More conservative value analysis of pointer equality #516
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A pointer "one past" a global "x" and a pointer to the beginning of a global "y" can be equal. This is undefined behavior in CompCert C, but defined behavior in ISO C. Thus, the value analysis should not infer that a pointer based at "x" and a pointer based at "y" cannot be equal. The only inference we keep is that a pointer based in the current stack block and a pointer based in a global variable cannot be equal.
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Consider two global variables
xandy. Currently, the value analysis considers that any pointer based onxcannot be equal to any pointer based ony. This reflects the CompCert C semantics for pointer equality: if the two pointers are within bounds, they must differ; and if one pointer is outside bounds (including if it is "one past"), comparison is undefined.However, a pointer "one past"
xand a pointer to the beginning ofycan be equal at the machine level, and comparing them for equality is not undefined behavior in ISO C, just an unpredictable result.This PR makes sure CompCert's value analysis says "I don't know" for equality tests between pointers based on different global variables. It still says "false or undefined" for equality between a pointer based in the current stack block and a pointer based elsewhere, as such pointers can never be equal even if "one past".