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[DirectX] Add GlobalDCE pass after finalize linkage pass in DirectX backend #151071

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[DirectX] Add GlobalDCE pass after finalize linkage pass in DirectX backend #151071

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bced13d
[LegacyPM] Restore the GlobalDCE Legacy pass and add it to the Direct…
farzonl Mar 28, 2025
3bca2c8
Make the dxil-finalize-linkage pass usable using the new pass manager…
farzonl Jul 28, 2025
08da645
convert external globals with no usage to internal linkage to be remo…
kmpeng Jul 16, 2025
d1b498e
update finalize_linkage test, update GlobalDCE comment
kmpeng Aug 7, 2025
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1 change: 1 addition & 0 deletions llvm/include/llvm/InitializePasses.h
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Expand Up @@ -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 &);
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2 changes: 2 additions & 0 deletions llvm/include/llvm/LinkAllPasses.h
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Expand Up @@ -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"
Expand Down Expand Up @@ -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();
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2 changes: 2 additions & 0 deletions llvm/include/llvm/Transforms/IPO/GlobalDCE.h
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Expand Up @@ -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> {
Expand Down Expand Up @@ -80,6 +81,7 @@ class GlobalDCEPass : public PassInfoMixin<GlobalDCEPass> {
void ComputeDependencies(Value *V, SmallPtrSetImpl<GlobalValue *> &U);
};

ModulePass *createGlobalDCEPass();
}

#endif // LLVM_TRANSFORMS_IPO_GLOBALDCE_H
1 change: 1 addition & 0 deletions llvm/lib/Target/DirectX/CMakeLists.txt
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Expand Up @@ -49,6 +49,7 @@ add_llvm_target(DirectXCodeGen
DirectXInfo
DirectXPointerTypeAnalysis
FrontendHLSL
IPO
MC
ScalarOpts
SelectionDAG
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8 changes: 4 additions & 4 deletions llvm/lib/Target/DirectX/DXILFinalizeLinkage.cpp
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Expand Up @@ -20,13 +20,13 @@ using namespace llvm;
static bool finalizeLinkage(Module &M) {
bool MadeChange = false;

// Convert private global variables to internal linkage.
for (GlobalVariable &GV : M.globals()) {
if (GV.hasPrivateLinkage()) {
// Convert private globals and external globals with no usage to internal
// linkage.
for (GlobalVariable &GV : M.globals())
if (GV.hasPrivateLinkage() || (GV.hasExternalLinkage() && GV.use_empty())) {
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For anyone else looking this is the crux of the change right here. GlobalDCE is conservative so it won't remove globals marked external. This is because Linker would handle these globals and usually something like the internalizer pass would run and do a similar conversion to what we are doing here.

In HLSL's case after finalize linkage we should be able to assume that it is safe to mark all external globals with no uses as internal. If we then run GlobalDCE after this these globals will get cleaned up.

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If we're just concerned with deleting unused external variables... we could do that as part of DXILFinalizeLinkage instead of running GlobalDCE in the backend.

Are there other cases that GlobalDCE catches that we need to catch late?

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Justin suggested we not do special casing of global_ctors and that GlobalDCE was a more complete solution. Hence why we closed the previous attempt at fixing this issue.

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We need to special case global constructors for library shaders. That's not optional.

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If we use GlobalDCE we don't need to special case, things just work by converting to internal.

GV.setLinkage(GlobalValue::InternalLinkage);
MadeChange = true;
}
}

SmallVector<Function *> Funcs;

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1 change: 1 addition & 0 deletions llvm/lib/Target/DirectX/DirectXPassRegistry.def
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Expand Up @@ -24,6 +24,7 @@ MODULE_ANALYSIS("dxil-root-signature-analysis", dxil::RootSignatureAnalysis())
#define MODULE_PASS(NAME, CREATE_PASS)
#endif
MODULE_PASS("dxil-cbuffer-access", DXILCBufferAccess())
MODULE_PASS("dxil-finalize-linkage", DXILFinalizeLinkage())
MODULE_PASS("dxil-data-scalarization", DXILDataScalarization())
MODULE_PASS("dxil-flatten-arrays", DXILFlattenArrays())
MODULE_PASS("dxil-intrinsic-expansion", DXILIntrinsicExpansion())
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4 changes: 4 additions & 0 deletions llvm/lib/Target/DirectX/DirectXTargetMachine.cpp
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Expand Up @@ -14,6 +14,7 @@
#include "DirectXTargetMachine.h"
#include "DXILCBufferAccess.h"
#include "DXILDataScalarization.h"
#include "DXILFinalizeLinkage.h"
#include "DXILFlattenArrays.h"
#include "DXILForwardHandleAccesses.h"
#include "DXILIntrinsicExpansion.h"
Expand Down Expand Up @@ -45,6 +46,7 @@
#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>

