[llvm] [BasicAA] Use nuw attribute of GEPs (PR #98608)

[Nikita Popov via llvm-commits]

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@@ -1843,6 +1858,11 @@ bool BasicAAResult::isValueEqualInPotentialCycles(const Value *V,
 void BasicAAResult::subtractDecomposedGEPs(DecomposedGEP &DestGEP,
                                            const DecomposedGEP &SrcGEP,
                                            const AAQueryInfo &AAQI) {
+  // Drop nuw flag from GEP if subtraction of constant offsets overflows in an
+  // unsigned sense.
+  if (DestGEP.Offset.ult(SrcGEP.Offset))
+    DestGEP.NWFlags = DestGEP.NWFlags->withoutNoUnsignedWrap();
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nikic wrote:

(Another reason to drop the std::optional: It's not really clear that this code here can't access nullopt).

https://github.com/llvm/llvm-project/pull/98608