[llvm] [GlobalISel] Constant-fold G_PTR_ADD with different type sizes (PR #81473)

[Pierre van Houtryve via llvm-commits]

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@@ -660,8 +660,17 @@ std::optional<APInt> llvm::ConstantFoldBinOp(unsigned Opcode,
   default:
     break;
   case TargetOpcode::G_ADD:
-  case TargetOpcode::G_PTR_ADD:
     return C1 + C2;
+  case TargetOpcode::G_PTR_ADD: {
+    // Types can be of different width here.
+    if (C1.getBitWidth() > C2.getBitWidth())
+      return C2.zext(C1.getBitWidth()) + C1;
+    // We always need to return something the same size as C1, so
+    // truncate in this case.
+    if (C1.getBitWidth() < C2.getBitWidth())
+      return C1 + C2.trunc(C1.getBitWidth());
+    return C1 + C2;
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Pierre-vh wrote:

I think sext is probably more correct as offsets can be negative, right?

https://github.com/llvm/llvm-project/pull/81473