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-rw-r--r--llvm/include/llvm/Transforms/IPO/GlobalDCE.h21
-rw-r--r--llvm/lib/Transforms/IPO/GlobalDCE.cpp143
2 files changed, 99 insertions, 65 deletions
diff --git a/llvm/include/llvm/Transforms/IPO/GlobalDCE.h b/llvm/include/llvm/Transforms/IPO/GlobalDCE.h
index 57e174c2a37..9ca939c15b6 100644
--- a/llvm/include/llvm/Transforms/IPO/GlobalDCE.h
+++ b/llvm/include/llvm/Transforms/IPO/GlobalDCE.h
@@ -18,6 +18,8 @@
#ifndef LLVM_TRANSFORMS_IPO_GLOBALDCE_H
#define LLVM_TRANSFORMS_IPO_GLOBALDCE_H
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallSet.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include <unordered_map>
@@ -31,14 +33,23 @@ public:
private:
SmallPtrSet<GlobalValue*, 32> AliveGlobals;
- SmallPtrSet<Constant *, 8> SeenConstants;
+
+ /// Global -> Global that uses this global.
+ std::unordered_multimap<GlobalValue *, GlobalValue *> GVDependencies;
+
+ /// Constant -> Globals that use this global cache.
+ std::unordered_map<Constant *, SmallPtrSet<GlobalValue *, 8>>
+ ConstantDependenciesCache;
+
+ /// Comdat -> Globals in that Comdat section.
std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
- /// Mark the specific global value as needed, and
- /// recursively mark anything that it uses as also needed.
- void GlobalIsNeeded(GlobalValue *GV);
- void MarkUsedGlobalsAsNeeded(Constant *C);
+ void UpdateGVDependencies(GlobalValue &GV);
+ void MarkLive(GlobalValue &GV,
+ SmallVectorImpl<GlobalValue *> *Updates = nullptr);
bool RemoveUnusedGlobalValue(GlobalValue &GV);
+
+ void ComputeDependencies(Value *V, SmallPtrSetImpl<GlobalValue *> &U);
};
}
diff --git a/llvm/lib/Transforms/IPO/GlobalDCE.cpp b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
index 67e9d20eb92..c91e8b45492 100644
--- a/llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -25,7 +25,7 @@
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/CtorUtils.h"
#include "llvm/Transforms/Utils/GlobalStatus.h"
-#include <unordered_map>
+
using namespace llvm;
#define DEBUG_TYPE "globaldce"
@@ -85,9 +85,67 @@ static bool isEmptyFunction(Function *F) {
return RI.getReturnValue() == nullptr;
}
+/// Compute the set of GlobalValue that depends from V.
+/// The recursion stops as soon as a GlobalValue is met.
+void GlobalDCEPass::ComputeDependencies(Value *V,
+ SmallPtrSetImpl<GlobalValue *> &Deps) {
+ if (auto *I = dyn_cast<Instruction>(V)) {
+ Function *Parent = I->getParent()->getParent();
+ Deps.insert(Parent);
+ } else if (auto *GV = dyn_cast<GlobalValue>(V)) {
+ Deps.insert(GV);
+ } else if (auto *CE = dyn_cast<Constant>(V)) {
+ // Avoid walking the whole tree of a big ConstantExprs multiple times.
+ auto Where = ConstantDependenciesCache.find(CE);
+ if (Where != ConstantDependenciesCache.end()) {
+ auto const &K = Where->second;
+ Deps.insert(K.begin(), K.end());
+ } else {
+ SmallPtrSetImpl<GlobalValue *> &LocalDeps = ConstantDependenciesCache[CE];
+ for (User *CEUser : CE->users())
+ ComputeDependencies(CEUser, LocalDeps);
+ Deps.insert(LocalDeps.begin(), LocalDeps.end());
+ }
+ }
+}
+
+void GlobalDCEPass::UpdateGVDependencies(GlobalValue &GV) {
+ SmallPtrSet<GlobalValue *, 8> Deps;
+ for (User *User : GV.users())
+ ComputeDependencies(User, Deps);
+ Deps.erase(&GV); // Remove self-reference.
+ for (GlobalValue *GVU : Deps) {
+ GVDependencies.insert(std::make_pair(GVU, &GV));
+ }
+}
+
+/// Mark Global value as Live
+void GlobalDCEPass::MarkLive(GlobalValue &GV,
+ SmallVectorImpl<GlobalValue *> *Updates) {
+ auto const Ret = AliveGlobals.insert(&GV);
+ if (!Ret.second)
+ return;
+
+ if (Updates)
+ Updates->push_back(&GV);
+ if (Comdat *C = GV.getComdat()) {
+ for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
+ MarkLive(*CM.second, Updates); // Recursion depth is only two because only
+ // globals in the same comdat are visited.
+ }
+}
+
PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
bool Changed = false;
+ // The algorithm first computes the set L of global variables that are
+ // trivially live. Then it walks the initialization of these variables to
+ // compute the globals used to initialize them, which effectively builds a
+ // directed graph where nodes are global variables, and an edge from A to B
+ // means B is used to initialize A. Finally, it propagates the liveness
+ // information through the graph starting from the nodes in L. Nodes note
+ // marked as alive are discarded.
