yaxunl marked 3 inline comments as done.
yaxunl added a comment.
> 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?
Thanks.
https://reviews.llvm.org/D26196
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