[llvm-branch-commits] [llvm] [InstCombine] Improve inbounds preservation for ADD+GEP -> GEP+GEP (PR #135155)

[Nikita Popov via llvm-branch-commits]

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@@ -3087,12 +3087,22 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
     return nullptr;
 
   if (GEP.getNumIndices() == 1) {
-    // We can only preserve inbounds if the original gep is inbounds, the add
-    // is nsw, and the add operands are non-negative.
-    auto CanPreserveInBounds = [&](bool AddIsNSW, Value *Idx1, Value *Idx2) {
+    auto CanPreserveNoWrapFlags = [&](bool AddIsNSW, bool AddIsNUW, Value *Idx1,
+                                      Value *Idx2) {
+      // Preserve "inbounds nuw" if the original gep is "inbounds nuw",
+      // and the add is "nuw".
+      if (GEP.isInBounds() && GEP.hasNoUnsignedWrap() && AddIsNUW)
+        return GEPNoWrapFlags::inBounds() | GEPNoWrapFlags::noUnsignedWrap();
+      // Preserve "inbounds" if the original gep is "inbounds", the add
+      // is "nsw", and the add operands are non-negative.
       SimplifyQuery Q = SQ.getWithInstruction(&GEP);
-      return GEP.isInBounds() && AddIsNSW && isKnownNonNegative(Idx1, Q) &&
-             isKnownNonNegative(Idx2, Q);
+      if (GEP.isInBounds() && AddIsNSW && isKnownNonNegative(Idx1, Q) &&
+          isKnownNonNegative(Idx2, Q))
+        return GEPNoWrapFlags::inBounds();
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nikic wrote:

Is it actually still necessary to explicitly handle this case? If we have an add nsw with nonneg operands, I think we should infer nuw on both add and gep and can then use the new code path?

https://github.com/llvm/llvm-project/pull/135155