[LegacyPM] Restore the GlobalDCE Legacy pass and add it to the DirectX backend

Author: farzonl

llvm/include/llvm/InitializePasses.h

@@ -127,6 +127,7 @@ LLVM_ABI void initializeGCEmptyBasicBlocksPass(PassRegistry &);
 LLVM_ABI void initializeGCMachineCodeAnalysisPass(PassRegistry &);
 LLVM_ABI void initializeGCModuleInfoPass(PassRegistry &);
 LLVM_ABI void initializeGVNLegacyPassPass(PassRegistry &);
+LLVM_ABI void initializeGlobalDCELegacyPassPass(PassRegistry &);
 LLVM_ABI void initializeGlobalMergeFuncPassWrapperPass(PassRegistry &);
 LLVM_ABI void initializeGlobalMergePass(PassRegistry &);
 LLVM_ABI void initializeGlobalsAAWrapperPassPass(PassRegistry &);

llvm/include/llvm/LinkAllPasses.h

@@ -38,6 +38,7 @@
 #include "llvm/Support/Valgrind.h"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/IPO/AlwaysInliner.h"
+#include "llvm/Transforms/IPO/GlobalDCE.h"
 #include "llvm/Transforms/InstCombine/InstCombine.h"
 #include "llvm/Transforms/ObjCARC.h"
 #include "llvm/Transforms/Scalar.h"
@@ -83,6 +84,7 @@ struct ForcePassLinking {
     (void)llvm::createDomOnlyViewerWrapperPassPass();
     (void)llvm::createDomViewerWrapperPassPass();
     (void)llvm::createAlwaysInlinerLegacyPass();
+    (void)llvm::createGlobalDCEPass();
     (void)llvm::createGlobalMergeFuncPass();
     (void)llvm::createGlobalsAAWrapperPass();
     (void)llvm::createInstSimplifyLegacyPass();

llvm/include/llvm/Transforms/IPO/GlobalDCE.h

@@ -32,6 +32,7 @@ class GlobalVariable;
 class Metadata;
 class Module;
 class Value;
+class ModulePass;
 
 /// Pass to remove unused function declarations.
 class GlobalDCEPass : public PassInfoMixin<GlobalDCEPass> {
@@ -80,6 +81,7 @@ class GlobalDCEPass : public PassInfoMixin<GlobalDCEPass> {
   void ComputeDependencies(Value *V, SmallPtrSetImpl<GlobalValue *> &U);
 };
 
+ModulePass *createGlobalDCEPass();
 }
 
 #endif // LLVM_TRANSFORMS_IPO_GLOBALDCE_H

llvm/lib/Target/DirectX/CMakeLists.txt

@@ -49,6 +49,7 @@ add_llvm_target(DirectXCodeGen
   DirectXInfo
   DirectXPointerTypeAnalysis
   FrontendHLSL
+  IPO
   MC
   ScalarOpts
   SelectionDAG

llvm/lib/Target/DirectX/DirectXTargetMachine.cpp

@@ -45,6 +45,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
+#include "llvm/Transforms/IPO/GlobalDCE.h"
 #include "llvm/Transforms/Scalar/Scalarizer.h"
 #include <optional>
 
@@ -62,6 +63,7 @@ extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeDirectXTarget() {
   initializeEmbedDXILPassPass(*PR);
   initializeWriteDXILPassPass(*PR);
   initializeDXContainerGlobalsPass(*PR);
+  initializeGlobalDCELegacyPassPass(*PR);
   initializeDXILOpLoweringLegacyPass(*PR);
   initializeDXILResourceAccessLegacyPass(*PR);
   initializeDXILResourceImplicitBindingLegacyPass(*PR);
@@ -103,6 +105,7 @@ class DirectXPassConfig : public TargetPassConfig {
   FunctionPass *createTargetRegisterAllocator(bool) override { return nullptr; }
   void addCodeGenPrepare() override {
     addPass(createDXILFinalizeLinkageLegacyPass());
+    addPass(createGlobalDCEPass());
     addPass(createDXILResourceAccessLegacyPass());
     addPass(createDXILIntrinsicExpansionLegacyPass());
     addPass(createDXILCBufferAccessLegacyPass());

llvm/lib/Transforms/IPO/GlobalDCE.cpp

@@ -21,6 +21,8 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/Utils/CtorUtils.h"
@@ -30,6 +32,36 @@ using namespace llvm;
 
 #define DEBUG_TYPE "globaldce"
 
+namespace {
+class GlobalDCELegacyPass : public ModulePass {
+public:
+  static char ID; // Pass identification, replacement for typeid
+  GlobalDCELegacyPass() : ModulePass(ID) {
+    initializeGlobalDCELegacyPassPass(*PassRegistry::getPassRegistry());
+  }
+  bool runOnModule(Module &M) override {
+    if (skipModule(M))
+      return false;
+    // Note: GlobalDCEPass does not use MAM. That
+    // means we can get away with init and pass
+    // as arg.
+    ModuleAnalysisManager MAM;
+    auto PA = Impl.run(M, MAM);
+    return !PA.areAllPreserved();
+  }
+
+private:
+  GlobalDCEPass Impl;
+};
+} // namespace
+
+char GlobalDCELegacyPass::ID = 0;
+INITIALIZE_PASS(GlobalDCELegacyPass, "globaldce", "Dead Global Elimination",
+                false, false)
+
+// Public interface to the GlobalDCEPass.
+ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCELegacyPass(); }
+
 static cl::opt<bool>
     ClEnableVFE("enable-vfe", cl::Hidden, cl::init(true),
                 cl::desc("Enable virtual function elimination"));

llvm/test/CodeGen/DirectX/llc-pipeline.ll

@@ -14,6 +14,7 @@
 
 ; CHECK-NEXT: ModulePass Manager
 ; CHECK-NEXT:   DXIL Finalize Linkage
+; CHECK-NEXT:   Dead Global Elimination
 ; CHECK-NEXT:   FunctionPass Manager
 ; CHECK-NEXT:     DXIL Resource Access
 ; CHECK-NEXT:   DXIL Intrinsic Expansion