Expand All @@ -62,6 +64,7 @@ extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeDirectXTarget() {
initializeEmbedDXILPassPass(*PR);
initializeWriteDXILPassPass(*PR);
initializeDXContainerGlobalsPass(*PR);
initializeGlobalDCELegacyPassPass(*PR);
initializeDXILOpLoweringLegacyPass(*PR);
initializeDXILResourceAccessLegacyPass(*PR);
initializeDXILResourceImplicitBindingLegacyPass(*PR);
Expand Down Expand Up @@ -103,6 +106,7 @@ class DirectXPassConfig : public TargetPassConfig {
FunctionPass *createTargetRegisterAllocator(bool) override { return nullptr; }
void addCodeGenPrepare() override {
addPass(createDXILFinalizeLinkageLegacyPass());
addPass(createGlobalDCEPass());
addPass(createDXILResourceAccessLegacyPass());
addPass(createDXILIntrinsicExpansionLegacyPass());
addPass(createDXILCBufferAccessLegacyPass());
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31 changes: 31 additions & 0 deletions llvm/lib/Transforms/IPO/GlobalDCE.cpp
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Expand Up @@ -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"
Expand All @@ -30,6 +32,35 @@ 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 any analyses, so we're safe to call the
// new-pm style pass with a default-initialized analysis manager here
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"));
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24 changes: 23 additions & 1 deletion llvm/test/CodeGen/DirectX/finalize_linkage.ll
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Expand Up @@ -4,7 +4,8 @@
target triple = "dxilv1.5-pc-shadermodel6.5-compute"

; DXILFinalizeLinkage changes linkage of all functions that are hidden to
; internal, and converts private global variables to internal linkage.
; internal, converts private globals to internal linkage, and converts external globals
; with no usage to internal linkage.

; CHECK: @switch.table = internal unnamed_addr constant [4 x i32]
@switch.table = private unnamed_addr constant [4 x i32] [i32 1, i32 257, i32 65793, i32 16843009], align 4
Expand All @@ -27,6 +28,27 @@ target triple = "dxilv1.5-pc-shadermodel6.5-compute"
; CHECK: @hidden_var = hidden global i32
@hidden_var = hidden global i32 1, align 4

; Running the whole pipeline should remove unused global variables

; CHECK: @aTile = internal addrspace(3) global
; CHECK-LLC-NOT: @aTile
@aTile = hidden addrspace(3) global [4 x [1 x i32]] zeroinitializer, align 4

; CHECK: @bTile = internal addrspace(3) global
; CHECK-LLC-NOT: @bTile
@bTile = hidden addrspace(3) global [1 x [1 x i32]] zeroinitializer, align 4

define void @anchor_function() #0 {
entry:
%0 = load i32, ptr @switch.table, align 4
%1 = load [3 x float], ptr @private_array, align 4
%2 = load i32, ptr @private_var, align 4
%3 = load i32, ptr @internal_var, align 4
%4 = load i32, ptr @external_var, align 4
%5 = load i32, ptr @hidden_var, align 4
ret void
}

; CHECK-NOT: define internal void @"?f1@@YAXXZ"()
define void @"?f1@@YAXXZ"() #0 {
entry:
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1 change: 1 addition & 0 deletions llvm/test/CodeGen/DirectX/llc-pipeline.ll
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion llvm/test/CodeGen/DirectX/scalar-data.ll
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@@ -1,4 +1,4 @@
; RUN: llc %s -mtriple=dxil-pc-shadermodel6.3-library --filetype=asm -o - | FileCheck %s
; RUN: opt -S -passes='dxil-data-scalarization,dxil-flatten-arrays' -mtriple=dxil-unknown-shadermodel6.5-compute %s | FileCheck %s
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Would it be better to modify the linkage of the globals in this test to be external rather than changing how we test this?

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@farzonl suggested changing it to opt - do you have thoughts on this?

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Yeah switched to opt b\c globalDCE would eliminate globals that were being used in this test.

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I meant do you have thoughts on if it would be better to modify the linkages instead of changing to opt?

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This test case is intended to be very limited to the effects of the array flattening and vector scalarization passes. Even if we change the linkage to external it would still get cleaned up by Global DCE. Since we know which two passes are responsible for the data transformations I think its fine to just run those two passes.


; Make sure we don't touch arrays without vectors and that can recurse and flatten multiple-dimension arrays of vectors

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1 change: 1 addition & 0 deletions llvm/test/tools/dxil-dis/opaque-value_as_metadata.ll
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Expand Up @@ -7,6 +7,7 @@ target triple = "dxil-unknown-shadermodel6.7-library"
@CBV = external constant %"$Globals"

define void @main() #0 {
%1 = load float, ptr @CBV, align 4
ret void
}

Expand Down