allow function declarations in @llvm.compiler.used

sameerds · August 9, 2019, 8:28am

Hi all,

CodeGenModule::addCompilerUsedGlobal() currently asserts if the global being added is a function
declaration.

llvm/llvm-project/blob/474d70b2204ee082854049ad691ccf435863709a/clang/lib/CodeGen/CodeGenModule.cpp#L1883


      
                                                         /* VarArgsArePassed */ false)}));
            }
          }
          
          
void CodeGenModule::addUsedGlobal(llvm::GlobalValue *GV) {
            assert(!GV->isDeclaration() &&
                   "Only globals with definition can force usage.");
            LLVMUsed.emplace_back(GV);
          }
          
          
void CodeGenModule::addCompilerUsedGlobal(llvm::GlobalValue *GV) {
            assert(!GV->isDeclaration() &&
                   "Only globals with definition can force usage.");
            LLVMCompilerUsed.emplace_back(GV);
          }
          
          
static void emitUsed(CodeGenModule &CGM, StringRef Name,
                               std::vector<llvm::WeakTrackingVH> &List) {
            // Don't create llvm.used if there is no need.
            if (List.empty())
              return;

Is there is any specific requirement that a global being added here must have a definition? Or in other words, what could go wrong if a function declaration was added to @llvm.compiler.used?

The Clang toolchain for HIP language provides a set of device libraries in bitcode format that are linked into the current module being compiled. But Linker::LinkOnlyNeeded causes unused functions to be dropped while linking in the device libraries. I am working on an implementation that requires specific functions to be retained until calls for them are later generated by an LLVM pass in the AMDGPU target. As far as I can see, the simplest way is to add their declarations to @llvm.compiler.used. This has to be done before LinkInModules() is invoked in CodeGenAction.

Sameer.