[PATCH] D32977: [OpenCL] Emit function-scope variable in constant address space as static variable

[John McCall via Phabricator via cfe-commits]

rjmccall 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);
----------------
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.


https://reviews.llvm.org/D32977