rjmccall added inline comments.
================
Comment at: lib/Sema/SemaExprCXX.cpp:4289
+ /*BasePath=*/nullptr, CCK)
+ .get();
----------------
Anastasia wrote:
> rjmccall wrote:
> > Anastasia wrote:
> > > rjmccall wrote:
> > > > Anastasia wrote:
> > > > > rjmccall wrote:
> > > > > > Okay. But if `ToType` *isn't* a reference type, this will never be
> > > > > > an address-space conversion. I feel like this code could be
> > > > > > written more clearly to express what it's trying to do.
> > > > > I hope it makes more sense now. Btw, it also applies to pointer type.
> > > > The logic is wrong for pointer types; if you're converting pointers,
> > > > you need to be checking the address space of the pointee type of the
> > > > from type.
> > > >
> > > > It sounds like this is totally inadequately tested; please flesh out
> > > > the test with all of these cases. While you're at it, please ensure
> > > > that there are tests verifying that we don't allowing address-space
> > > > changes in nested positions.
> > > Thanks for spotting this bug! The generated IR for the test was still
> > > correct because AS of `FromType` happened to correctly mismatch AS of
> > > pointee of `ToType`.
> > >
> > > I failed to construct the test case where it would miss classifying
> > > `addrspacecast` due to OpenCL or C++ sema rules but I managed to add a
> > > case in which `addrspacecast` was incorrectly added for pointers where it
> > > wasn't needed (see line 36 of the test). I think this code is covered now.
> > >
> > > As for the address space position in pointers, the following test checks
> > > the address spaces of pointers in `addrspacecast`. For the other program
> > > paths we also have a test with similar checks in
> > > `test/CodeGenOpenCL/address-spaces-conversions.cl` that we now run for
> > > C++ mode too.
> > >
> > > BTW, while trying to construct a test case for the bug, I have discovered
> > > that multiple pointer indirection casting isn't working correctly. I.e.
> > > for the following program:
> > > kernel void foo(){
> > > __private int** loc;
> > > int** loc_p = loc;
> > > **loc_p = 1;
> > > }
> > > We generate:
> > > bitcast i32* addrspace(4)* %0 to i32 addrspace(4)* addrspace(4)*
> > > in OpenCL C and then perform `store` over pointer in AS 4 (generic). We
> > > have now lost the information that the original pointer was in `private`
> > > AS and that the adjustment of AS segment has to be performed before
> > > accessing memory pointed by the pointer. Based on the current
> > > specification of `addrspacecast` in
> > > https://llvm.org/docs/LangRef.html#addrspacecast-to-instruction I am not
> > > very clear whether it can be used for this case without any modifications
> > > or clarifications and also what would happen if there are multiple AS
> > > mismatches. I am going to look at this issue separately in more details.
> > > In OpenCL C++ an ICE is triggered for this though. Let me know if you
> > > have any thoughts on this.
> > Thanks, the check looks good now.
> >
> > > BTW, while trying to construct a test case for the bug, I have discovered
> > > that multiple pointer indirection casting isn't working correctly.
> >
> > This needs to be an error in Sema. The only qualification conversions that
> > should be allowed in general on nested pointers (i.e. on `T` in `T**` or
> > `T*&`) are the basic C qualifiers: `const`, `volatile`, and `restrict`; any
> > other qualification change there is unsound.
> I see. I guess it's because C++ rules don't cover address spaces.
>
> It feels like it would be a regression for OpenCL C++ vs OpenCL C to reject
> nested pointers with address spaces because it was allowed before. :(
>
> However, the generation for OpenCL C and C are incorrect currently. I will
> try to sort that all out as a separate patch though, if it makes sense?
C++'s rules assume that qualifiers don't introduce real representation
differences and that operations on qualified types are compatible with
operations on unqualified types. That's not true of qualifiers in general:
address space qualifiers can change representations, ARC qualifiers can have
incompatible semantics, etc. There is no way to soundly implement a conversion
from `__private int **` to `__generic int **`, just there's no way to soundly
implement a conversion from `Derived **` to `Base **`.
If you want to allow this conversion anyway for source-compatibility reasons
(and I don't think that's a good idea), it should be a bitcast.
https://reviews.llvm.org/D53764
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