+
// Remove empty functions from the global ctors list.
Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
@@ -110,21 +168,39 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
// initializer.
if (!GO.isDeclaration() && !GO.hasAvailableExternallyLinkage())
if (!GO.isDiscardableIfUnused())
- GlobalIsNeeded(&GO);
+ MarkLive(GO);
+
+ UpdateGVDependencies(GO);
}
+ // Compute direct dependencies of aliases.
for (GlobalAlias &GA : M.aliases()) {
Changed |= RemoveUnusedGlobalValue(GA);
// Externally visible aliases are needed.
if (!GA.isDiscardableIfUnused())
- GlobalIsNeeded(&GA);
+ MarkLive(GA);
+
+ UpdateGVDependencies(GA);
}
+ // Compute direct dependencies of ifuncs.
for (GlobalIFunc &GIF : M.ifuncs()) {
Changed |= RemoveUnusedGlobalValue(GIF);
// Externally visible ifuncs are needed.
if (!GIF.isDiscardableIfUnused())
- GlobalIsNeeded(&GIF);
+ MarkLive(GIF);
+
+ UpdateGVDependencies(GIF);
+ }
+
+ // Propagate liveness from collected Global Values through the computed
+ // dependencies.
+ SmallVector<GlobalValue *, 8> NewLiveGVs{AliveGlobals.begin(),
+ AliveGlobals.end()};
+ while (!NewLiveGVs.empty()) {
+ GlobalValue *LGV = NewLiveGVs.pop_back_val();
+ for (auto &&GVD : make_range(GVDependencies.equal_range(LGV)))
+ MarkLive(*GVD.second, &NewLiveGVs);
}
// Now that all globals which are needed are in the AliveGlobals set, we loop
@@ -161,7 +237,7 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
GA.setAliasee(nullptr);
}
- // The third pass drops targets of ifuncs which are dead...
+ // The fourth pass drops targets of ifuncs which are dead...
std::vector<GlobalIFunc*> DeadIFuncs;
for (GlobalIFunc &GIF : M.ifuncs())
if (!AliveGlobals.count(&GIF)) {
@@ -195,7 +271,8 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
// Make sure that all memory is released
AliveGlobals.clear();
- SeenConstants.clear();
+ ConstantDependenciesCache.clear();
+ GVDependencies.clear();
ComdatMembers.clear();
if (Changed)
@@ -203,60 +280,6 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
return PreservedAnalyses::all();
}
-/// GlobalIsNeeded - the specific global value as needed, and
-/// recursively mark anything that it uses as also needed.
-void GlobalDCEPass::GlobalIsNeeded(GlobalValue *G) {
- // If the global is already in the set, no need to reprocess it.
- if (!AliveGlobals.insert(G).second)
- return;
-
- if (Comdat *C = G->getComdat()) {
- for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
- GlobalIsNeeded(CM.second);
- }
-
- if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
- // If this is a global variable, we must make sure to add any global values
- // referenced by the initializer to the alive set.
- if (GV->hasInitializer())
- MarkUsedGlobalsAsNeeded(GV->getInitializer());
- } else if (GlobalIndirectSymbol *GIS = dyn_cast<GlobalIndirectSymbol>(G)) {
- // The target of a global alias or ifunc is needed.
- MarkUsedGlobalsAsNeeded(GIS->getIndirectSymbol());
- } else {
- // Otherwise this must be a function object. We have to scan the body of
- // the function looking for constants and global values which are used as
- // operands. Any operands of these types must be processed to ensure that
- // any globals used will be marked as needed.
- Function *F = cast<Function>(G);
-
- for (Use &U : F->operands())
- MarkUsedGlobalsAsNeeded(cast<Constant>(U.get()));
-
- for (BasicBlock &BB : *F)
- for (Instruction &I : BB)
- for (Use &U : I.operands())
- if (GlobalValue *GV = dyn_cast<GlobalValue>(U))
- GlobalIsNeeded(GV);
- else if (Constant *C = dyn_cast<Constant>(U))
- MarkUsedGlobalsAsNeeded(C);
- }
-}
-
-void GlobalDCEPass::MarkUsedGlobalsAsNeeded(Constant *C) {
- if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
- return GlobalIsNeeded(GV);
-
- // Loop over all of the operands of the constant, adding any globals they
- // use to the list of needed globals.
- for (Use &U : C->operands()) {
- // If we've already processed this constant there's no need to do it again.
- Constant *Op = dyn_cast<Constant>(U);
- if (Op && SeenConstants.insert(Op).second)
- MarkUsedGlobalsAsNeeded(Op);
- }
-}
-
// RemoveUnusedGlobalValue - Loop over all of the uses of the specified
// GlobalValue, looking for the constant pointer ref that may be pointing to it.
// If found, check to see if the constant pointer ref is safe to destroy, and if
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