[clang] [lld] [llvm] [WIP][IR][Constants] Change the semantic of `ConstantPointerNull` to represent an actual `nullptr` instead of a zero-value pointer (PR #166667)

[Shilei Tian via llvm-commits]

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@@ -4275,8 +4275,15 @@ static void emitGlobalConstantImpl(const DataLayout &DL, const Constant *CV,
       return emitGlobalConstantFP(CFP, AP);
   }
 
-  if (isa<ConstantPointerNull>(CV)) {
-    AP.OutStreamer->emitIntValue(0, Size);
+  if (auto *NullPtr = dyn_cast<ConstantPointerNull>(CV)) {
+    if (std::optional<APInt> NullPtrVal =
+            DL.getNullPtrValue(NullPtr->getType()->getPointerAddressSpace())) {
+      AP.OutStreamer->emitIntValue(NullPtrVal->getSExtValue(), Size);
+    } else {
+      // We fall back to the default behavior of emitting a zero value if we
+      // can't get the null pointer value from the data layout.
+      AP.OutStreamer->emitIntValue(0, Size);
+    }
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shiltian wrote:

That sounds like a good idea. I can do that but I guess we'd need an RFC.

https://github.com/llvm/llvm-project/pull/166667