[llvm] [TargetLowering] Add a new function `getNullPtrValue` (PR #126665)

[Shilei Tian via llvm-commits]

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@@ -1807,8 +1807,7 @@ SDValue SelectionDAGBuilder::getValueImpl(const Value *V) {
 
     if (isa<ConstantPointerNull>(C)) {
       unsigned AS = V->getType()->getPointerAddressSpace();
-      return DAG.getConstant(0, getCurSDLoc(),
-                             TLI.getPointerTy(DAG.getDataLayout(), AS));
+      return TLI.getNullPtrValue(AS, getCurSDLoc(), DAG);
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shiltian wrote:

I don't believe that is the case. For example,

```
%6 = icmp eq ptr addrspace(5) %5, null
```

If you check `operand(1)`, it is `ptr addrspace(5) null`, which is recognized as `ConstantPointerNull`. Unless you’re suggesting that `ptr addrspace(5) null` might not be a valid or sentinel pointer, while `addrspacecast (ptr null to ptr addrspace(5))` is, then InstCombine is handling this incorrectly by treating them as equivalent.

https://github.com/llvm/llvm-project/pull/126665