Anastasia added inline comments.
================
Comment at: lib/CodeGen/CGDecl.cpp:1105-1119
+ // Alloca always returns a pointer in alloca address space, which may
+ // be different from the type defined by the language. For example,
+ // in C++ the auto variables are in the default address space. Therefore
+ // cast alloca to the expected address space when necessary.
+ auto Addr = address.getPointer();
+ auto AddrTy = cast<llvm::PointerType>(Addr->getType());
+ auto ExpectedAddrSpace =
CGM.getTypes().getVariableType(D)->getAddressSpace();
----------------
rjmccall wrote:
> yaxunl wrote:
> > yaxunl wrote:
> > > rjmccall wrote:
> > > > Anastasia wrote:
> > > > > yaxunl wrote:
> > > > > > yaxunl wrote:
> > > > > > > t-tye wrote:
> > > > > > > > Is any assert done to ensure that it is legal to address space
> > > > > > > > cast from variable address space to expected address space?
> > > > > > > > Presumably the language, by definition, will only be causing
> > > > > > > > legal casts. For example from alloca address space to generic
> > > > > > > > (which includes the alloca address space).
> > > > > > > >
> > > > > > > > For OpenCL, can you explain how the local variable can have the
> > > > > > > > constant address space and use an alloca for allocation?
> > > > > > > > Wouldn't a constant address space mean it was static and so
> > > > > > > > should not be using alloca? And if it is using an alloca, how
> > > > > > > > can it then be accessed as if it was in constant address space?
> > > > > > > If the auto var has address space qualifier specified through
> > > > > > > `__attribute__((address_space(n)))`, there is not much we can
> > > > > > > check in clang since it is target dependent. We will just emit
> > > > > > > address space cast when necessary and let the backend check the
> > > > > > > validity of the address space cast.
> > > > > > >
> > > > > > > Otherwise, for OpenCL, we can assert the expected address space
> > > > > > > is default (for OpenCL default address space in AST represents
> > > > > > > private address space in source language) or constant. For other
> > > > > > > languages we can assert the expected address space qualifier is
> > > > > > > default (no address space qualifier). It is not convenient to
> > > > > > > further check whether the emitted LLVM address space cast
> > > > > > > instruction is valid since it requires target specific
> > > > > > > information, therefore such check is better deferred to the
> > > > > > > backend.
> > > > > > >
> > > > > > > For OpenCL, currently automatic variable in constant address
> > > > > > > space is emitted in private address space. For example, currently
> > > > > > > Clang does not diagnose the following code
> > > > > > >
> > > > > > > ```
> > > > > > > void f(global int* a) {
> > > > > > > global int* constant p = a;
> > > > > > > }
> > > > > > >
> > > > > > > ```
> > > > > > > Instead, it emits alloca for p, essentially treats it as `global
> > > > > > > int* const p`. This seems to be a bug to me (or maybe we can call
> > > > > > > it a feature? since there seems no better way to translate this
> > > > > > > to LLVM IR, or simply diagnose this as an error). However, this
> > > > > > > is better addressed by another patch.
> > > > > >
> > > > > > Hi Anastasia,
> > > > > >
> > > > > > Any comments about the automatic variable in constant address
> > > > > > space? Thanks.
> > > > > From the spec s6.5.3 it feels like we should follow the same
> > > > > implementation path in Clang for constant AS inside kernel function
> > > > > as local AS. Because constant AS objects are essentially global
> > > > > objects.
> > > > >
> > > > > Although, we didn't have any issues up to now because const just
> > > > > does the trick of optimising the objects out eventually. I am not
> > > > > clear if this creates any issue now with your allocation change. It
> > > > > feels though that it should probably work fine just as is?
> > > > If these __constant locals are required to be const (or are implicitly
> > > > const?) and have constant initializers, it seems to me the
> > > > implementation obviously intended by the spec is that you allocate them
> > > > statically in the constant address space. It's likely that just
> > > > implicitly making the variable static in Sema, the same way you make it
> > > > implicitly const, will make IRGen and various other parts of the
> > > > compiler just do the right thing.
> > > My patch does not change the current behaviour of Clang regarding
> > > function-scope variable in constant address space. Basically there is no
> > > issue if there is no address taken. However, if there is address taken,
> > > there will be assertion due to casting private pointer to constant
> > > address space. I think probably it is better to emit function-scope
> > > variable in constant address space as global variable in constant address
> > > space instead of alloca, as John suggested.
> > >
> > I agree function-scope variable in constant address space should be emitted
> > as global variable in constant address space instead of alloca. However, in
> > OpenCL 1.2 section 6.8, "The static storage-class specifier can only be
> > used for non-kernel functions and global variables declared in program
> > scope." Currently Clang diagnoses function-scope variables with static
> > storage class for OpenCL 1.2. Therefore we cannot make function-scope
> > variable in constant address space have static storage class. However, I
> > think we can still emit function-scope variable in constant address space
> > as global variable in CodeGen.
> There's precedent for treating something as implicitly static in the AST.
> thread_local variables in C++ are considered implicitly static.
>
> You'll need to change VarDecl::hasLocalStorage() and isStaticLocal(), and you
> should look around for other uses of getTSCSpec() that look like they're
> implementing the same logic.
I belive we already do similar logic for stack variables with `__local` AS.
Because they are emmited as static globals. There should be no contradictions
with spec to do the same for `__constant `AS objects. They are essentially
intended to be located in a "special" memory sections and not on a stack. Also
constant AS local variable can only be declared in the kernel functions.
https://reviews.llvm.org/D32248
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