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 #define DEBUG_TYPE "prune-eh"
18 #include "llvm/Transforms/IPO.h"
19 #include "llvm/CallGraphSCCPass.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Function.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/Analysis/CallGraph.h"
24 #include "llvm/ADT/SmallPtrSet.h"
25 #include "llvm/ADT/SmallVector.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/Support/CFG.h"
28 #include "llvm/Support/Compiler.h"
33 STATISTIC(NumRemoved, "Number of invokes removed");
34 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
37 struct VISIBILITY_HIDDEN PruneEH : public CallGraphSCCPass {
38 static char ID; // Pass identification, replacement for typeid
39 PruneEH() : CallGraphSCCPass(&ID) {}
41 // runOnSCC - Analyze the SCC, performing the transformation if possible.
42 bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
44 bool SimplifyFunction(Function *F);
45 void DeleteBasicBlock(BasicBlock *BB);
50 static RegisterPass<PruneEH>
51 X("prune-eh", "Remove unused exception handling info");
53 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
56 bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
57 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
58 CallGraph &CG = getAnalysis<CallGraph>();
59 bool MadeChange = false;
61 // Fill SCCNodes with the elements of the SCC. Used for quickly
62 // looking up whether a given CallGraphNode is in this SCC.
63 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
64 SCCNodes.insert(SCC[i]);
66 // First pass, scan all of the functions in the SCC, simplifying them
67 // according to what we know.
68 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
69 if (Function *F = SCC[i]->getFunction())
70 MadeChange |= SimplifyFunction(F);
72 // Next, check to see if any callees might throw or if there are any external
73 // functions in this SCC: if so, we cannot prune any functions in this SCC.
74 // Definitions that are weak and not declared non-throwing might be
75 // overridden at linktime with something that throws, so assume that.
76 // If this SCC includes the unwind instruction, we KNOW it throws, so
77 // obviously the SCC might throw.
79 bool SCCMightUnwind = false, SCCMightReturn = false;
80 for (unsigned i = 0, e = SCC.size();
81 (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
82 Function *F = SCC[i]->getFunction();
84 SCCMightUnwind = true;
85 SCCMightReturn = true;
86 } else if (F->isDeclaration() || F->mayBeOverridden()) {
87 SCCMightUnwind |= !F->doesNotThrow();
88 SCCMightReturn |= !F->doesNotReturn();
90 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
91 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
93 if (!CheckUnwind && !CheckReturn)
96 // Check to see if this function performs an unwind or calls an
97 // unwinding function.
98 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
99 if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
101 SCCMightUnwind = true;
102 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
103 SCCMightReturn = true;
106 // Invoke instructions don't allow unwinding to continue, so we are
107 // only interested in call instructions.
108 if (CheckUnwind && !SCCMightUnwind)
109 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
110 if (CallInst *CI = dyn_cast<CallInst>(I)) {
111 if (CI->doesNotThrow()) {
112 // This call cannot throw.
113 } else if (Function *Callee = CI->getCalledFunction()) {
114 CallGraphNode *CalleeNode = CG[Callee];
115 // If the callee is outside our current SCC then we may
116 // throw because it might.
117 if (!SCCNodes.count(CalleeNode)) {
118 SCCMightUnwind = true;
122 // Indirect call, it might throw.
123 SCCMightUnwind = true;
127 if (SCCMightUnwind && SCCMightReturn) break;
132 // If the SCC doesn't unwind or doesn't throw, note this fact.
133 if (!SCCMightUnwind || !SCCMightReturn)
134 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
135 Attributes NewAttributes = Attribute::None;
138 NewAttributes |= Attribute::NoUnwind;
140 NewAttributes |= Attribute::NoReturn;
142 const AttrListPtr &PAL = SCC[i]->getFunction()->getAttributes();
143 const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
146 SCC[i]->getFunction()->setAttributes(NPAL);
150 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
151 // Convert any invoke instructions to non-throwing functions in this node
152 // into call instructions with a branch. This makes the exception blocks
154 if (Function *F = SCC[i]->getFunction())
155 MadeChange |= SimplifyFunction(F);
162 // SimplifyFunction - Given information about callees, simplify the specified
163 // function if we have invokes to non-unwinding functions or code after calls to
164 // no-return functions.
165 bool PruneEH::SimplifyFunction(Function *F) {
166 CallGraph &CG = getAnalysis<CallGraph>();
167 CallGraphNode *CGN = CG[F];
169 bool MadeChange = false;
170 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
171 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
172 if (II->doesNotThrow()) {
173 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
174 // Insert a call instruction before the invoke.
175 CallInst *Call = CallInst::Create(II->getCalledValue(),
176 Args.begin(), Args.end(), "", II);
178 Call->setCallingConv(II->getCallingConv());
179 Call->setAttributes(II->getAttributes());
181 // Anything that used the value produced by the invoke instruction
182 // now uses the value produced by the call instruction.
183 II->replaceAllUsesWith(Call);
184 BasicBlock *UnwindBlock = II->getUnwindDest();
185 UnwindBlock->removePredecessor(II->getParent());
187 // Fix up the call graph.
188 CGN->replaceCallSite(II, Call);
190 // Insert a branch to the normal destination right before the
192 BranchInst::Create(II->getNormalDest(), II);
194 // Finally, delete the invoke instruction!
195 BB->getInstList().pop_back();
197 // If the unwind block is now dead, nuke it.
198 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
199 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
205 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
206 if (CallInst *CI = dyn_cast<CallInst>(I++))
207 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
208 // This call calls a function that cannot return. Insert an
209 // unreachable instruction after it and simplify the code. Do this
210 // by splitting the BB, adding the unreachable, then deleting the
212 BasicBlock *New = BB->splitBasicBlock(I);
214 // Remove the uncond branch and add an unreachable.
215 BB->getInstList().pop_back();
216 new UnreachableInst(BB);
218 DeleteBasicBlock(New); // Delete the new BB.
228 /// DeleteBasicBlock - remove the specified basic block from the program,
229 /// updating the callgraph to reflect any now-obsolete edges due to calls that
231 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
232 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
233 CallGraph &CG = getAnalysis<CallGraph>();
235 CallGraphNode *CGN = CG[BB->getParent()];
236 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
238 if (CallInst *CI = dyn_cast<CallInst>(I))
239 CGN->removeCallEdgeFor(CI);
240 else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
241 CGN->removeCallEdgeFor(II);
243 I->replaceAllUsesWith(UndefValue::get(I->getType()));
246 // Get the list of successors of this block.
247 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
249 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
250 Succs[i]->removePredecessor(BB);
252 BB->eraseFromParent();