1 //===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a simple interprocedural pass which walks the
11 // call-graph, turning invoke instructions into calls, iff the callee cannot
12 // throw an exception, and marking functions 'nounwind' if they cannot throw.
13 // It implements this as a bottom-up traversal of the call-graph.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/IPO.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Support/raw_ostream.h"
22 #include "llvm/Analysis/CallGraph.h"
23 #include "llvm/Analysis/CallGraphSCCPass.h"
24 #include "llvm/Analysis/EHPersonalities.h"
25 #include "llvm/IR/CFG.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/IR/InlineAsm.h"
29 #include "llvm/IR/Instructions.h"
30 #include "llvm/IR/IntrinsicInst.h"
31 #include "llvm/IR/LLVMContext.h"
32 #include "llvm/Transforms/Utils/Local.h"
36 #define DEBUG_TYPE "prune-eh"
38 STATISTIC(NumRemoved, "Number of invokes removed");
39 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
42 struct PruneEH : public CallGraphSCCPass {
43 static char ID; // Pass identification, replacement for typeid
44 PruneEH() : CallGraphSCCPass(ID) {
45 initializePruneEHPass(*PassRegistry::getPassRegistry());
48 // runOnSCC - Analyze the SCC, performing the transformation if possible.
49 bool runOnSCC(CallGraphSCC &SCC) override;
51 bool SimplifyFunction(Function *F);
52 void DeleteBasicBlock(BasicBlock *BB);
57 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
58 "Remove unused exception handling info", false, false)
59 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
60 INITIALIZE_PASS_END(PruneEH, "prune-eh",
61 "Remove unused exception handling info", false, false)
63 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
66 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
67 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
68 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
69 bool MadeChange = false;
71 // Fill SCCNodes with the elements of the SCC. Used for quickly
72 // looking up whether a given CallGraphNode is in this SCC.
73 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
76 // First pass, scan all of the functions in the SCC, simplifying them
77 // according to what we know.
78 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
79 if (Function *F = (*I)->getFunction())
80 MadeChange |= SimplifyFunction(F);
82 // Next, check to see if any callees might throw or if there are any external
83 // functions in this SCC: if so, we cannot prune any functions in this SCC.
84 // Definitions that are weak and not declared non-throwing might be
85 // overridden at linktime with something that throws, so assume that.
86 // If this SCC includes the unwind instruction, we KNOW it throws, so
87 // obviously the SCC might throw.
89 bool SCCMightUnwind = false, SCCMightReturn = false;
90 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
91 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
92 Function *F = (*I)->getFunction();
94 SCCMightUnwind = true;
95 SCCMightReturn = true;
96 } else if (F->isDeclaration() || F->mayBeOverridden()) {
97 SCCMightUnwind |= !F->doesNotThrow();
98 SCCMightReturn |= !F->doesNotReturn();
100 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
101 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
102 // Determine if we should scan for InlineAsm in a naked function as it
103 // is the only way to return without a ReturnInst. Only do this for
104 // no-inline functions as functions which may be inlined cannot
105 // meaningfully return via assembly.
106 bool CheckReturnViaAsm = CheckReturn &&
107 F->hasFnAttribute(Attribute::Naked) &&
108 F->hasFnAttribute(Attribute::NoInline);
110 if (!CheckUnwind && !CheckReturn)
113 for (const BasicBlock &BB : *F) {
114 const TerminatorInst *TI = BB.getTerminator();
115 if (CheckUnwind && TI->mayThrow()) {
116 SCCMightUnwind = true;
117 } else if (CheckReturn && isa<ReturnInst>(TI)) {
118 SCCMightReturn = true;
121 for (const Instruction &I : BB) {
122 if ((!CheckUnwind || SCCMightUnwind) &&
123 (!CheckReturnViaAsm || SCCMightReturn))
126 // Check to see if this function performs an unwind or calls an
127 // unwinding function.
128 if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
129 bool InstMightUnwind = true;
130 if (const auto *CI = dyn_cast<CallInst>(&I)) {
131 if (Function *Callee = CI->getCalledFunction()) {
132 CallGraphNode *CalleeNode = CG[Callee];
133 // If the callee is outside our current SCC then we may throw
134 // because it might. If it is inside, do nothing.
135 if (SCCNodes.count(CalleeNode) > 0)
136 InstMightUnwind = false;
139 SCCMightUnwind |= InstMightUnwind;
141 if (CheckReturnViaAsm && !SCCMightReturn)
142 if (auto ICS = ImmutableCallSite(&I))
143 if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
144 if (IA->hasSideEffects())
145 SCCMightReturn = true;
148 if (SCCMightUnwind && SCCMightReturn)
154 // If the SCC doesn't unwind or doesn't throw, note this fact.
155 if (!SCCMightUnwind || !SCCMightReturn)
156 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
157 Function *F = (*I)->getFunction();
159 if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
160 F->addFnAttr(Attribute::NoUnwind);
164 if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
165 F->addFnAttr(Attribute::NoReturn);
170 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
171 // Convert any invoke instructions to non-throwing functions in this node
172 // into call instructions with a branch. This makes the exception blocks
174 if (Function *F = (*I)->getFunction())
175 MadeChange |= SimplifyFunction(F);
182 // SimplifyFunction - Given information about callees, simplify the specified
183 // function if we have invokes to non-unwinding functions or code after calls to
184 // no-return functions.
185 bool PruneEH::SimplifyFunction(Function *F) {
186 bool MadeChange = false;
187 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
188 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
189 if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
190 BasicBlock *UnwindBlock = II->getUnwindDest();
191 removeUnwindEdge(&*BB);
193 // If the unwind block is now dead, nuke it.
194 if (pred_empty(UnwindBlock))
195 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
201 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
202 if (CallInst *CI = dyn_cast<CallInst>(I++))
203 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
204 // This call calls a function that cannot return. Insert an
205 // unreachable instruction after it and simplify the code. Do this
206 // by splitting the BB, adding the unreachable, then deleting the
208 BasicBlock *New = BB->splitBasicBlock(I);
210 // Remove the uncond branch and add an unreachable.
211 BB->getInstList().pop_back();
212 new UnreachableInst(BB->getContext(), &*BB);
214 DeleteBasicBlock(New); // Delete the new BB.
224 /// DeleteBasicBlock - remove the specified basic block from the program,
225 /// updating the callgraph to reflect any now-obsolete edges due to calls that
227 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
228 assert(pred_empty(BB) && "BB is not dead!");
229 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
231 Instruction *TokenInst = nullptr;
233 CallGraphNode *CGN = CG[BB->getParent()];
234 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
237 if (I->getType()->isTokenTy()) {
242 if (auto CS = CallSite (&*I)) {
243 const Function *Callee = CS.getCalledFunction();
244 if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID()))
245 CGN->removeCallEdgeFor(CS);
246 else if (!Callee->isIntrinsic())
247 CGN->removeCallEdgeFor(CS);
251 I->replaceAllUsesWith(UndefValue::get(I->getType()));
255 if (!isa<TerminatorInst>(TokenInst))
256 changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false);
258 // Get the list of successors of this block.
259 std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
261 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
262 Succs[i]->removePredecessor(BB);
264 BB->eraseFromParent();