it to be optimized to something like the following:

void *myfunc() {
    return malloc(200);
}

Why is neither Clang nor GCC optimizing it out? - Are they not allowed to do so?

question from:https://stackoverflow.com/questions/53373421/are-compilers-allowed-to-optimize-out-realloc

Answer 1

Are they not allowed to do so?

Maybe, but optimization not done in this case may be due to corner functional differences.

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If 150 bytes of allocatable memory remain,
data = malloc(100); data = realloc(data, 200); returns NULL with 100 bytes consumed (and leaked) and 50 remain.

data = malloc(200); returns NULL with 0 bytes consumed (none leaked) and 150 remain.

Different functionality in this narrow case may prevent optimization.

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Are compilers allowed to optimize-out realloc?

Perhaps - I would expect it is allowed. Yet it may not be worth the effect to enhance the compiler to determine when it can.

Successful malloc(n); ... realloc(p, 2*n) differs from malloc(2*n); when ... may have set some of the memory.

It might be beyond that compiler's design to ensure ..., even if empty code, did not set any memory.