================
@@ -1561,6 +1583,24 @@ Constant *llvm::ConstantFoldCastOperand(unsigned Opcode,
Constant *C,
}
}
break;
+ case Instruction::AddrSpaceCast:
+ // A null pointer (`ptr addrspace(N) null` in IR presentation,
+ // `ConstantPointerNull` in LLVM class, not `nullptr` in C/C++) used to
+ // represent a zero-value pointer in the corresponding address space.
+ // Therefore, we can't simply fold an address space cast of a null pointer
+ // from one address space to another, because on some targets, the nullptr
+ // of an address space could be non-zero.
+ //
+ // Recently, the semantic of `ptr addrspace(N) null` is changed to
represent
+ // the actual nullptr in the corresponding address space. It can be zero or
+ // non-zero, depending on the target. Therefore, we can fold an address
+ // space cast of a nullptr from one address space to another.
+
+ // If the input is a nullptr, we can fold it to the corresponding
+ // nullptr in the destination address space.
+ if (C->isNullValue())
----------------
shiltian wrote:
> Is this actually true? Are null values in one address space always null in
> all others? I imagine this is almost always true, but maybe you could have
> some where this conversion is not valid?
This is actually the whole point of this PR. The semantic would be, convert the
`nullptr` in AS X to AS Y, so it would still be a `nullptr` in AS Y. I don't
think the semantic should be convert the value of `nullptr` in AS X to the
corresponding pointer value in AS Y.
I'll clarify this in the LangRef.
https://github.com/llvm/llvm-project/pull/166667
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