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/SmallVector.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/Support/CFG.h"
27 #include "llvm/Support/Compiler.h"
28 #include "llvm/ParamAttrsList.h"
33 STATISTIC(NumRemoved, "Number of invokes removed");
34 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
35 STATISTIC(NumBBUnwind, "Number of unwind_to removed from blocks");
38 struct VISIBILITY_HIDDEN PruneEH : public CallGraphSCCPass {
39 static char ID; // Pass identification, replacement for typeid
40 PruneEH() : CallGraphSCCPass((intptr_t)&ID) {}
42 // runOnSCC - Analyze the SCC, performing the transformation if possible.
43 bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
45 bool SimplifyFunction(Function *F);
46 void DeleteBasicBlock(BasicBlock *BB);
50 RegisterPass<PruneEH> 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 CallGraph &CG = getAnalysis<CallGraph>();
58 bool MadeChange = false;
60 // First pass, scan all of the functions in the SCC, simplifying them
61 // according to what we know.
62 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
63 if (Function *F = SCC[i]->getFunction())
64 MadeChange |= SimplifyFunction(F);
66 // Next, check to see if any callees might throw or if there are any external
67 // functions in this SCC: if so, we cannot prune any functions in this SCC.
68 // If this SCC includes the unwind instruction, we KNOW it throws, so
69 // obviously the SCC might throw.
71 bool SCCMightUnwind = false, SCCMightReturn = false;
72 for (unsigned i = 0, e = SCC.size();
73 (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
74 Function *F = SCC[i]->getFunction();
76 SCCMightUnwind = true;
77 SCCMightReturn = true;
78 } else if (F->isDeclaration()) {
79 SCCMightUnwind |= !F->doesNotThrow();
80 SCCMightReturn |= !F->doesNotReturn();
82 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
83 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
85 if (!CheckUnwind && !CheckReturn)
88 // Check to see if this function performs an unwind or calls an
89 // unwinding function.
90 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
91 if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
93 SCCMightUnwind = true;
94 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
95 SCCMightReturn = true;
98 // Invoke instructions don't allow unwinding to continue, so we are
99 // only interested in call instructions.
100 if (CheckUnwind && !SCCMightUnwind)
101 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
102 if (CallInst *CI = dyn_cast<CallInst>(I)) {
103 if (CI->doesNotThrow()) {
104 // This call cannot throw.
105 } else if (Function *Callee = CI->getCalledFunction()) {
106 CallGraphNode *CalleeNode = CG[Callee];
107 // If the callee is outside our current SCC then we may
108 // throw because it might.
109 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()){
110 SCCMightUnwind = true;
114 // Indirect call, it might throw.
115 SCCMightUnwind = true;
119 if (SCCMightUnwind && SCCMightReturn) break;
124 // If the SCC doesn't unwind or doesn't throw, note this fact.
125 if (!SCCMightUnwind || !SCCMightReturn)
126 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
127 ParameterAttributes NewAttributes = ParamAttr::None;
130 NewAttributes |= ParamAttr::NoUnwind;
132 NewAttributes |= ParamAttr::NoReturn;
134 const ParamAttrsList *PAL = SCC[i]->getFunction()->getParamAttrs();
135 PAL = ParamAttrsList::includeAttrs(PAL, 0, NewAttributes);
136 SCC[i]->getFunction()->setParamAttrs(PAL);
139 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
140 // Convert any invoke instructions to non-throwing functions in this node
141 // into call instructions with a branch. This makes the exception blocks
143 if (Function *F = SCC[i]->getFunction())
144 MadeChange |= SimplifyFunction(F);
151 // SimplifyFunction - Given information about callees, simplify the specified
152 // function if we have invokes to non-unwinding functions or code after calls to
153 // no-return functions.
154 bool PruneEH::SimplifyFunction(Function *F) {
155 bool MadeChange = false;
156 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
157 bool couldUnwind = false;
159 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
160 if (II->doesNotThrow()) {
161 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
162 // Insert a call instruction before the invoke.
163 CallInst *Call = new CallInst(II->getCalledValue(),
164 Args.begin(), Args.end(), "", II);
166 Call->setCallingConv(II->getCallingConv());
167 Call->setParamAttrs(II->getParamAttrs());
169 // Anything that used the value produced by the invoke instruction
170 // now uses the value produced by the call instruction.
171 II->replaceAllUsesWith(Call);
172 BasicBlock *UnwindBlock = II->getUnwindDest();
173 UnwindBlock->removePredecessor(II->getParent());
175 // Insert a branch to the normal destination right before the
177 new BranchInst(II->getNormalDest(), II);
179 // Finally, delete the invoke instruction!
180 BB->getInstList().pop_back();
182 // If the unwind block is now dead, nuke it.
183 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
184 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
191 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
192 if (CallInst *CI = dyn_cast<CallInst>(I++)) {
193 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
194 // This call calls a function that cannot return. Insert an
195 // unreachable instruction after it and simplify the code. Do this
196 // by splitting the BB, adding the unreachable, then deleting the
198 BasicBlock *New = BB->splitBasicBlock(I);
200 // Remove the uncond branch and add an unreachable.
201 BB->getInstList().pop_back();
202 new UnreachableInst(BB);
204 DeleteBasicBlock(New); // Delete the new BB.
208 } else if (!CI->doesNotThrow())
212 // Strip 'unwindTo' off of BBs that have no calls/invokes without nounwind.
213 if (!couldUnwind && BB->getUnwindDest()) {
216 BB->getUnwindDest()->removePredecessor(BB, false, true);
217 BB->setUnwindDest(NULL);
223 /// DeleteBasicBlock - remove the specified basic block from the program,
224 /// updating the callgraph to reflect any now-obsolete edges due to calls that
226 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
227 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
228 CallGraph &CG = getAnalysis<CallGraph>();
230 CallGraphNode *CGN = CG[BB->getParent()];
231 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
233 if (CallInst *CI = dyn_cast<CallInst>(I)) {
234 if (Function *Callee = CI->getCalledFunction())
235 CGN->removeCallEdgeTo(CG[Callee]);
236 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
237 if (Function *Callee = II->getCalledFunction())
238 CGN->removeCallEdgeTo(CG[Callee]);
241 I->replaceAllUsesWith(UndefValue::get(I->getType()));
244 // Get the list of successors of this block.
245 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
247 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
248 Succs[i]->removePredecessor(BB);
250 BB->eraseFromParent();