[llvm-dev] Semantics of any out-of-bounds address returned by GEP

Gautam Chakrabarti via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 6 22:36:49 PST 2020


I am looking to get some clarification about out-of-bounds address that may be returned by a GEP.

My understanding is that in the absence of the inbounds keyword, a GEP may return a pointer that is out-of-bounds of the underlying allocated object. Is it correct that any such out-of-bounds address must not be dereferenced?

I am a bit confused by the current wordings on this.

https://llvm.org/docs/LangRef.html#getelementptr-instruction says:

"If the inbounds keyword is not present, .... The result value of the getelementptr may be outside the object pointed to by the base pointer. The result value may not necessarily be used to access memory though, even if it happens to point into allocated storage."

This seems to say that the out-of-bounds pointer may or may not be used to access memory.

However, https://llvm.org/docs/GetElementPtr.html#how-is-gep-different-from-ptrtoint-arithmetic-and-inttoptr says:

"Also, GEP carries additional pointer aliasing rules. It's invalid to take a GEP from one object, address into a different separately allocated object, and dereference it. IR producers (front-ends) must follow this rule, and consumers (optimizers, specifically alias analysis) benefit from being able to rely on it."

This quite clearly says the out-of-bounds address shall not be dereferenced through. This also seems to be the assumption in AliasAnalysis implementation (as already stated in above documentation).


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