yaxunl added inline comments.
================
Comment at: lib/Sema/SemaDecl.cpp:10286
+ // these variables must be a compile time constant.
+ VDecl->getType().getAddressSpace() == LangAS::opencl_constant)
CheckForConstantInitializer(Init, DclT);
----------------
rjmccall wrote:
> yaxunl wrote:
> > Anastasia wrote:
> > > yaxunl wrote:
> > > > rjmccall wrote:
> > > > > yaxunl wrote:
> > > > > > rjmccall wrote:
> > > > > > > Should this rule apply even in C++ mode? I can't remember if
> > > > > > > there are any OpenCL/C++ hybrids.
> > > > > > No. This rule (static local variable needs to be initialised with
> > > > > > compile-time constant) only applies to C and OpenCL.
> > > > > > In C++ static local variable can be initialised with
> > > > > > non-compile-time constant, in which case Clang will emit atomic
> > > > > > instructions to make sure it is only initialised once.
> > > > > >
> > > > > > Currently OpenCL 2.2 defines OpenCL C++ but clang does not support
> > > > > > it.
> > > > > Yes, I understand that C++ generally allows static locals to be
> > > > > lazily initialized, and that that rule would (probably) still apply
> > > > > to ordinary static locals in OpenCL C++. However, I would expect
> > > > > that OpenCL C++ rule is that __constant local variables still need to
> > > > > be statically initialized, because there's no plausible way in the
> > > > > OpenCL implementation model to actually put lazily-initialized
> > > > > variables in the constant address space. Assuming that's correct,
> > > > > then I would recommend reworking this whole chain of conditions to:
> > > > >
> > > > > // Don't check the initializer if the declaration is malformed.
> > > > > if (VDecl->isInvalidDecl()) {
> > > > > // do nothing
> > > > >
> > > > > // OpenCL __constant locals must be constant-initialized, even in
> > > > > OpenCL C++.
> > > > > } else if (VDecl->getType().getAddressSpace() ==
> > > > > LangAS::opencl_constant) {
> > > > > CheckForConstantInitializer(Init, DclT);
> > > > >
> > > > > // Otherwise, C++ does not restrict the initializer.
> > > > > } else if (getLangOpts().CPlusPlus) {
> > > > > // do nothing
> > > > >
> > > > > // C99 6.7.8p4: All the expressions in an initializer for an object
> > > > > that has
> > > > > // static storage duration shall be constant expressions or string
> > > > > literals.
> > > > > } else if (VDecl->getStorageClass() == SC_Static) {
> > > > > CheckForConstantInitializer(Init, DclT);
> > > > >
> > > > > // C89 is stricter than C99 for aggregate initializers.
> > > > > // C89 6.5.7p3: All the expressions [...] in an initializer list
> > > > > // for an object that has aggregate or union type shall be
> > > > > // constant expressions.
> > > > > } else if (!getLangOpts().C99 &&
> > > > > VDecl->getType()->isAggregateType() && isa<InitListExpr>(Init)) {
> > > > > Expr *Culprit;
> > > > > if (!Init->isConstantInitializer(Context, false, &Culprit)) {
> > > > > ...
> > > > > }
> > > > > }
> > > > Agree that even OpenCL C++ is unable to lazy initialise function-scope
> > > > var in constant addr space. Will do.
> > > I think the original way would be much simpler to read and understand
> > > though.
> > >
> > > To be honest I wouldn't complicate things now for the feature we don't
> > > support. I feel OpenCL C++ should be represented as a separate LangOpt
> > > since there are some places that will require special handling due to
> > > deviation from C++. I would rather extend things later in more systematic
> > > way.
> > I will delete the comment about OpenCL C++ when committing.
> I disagree. Simple chains like this are almost always superior to building
> up complex logical conditions: the priorities between conditions are clearer
> (such as the interaction between __constant and C++ here), each condition can
> be conveniently documented without having to add comments to the middle of an
> expression, and there's less need to build up (A && !B) conditions just to
> make sure that cases are routed to the right place. If the body of a clause
> is complex, it's usually a good idea to extract it into a separate function
> anyway, as has already been done here with CheckForConstantInitializer.
>
> Deleting the comment about OpenCL C++ seems silly. The comment is correct
> and explains why the clauses need to be ordered the way they are, and someone
> implementing OpenCL C++ support later will not think to add it back.
>
> Please trust me that you would not want to use a different LangOpt for OpenCL
> C++. OpenCL C++ may feel a little different from normal C++ to a user, but
> for the compiler its deviations are tiny compared to the number of ways in
> which C++ deviates from C. The major C++ features that really cut across the
> frontend are all still there: templates, references, type members, function
> overloading, operator overloading, totally different lookup rules, etc.
I double checked OpenCL 2.2 C++ language spec and did some experiment with the
khronos implementation:
https://www.khronos.org/registry/OpenCL/specs/opencl-2.2-cplusplus.pdf
https://github.com/KhronosGroup/libclcxx
I think you are right about the constant address space in OpenCL C++. Although
OpenCL C++ uses class templates to implement address space, it does use
`__constant` to implement constant class template internally, and supports
`__constant` address space qualifier. Therefore the comment about OpenCL C++
should be correct.
https://reviews.llvm.org/D32977
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