[llvm] [ValueTracking] Allow getUnderlyingPointer to look through inttoptr/ptrtoint round trip casts (PR #146432)

[Nikita Popov via llvm-commits]

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@@ -1602,6 +1602,8 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
   // Figure out what objects these things are pointing to if we can.
   const Value *O1 = getUnderlyingObject(V1, MaxLookupSearchDepth);
   const Value *O2 = getUnderlyingObject(V2, MaxLookupSearchDepth);
+  removeRoundTripCasts(O1);
+  removeRoundTripCasts(O2);
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nikic wrote:

It's the same problem as in the general case. You can't look through inttoptr/ptrtoint cast in any code that does provenance-based reasoning, which BasicAA does (e.g. via isIdentifiedObject).

It *is* possible to look through these casts, but you can only reason about the *address* of the pointer after you do. This means that in your example, we would not be able to prove these are NoAlias because the underlying object is different -- but we *could* prove that the accessed address ranges must be disjoint.

It's possible, but this would need substantial changes to BasicAA and the AA API.

https://github.com/llvm/llvm-project/pull/146432