yaxunl added a comment. In https://reviews.llvm.org/D44985#1050876, @rjmccall wrote:
> In https://reviews.llvm.org/D44985#1050674, @yaxunl wrote: > > > In https://reviews.llvm.org/D44985#1050670, @rjmccall wrote: > > > > > What exactly are you trying to express here? Are you just trying to make > > > these external declarations when compiling for the device because > > > `__shared__` variables are actually defined on the host? That should be > > > handled by the frontend by setting up the AST so that these declarations > > > are not definitions. > > > > > > No. These variables are not like external symbols defined on the host. They > > behave like global variables in the kernel code but never initialized. > > Currently no targets are able to initialize them and it is users' > > responsibility to initialize them explicitly. > > > > Giving them an initial value will cause error in some backends since they > > cannot handle them, therefore put undef as initializer. > > > So undef is being used as a special marker to the backends that it's okay not > to try to initialize these variables? I think undef as the initializer tells the llvm passes and backend that this global variable contains undefined value. I am not sure if this is better than without an initializer. I saw code in CodeGenModule::getOrCreateStaticVarDecl // Local address space cannot have an initializer. llvm::Constant *Init = nullptr; if (Ty.getAddressSpace() != LangAS::opencl_local) Init = EmitNullConstant(Ty); else Init = llvm::UndefValue::get(LTy); which means OpenCL static variable in local address space (equivalent to CUDA shared address space) gets an undef initializer. For CUDA shared variable, in CodeGenFunction::EmitStaticVarDecl, it first goes through call of CodeGenModule::getOrCreateStaticVarDecl and gets a zeroinitializer, then it reaches line 400 // Whatever initializer such variable may have when it gets here is // a no-op and should not be emitted. bool isCudaSharedVar = getLangOpts().CUDA && getLangOpts().CUDAIsDevice && D.hasAttr<CUDASharedAttr>(); // If this value has an initializer, emit it. if (D.getInit() && !isCudaSharedVar) var = AddInitializerToStaticVarDecl(D, var); Although this disables adding initializer from D, var already has a zeroinitializer from CodeGenModule::getOrCreateStaticVarDecl, therefore its initializer needs to be overwritten by undef. Probably a better solution would be do it in CodeGenModule::getOrCreateStaticVarDecl, side by side by the OpenCL code. https://reviews.llvm.org/D44985 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
