[clang] [Clang][CodeGen] Check `isUnderlyingBasePointerConstantNull` in `emitPointerArithmetic` (PR #137849)

[Eli Friedman via cfe-commits]

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@@ -4238,7 +4238,8 @@ static Value *emitPointerArithmetic(CodeGenFunction &CGF,
   else
     elemTy = CGF.ConvertTypeForMem(elementType);
 
-  if (CGF.getLangOpts().PointerOverflowDefined)
+  if (CGF.getLangOpts().PointerOverflowDefined ||
+      CGF.isUnderlyingBasePointerConstantNull(pointerOperand))
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efriedma-quic wrote:

I guess it's fine to have different workarounds given the context isn't the same, but I'd like a comment in the code explaining the relationship between the two checks, at least.

https://github.com/llvm/llvm-project/pull/137849