[llvm] [CodeGenPrepare] Drop nuw on gep unmerging if the new index is negative (PR #193488)

[Antonio Frighetto via llvm-commits]

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@@ -8801,10 +8801,13 @@ static bool tryUnmergingGEPsAcrossIndirectBr(GetElementPtrInst *GEPI,
   }
   if (UGEPIs.size() == 0)
     return false;
-  // Check the materializing cost of (Uidx-Idx).
+  // Check materializing cost of (Uidx-Idx) and drop nuw flag if necessary.
   for (GetElementPtrInst *UGEPI : UGEPIs) {
     ConstantInt *UGEPIIdx = cast<ConstantInt>(UGEPI->getOperand(1));
     APInt NewIdx = UGEPIIdx->getValue() - GEPIIdx->getValue();
+    if (NewIdx.isNegative() && UGEPI->hasNoUnsignedWrap())
+      UGEPI->setNoWrapFlags(GEPNoWrapFlags::none());
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antoniofrighetto wrote:

Ugh, right, better always to check the LangRef.

https://github.com/llvm/llvm-project/pull/193488