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/Analysis/CallGraph.h"
22 #include "llvm/Analysis/CallGraphSCCPass.h"
23 #include "llvm/IR/CFG.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/Function.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/IntrinsicInst.h"
28 #include "llvm/IR/LLVMContext.h"
32 #define DEBUG_TYPE "prune-eh"
34 STATISTIC(NumRemoved, "Number of invokes removed");
35 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
38 struct PruneEH : public CallGraphSCCPass {
39 static char ID; // Pass identification, replacement for typeid
40 PruneEH() : CallGraphSCCPass(ID) {
41 initializePruneEHPass(*PassRegistry::getPassRegistry());
44 // runOnSCC - Analyze the SCC, performing the transformation if possible.
45 bool runOnSCC(CallGraphSCC &SCC) override;
47 bool SimplifyFunction(Function *F);
48 void DeleteBasicBlock(BasicBlock *BB);
53 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
54 "Remove unused exception handling info", false, false)
55 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
56 INITIALIZE_PASS_END(PruneEH, "prune-eh",
57 "Remove unused exception handling info", false, false)
59 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
62 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
63 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
64 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
65 bool MadeChange = false;
67 // Fill SCCNodes with the elements of the SCC. Used for quickly
68 // looking up whether a given CallGraphNode is in this SCC.
69 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
72 // First pass, scan all of the functions in the SCC, simplifying them
73 // according to what we know.
74 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
75 if (Function *F = (*I)->getFunction())
76 MadeChange |= SimplifyFunction(F);
78 // Next, check to see if any callees might throw or if there are any external
79 // functions in this SCC: if so, we cannot prune any functions in this SCC.
80 // Definitions that are weak and not declared non-throwing might be
81 // overridden at linktime with something that throws, so assume that.
82 // If this SCC includes the unwind instruction, we KNOW it throws, so
83 // obviously the SCC might throw.
85 bool SCCMightUnwind = false, SCCMightReturn = false;
86 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
87 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
88 Function *F = (*I)->getFunction();
90 SCCMightUnwind = true;
91 SCCMightReturn = true;
92 } else if (F->isDeclaration() || F->mayBeOverridden()) {
93 SCCMightUnwind |= !F->doesNotThrow();
94 SCCMightReturn |= !F->doesNotReturn();
96 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
97 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
99 if (!CheckUnwind && !CheckReturn)
102 // Check to see if this function performs an unwind or calls an
103 // unwinding function.
104 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
105 if (CheckUnwind && isa<ResumeInst>(BB->getTerminator())) {
106 // Uses unwind / resume!
107 SCCMightUnwind = true;
108 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
109 SCCMightReturn = true;
112 // Invoke instructions don't allow unwinding to continue, so we are
113 // only interested in call instructions.
114 if (CheckUnwind && !SCCMightUnwind)
115 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
116 if (CallInst *CI = dyn_cast<CallInst>(I)) {
117 if (CI->doesNotThrow()) {
118 // This call cannot throw.
119 } else if (Function *Callee = CI->getCalledFunction()) {
120 CallGraphNode *CalleeNode = CG[Callee];
121 // If the callee is outside our current SCC then we may
122 // throw because it might.
123 if (!SCCNodes.count(CalleeNode)) {
124 SCCMightUnwind = true;
128 // Indirect call, it might throw.
129 SCCMightUnwind = true;
133 if (SCCMightUnwind && SCCMightReturn) break;
138 // If the SCC doesn't unwind or doesn't throw, note this fact.
139 if (!SCCMightUnwind || !SCCMightReturn)
140 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
141 AttrBuilder NewAttributes;
144 NewAttributes.addAttribute(Attribute::NoUnwind);
146 NewAttributes.addAttribute(Attribute::NoReturn);
148 Function *F = (*I)->getFunction();
149 const AttributeSet &PAL = F->getAttributes().getFnAttributes();
150 const AttributeSet &NPAL = AttributeSet::get(
151 F->getContext(), AttributeSet::FunctionIndex, NewAttributes);
155 F->addAttributes(AttributeSet::FunctionIndex, NPAL);
159 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
160 // Convert any invoke instructions to non-throwing functions in this node
161 // into call instructions with a branch. This makes the exception blocks
163 if (Function *F = (*I)->getFunction())
164 MadeChange |= SimplifyFunction(F);
171 // SimplifyFunction - Given information about callees, simplify the specified
172 // function if we have invokes to non-unwinding functions or code after calls to
173 // no-return functions.
174 bool PruneEH::SimplifyFunction(Function *F) {
175 bool MadeChange = false;
176 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
177 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
178 if (II->doesNotThrow()) {
179 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
180 // Insert a call instruction before the invoke.
181 CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
183 Call->setCallingConv(II->getCallingConv());
184 Call->setAttributes(II->getAttributes());
185 Call->setDebugLoc(II->getDebugLoc());
187 // Anything that used the value produced by the invoke instruction
188 // now uses the value produced by the call instruction. Note that we
189 // do this even for void functions and calls with no uses so that the
190 // callgraph edge is updated.
191 II->replaceAllUsesWith(Call);
192 BasicBlock *UnwindBlock = II->getUnwindDest();
193 UnwindBlock->removePredecessor(II->getParent());
195 // Insert a branch to the normal destination right before the
197 BranchInst::Create(II->getNormalDest(), II);
199 // Finally, delete the invoke instruction!
200 BB->getInstList().pop_back();
202 // If the unwind block is now dead, nuke it.
203 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
204 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
210 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
211 if (CallInst *CI = dyn_cast<CallInst>(I++))
212 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
213 // This call calls a function that cannot return. Insert an
214 // unreachable instruction after it and simplify the code. Do this
215 // by splitting the BB, adding the unreachable, then deleting the
217 BasicBlock *New = BB->splitBasicBlock(I);
219 // Remove the uncond branch and add an unreachable.
220 BB->getInstList().pop_back();
221 new UnreachableInst(BB->getContext(), BB);
223 DeleteBasicBlock(New); // Delete the new BB.
233 /// DeleteBasicBlock - remove the specified basic block from the program,
234 /// updating the callgraph to reflect any now-obsolete edges due to calls that
236 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
237 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
238 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
240 CallGraphNode *CGN = CG[BB->getParent()];
241 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
243 if (CallInst *CI = dyn_cast<CallInst>(I)) {
244 if (!isa<IntrinsicInst>(I))
245 CGN->removeCallEdgeFor(CI);
246 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
247 CGN->removeCallEdgeFor(II);
249 I->replaceAllUsesWith(UndefValue::get(I->getType()));
252 // Get the list of successors of this block.
253 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
255 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
256 Succs[i]->removePredecessor(BB);
258 BB->eraseFromParent();