[amdgpu] Remove the GlobalDCE pass prior to the internalization pass.

[hliao]

Summary

• In D98783, an extra GlobalDCE pass is inserted before the internalization pass to ensure a global variable without users could be internalized even if there are dead users. Instead of inserting a dedicated optimization pass, the dead user checking, i.e. 'use_empty()', should be preceeded with constant dead user removal to ensure an accurate result.

[rampitec]

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

434

Maybe do this in the InternalizePass itself? This is a predicate function and it is not expected to change the IR.

[hliao]

Instead of inserting GlobalDCE to remove all constant users, call 'removeDeadConstantUsers' directly before 'use_empty' check. That should be the standard way to check whether a global is still in use.

[hliao]

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

434

In fact, even there are dead constants removed, the IR won't be changed. IR printer won't be able to print those dead constant users from this GV.

[hliao]

Rebase

[foad]

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

434

Agreed, I think this is the right way to do this.

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

return FSAttr.isValid() ? FSAttr.getValueAsString()
                           : getTargetFeatureString();
 }
 
 /// Predicate for Internalize pass.
 static bool mustPreserveGV(const GlobalValue &GV) {
   if (const Function *F = dyn_cast<Function>(&GV))
     return F->isDeclaration() || AMDGPU::isEntryFunctionCC(F->getCallingConv());
 
+  GV.removeDeadConstantUsers();
   return !GV.use_empty();
 }
 
 void AMDGPUTargetMachine::adjustPassManager(PassManagerBuilder &Builder) {
   Builder.DivergentTarget = true;
 
   bool EnableOpt = getOptLevel() > CodeGenOpt::None;
   bool Internalize = InternalizeSymbols;
PB.registerPipelineEarlySimplificationEPCallback(
       [this](ModulePassManager &PM, PassBuilder::OptimizationLevel Level) {
         if (Level == PassBuilder::OptimizationLevel::O0)
           return;
 
         PM.addPass(AMDGPUUnifyMetadataPass());
         PM.addPass(AMDGPUPrintfRuntimeBindingPass());
 
         if (InternalizeSymbols) {
-          // Global variables may have dead uses which need to be removed.
-          // Otherwise these useless global variables will not get internalized.
-          PM.addPass(GlobalDCEPass());
           PM.addPass(InternalizePass(mustPreserveGV));
         }
         PM.addPass(AMDGPUPropagateAttributesLatePass(*this));
         if (InternalizeSymbols) {
           PM.addPass(GlobalDCEPass());
         }
         if (EarlyInlineAll && !EnableFunctionCalls)
           PM.addPass(AMDGPUAlwaysInlinePass());