[llvm] [Attributor] Don't replace `AddrSpaceCast` with `ConstantPointerNull` (PR #126779)

[Shilei Tian via llvm-commits]

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@@ -10970,6 +10970,15 @@ struct AAPotentialValuesImpl : AAPotentialValues {
       Value *NewV = getSingleValue(A, *this, getIRPosition(), Values);
       if (!NewV || NewV == &OldV)
         continue;
+      // FIXME: ConstantPointerNull represents a pointer with value 0, but it
+      // doesn’t necessarily mean a nullptr. `ptr addrspace(x) null` is not the
+      // same as `addrspacecast (ptr null to ptr addrspace(x))` if the nullptr
+      // in AS x is not zero. Therefore, we can't simply replace it.
+      if (isa<ConstantPointerNull>(NewV) && isa<ConstantExpr>(OldV)) {
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shiltian wrote:

Underlying object works on top of potential values, but it doesn't directly rely on the literal value of a pointer. My understanding is, whether the value would be the same after address space cast would not make any difference, as long as we don't replace the value directly, which is what this PR is trying to fix.

https://github.com/llvm/llvm-project/pull/126779