rjmccall added a comment.
In https://reviews.llvm.org/D26196#602660, @yaxunl wrote:
> > But if you do need to support these conversions for some reason, the
> > correct behavior is to ensure that null is mapped to null.
>
> I only need to do this for constant folding, right? I found that the current
> implementation is already able to cast null pointer to the correct
> representation of null pointer in another address space for constant
> expression. e.g. file-scope variables
>
> generic char *gen = (global char*)(generic char*)(private char*)0;
> private char *priv = (private char*)(generic char*)(global char*)0;
> global char *glob = (global char*)(generic char*)(global char*)0;
>
>
>
>
> are translated to
>
> @gen = addrspace(1) global i8 addrspace(4)* null, align 4
> @priv = addrspace(1) global i8* addrspacecast (i8 addrspace(4)* null to
> i8*), align 4
> @glob = addrspace(1) global i8 addrspace(1)* null, align 4
>
>
> This is because null pointers are handled in APValue and
> PointerExprEvaluator::VisitCastExpr. Once a null pointer is identified, it
> can survive passing through addrspacecast and bitcast. When it gets folded,
> it becomes the target-specific null pointer representation in the target
> address space.
>
> However, if an addrspacecast is not going through constant folding, it will
> be translated to LLVM addrspacecast, e.g.
>
> void test_cast(void) {
> global char* glob = (global char*)(generic char*)0;
> }
>
>
> Since the initializer of the function-scope variable does not go through
> constant folding, it is not translated to target-specific null pointer
> representation in the target address space. Instead, it is simply an
> addrspacecast of the original null pointer:
>
> define void @test_cast() #0 {
> entry:
> %glob = alloca i8 addrspace(1)*, align 4
> store i8 addrspace(1)* addrspacecast (i8 addrspace(4)* null to i8
> addrspace(1)*), i8 addrspace(1)** %glob, align 4
> ret void
> }
>
>
> This is still correct translation, since backend should be able to understand
> addrspacecast of null pointer.
>
> Do you think in the last case I should try to fold the addrspacecast?
Yes. I think it would be wrong for an addrspacecast of a literal
llvm::ConstantPointerNull to have different semantics from an addrspacecast of
an opaque value that happens to be an llvm::ConstantPointerNull.
https://reviews.llvm.org/D26196
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