[LLVMdev] Uninitialized variables -- LLVM bug?

[Riyaz Puthiyapurayil]

Consider the following C code. When it is compiled using clang with ' -O', the assert fires. Gcc seems to do the opposite.

#include <assert.h>
int main()
{
    unsigned a;     // uninitialized!
    unsigned b = a;
    assert(b == a); // shouldn't this be always true?
    return 0;
}

Would you consider this a LLVM bug? I think GVN is responsible for this. I don't know if it falls within the realm of "undefined" or "implementation specific" behavior but there are probably examples of code where one can "correctly" skip initialization. The Briggs-Torczon sparse set data structure comes to mind (where you are not comparing two uninitialized variables but an uninitialized variable with an initialized variable-LLVM seems to do the right thing in that case, though).


n  Riyaz


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