[flang-commits] [flang] [Flang] Apply nusw nuw flags on array_coor gep's (PR #184573)

[Tom Eccles via flang-commits]

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@@ -2713,6 +2713,25 @@ struct XArrayCoorOpConversion
         baseIsBoxed ? getBoxTypePair(coor.getMemref().getType()) : TypePair{};
     mlir::LLVM::IntegerOverflowFlags nsw =
         mlir::LLVM::IntegerOverflowFlags::nsw;
+    mlir::LLVM::IntegerOverflowFlags nuw =
+        mlir::LLVM::IntegerOverflowFlags::nuw;
+    mlir::LLVM::IntegerOverflowFlags subFlags = nsw;
+    mlir::LLVM::IntegerOverflowFlags addMulFlags = nsw;
+    mlir::LLVM::GEPNoWrapFlags gepFlags = mlir::LLVM::GEPNoWrapFlags::none;
+
+    // In certain cases, where unsigned wrapping is known not to not occur, we
+    // can apply the nuw flag to Add/Mul operations, and `nusw nuw` flags to
+    // getelementptr's. By doing so, this enables better optimization through
+    // slp-vectorizer later in the LLVM pipeline.
+    const bool canUseNuw = !baseIsBoxed && !isShifted && !isSliced &&
+                           coor.getSubcomponent().empty() &&
+                           coor.getLenParams().empty() &&
+                           !coor.getShape().empty();
+    if (canUseNuw) {
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tblah wrote:

Thanks it makes sense that unsigned arithmetic would easily wrap small negative numbers. But I wonder if using unsigned arithmetic would ever produce a sensible result in this case. Could you give an example of where this would happen?

Should we be checking for negative values in all lower bounds then?

https://github.com/llvm/llvm-project/pull/184573