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/LLVMContext.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/Analysis/CallGraph.h"
26 #include "llvm/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Support/CFG.h"
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) {}
42 // runOnSCC - Analyze the SCC, performing the transformation if possible.
43 bool runOnSCC(CallGraphSCC &SCC);
45 bool SimplifyFunction(Function *F);
46 void DeleteBasicBlock(BasicBlock *BB);
51 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
52 "Remove unused exception handling info", false, false)
53 INITIALIZE_AG_DEPENDENCY(CallGraph)
54 INITIALIZE_PASS_END(PruneEH, "prune-eh",
55 "Remove unused exception handling info", false, false)
57 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
60 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
61 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
62 CallGraph &CG = getAnalysis<CallGraph>();
63 bool MadeChange = false;
65 // Fill SCCNodes with the elements of the SCC. Used for quickly
66 // looking up whether a given CallGraphNode is in this SCC.
67 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
70 // First pass, scan all of the functions in the SCC, simplifying them
71 // according to what we know.
72 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
73 if (Function *F = (*I)->getFunction())
74 MadeChange |= SimplifyFunction(F);
76 // Next, check to see if any callees might throw or if there are any external
77 // functions in this SCC: if so, we cannot prune any functions in this SCC.
78 // Definitions that are weak and not declared non-throwing might be
79 // overridden at linktime with something that throws, so assume that.
80 // If this SCC includes the unwind instruction, we KNOW it throws, so
81 // obviously the SCC might throw.
83 bool SCCMightUnwind = false, SCCMightReturn = false;
84 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
85 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
86 Function *F = (*I)->getFunction();
88 SCCMightUnwind = true;
89 SCCMightReturn = true;
90 } else if (F->isDeclaration() || F->mayBeOverridden()) {
91 SCCMightUnwind |= !F->doesNotThrow();
92 SCCMightReturn |= !F->doesNotReturn();
94 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
95 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
97 if (!CheckUnwind && !CheckReturn)
100 // Check to see if this function performs an unwind or calls an
101 // unwinding function.
102 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
103 if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
105 SCCMightUnwind = true;
106 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
107 SCCMightReturn = true;
110 // Invoke instructions don't allow unwinding to continue, so we are
111 // only interested in call instructions.
112 if (CheckUnwind && !SCCMightUnwind)
113 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
114 if (CallInst *CI = dyn_cast<CallInst>(I)) {
115 if (CI->doesNotThrow()) {
116 // This call cannot throw.
117 } else if (Function *Callee = CI->getCalledFunction()) {
118 CallGraphNode *CalleeNode = CG[Callee];
119 // If the callee is outside our current SCC then we may
120 // throw because it might.
121 if (!SCCNodes.count(CalleeNode)) {
122 SCCMightUnwind = true;
126 // Indirect call, it might throw.
127 SCCMightUnwind = true;
131 if (SCCMightUnwind && SCCMightReturn) break;
136 // If the SCC doesn't unwind or doesn't throw, note this fact.
137 if (!SCCMightUnwind || !SCCMightReturn)
138 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
139 Attributes NewAttributes = Attribute::None;
142 NewAttributes |= Attribute::NoUnwind;
144 NewAttributes |= Attribute::NoReturn;
146 Function *F = (*I)->getFunction();
147 const AttrListPtr &PAL = F->getAttributes();
148 const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
151 F->setAttributes(NPAL);
155 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
156 // Convert any invoke instructions to non-throwing functions in this node
157 // into call instructions with a branch. This makes the exception blocks
159 if (Function *F = (*I)->getFunction())
160 MadeChange |= SimplifyFunction(F);
167 // SimplifyFunction - Given information about callees, simplify the specified
168 // function if we have invokes to non-unwinding functions or code after calls to
169 // no-return functions.
170 bool PruneEH::SimplifyFunction(Function *F) {
171 bool MadeChange = false;
172 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
173 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
174 if (II->doesNotThrow()) {
175 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
176 // Insert a call instruction before the invoke.
177 CallInst *Call = CallInst::Create(II->getCalledValue(),
178 Args.begin(), Args.end(), "", II);
180 Call->setCallingConv(II->getCallingConv());
181 Call->setAttributes(II->getAttributes());
183 // Anything that used the value produced by the invoke instruction
184 // now uses the value produced by the call instruction. Note that we
185 // do this even for void functions and calls with no uses so that the
186 // callgraph edge is updated.
187 II->replaceAllUsesWith(Call);
188 BasicBlock *UnwindBlock = II->getUnwindDest();
189 UnwindBlock->removePredecessor(II->getParent());
191 // Insert a branch to the normal destination right before the
193 BranchInst::Create(II->getNormalDest(), II);
195 // Finally, delete the invoke instruction!
196 BB->getInstList().pop_back();
198 // If the unwind block is now dead, nuke it.
199 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
200 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
206 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
207 if (CallInst *CI = dyn_cast<CallInst>(I++))
208 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
209 // This call calls a function that cannot return. Insert an
210 // unreachable instruction after it and simplify the code. Do this
211 // by splitting the BB, adding the unreachable, then deleting the
213 BasicBlock *New = BB->splitBasicBlock(I);
215 // Remove the uncond branch and add an unreachable.
216 BB->getInstList().pop_back();
217 new UnreachableInst(BB->getContext(), BB);
219 DeleteBasicBlock(New); // Delete the new BB.
229 /// DeleteBasicBlock - remove the specified basic block from the program,
230 /// updating the callgraph to reflect any now-obsolete edges due to calls that
232 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
233 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
234 CallGraph &CG = getAnalysis<CallGraph>();
236 CallGraphNode *CGN = CG[BB->getParent()];
237 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
239 if (CallInst *CI = dyn_cast<CallInst>(I)) {
240 if (!isa<DbgInfoIntrinsic>(I))
241 CGN->removeCallEdgeFor(CI);
242 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
243 CGN->removeCallEdgeFor(II);
245 I->replaceAllUsesWith(UndefValue::get(I->getType()));
248 // Get the list of successors of this block.
249 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
251 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
252 Succs[i]->removePredecessor(BB);
254 BB->eraseFromParent();