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 static RegisterPass<PruneEH>
52 X("prune-eh", "Remove unused exception handling info");
54 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
57 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
58 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
59 CallGraph &CG = getAnalysis<CallGraph>();
60 bool MadeChange = false;
62 // Fill SCCNodes with the elements of the SCC. Used for quickly
63 // looking up whether a given CallGraphNode is in this SCC.
64 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
67 // First pass, scan all of the functions in the SCC, simplifying them
68 // according to what we know.
69 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
70 if (Function *F = (*I)->getFunction())
71 MadeChange |= SimplifyFunction(F);
73 // Next, check to see if any callees might throw or if there are any external
74 // functions in this SCC: if so, we cannot prune any functions in this SCC.
75 // Definitions that are weak and not declared non-throwing might be
76 // overridden at linktime with something that throws, so assume that.
77 // If this SCC includes the unwind instruction, we KNOW it throws, so
78 // obviously the SCC might throw.
80 bool SCCMightUnwind = false, SCCMightReturn = false;
81 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
82 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
83 Function *F = (*I)->getFunction();
85 SCCMightUnwind = true;
86 SCCMightReturn = true;
87 } else if (F->isDeclaration() || F->mayBeOverridden()) {
88 SCCMightUnwind |= !F->doesNotThrow();
89 SCCMightReturn |= !F->doesNotReturn();
91 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
92 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
94 if (!CheckUnwind && !CheckReturn)
97 // Check to see if this function performs an unwind or calls an
98 // unwinding function.
99 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
100 if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
102 SCCMightUnwind = true;
103 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
104 SCCMightReturn = true;
107 // Invoke instructions don't allow unwinding to continue, so we are
108 // only interested in call instructions.
109 if (CheckUnwind && !SCCMightUnwind)
110 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
111 if (CallInst *CI = dyn_cast<CallInst>(I)) {
112 if (CI->doesNotThrow()) {
113 // This call cannot throw.
114 } else if (Function *Callee = CI->getCalledFunction()) {
115 CallGraphNode *CalleeNode = CG[Callee];
116 // If the callee is outside our current SCC then we may
117 // throw because it might.
118 if (!SCCNodes.count(CalleeNode)) {
119 SCCMightUnwind = true;
123 // Indirect call, it might throw.
124 SCCMightUnwind = true;
128 if (SCCMightUnwind && SCCMightReturn) break;
133 // If the SCC doesn't unwind or doesn't throw, note this fact.
134 if (!SCCMightUnwind || !SCCMightReturn)
135 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
136 Attributes NewAttributes = Attribute::None;
139 NewAttributes |= Attribute::NoUnwind;
141 NewAttributes |= Attribute::NoReturn;
143 Function *F = (*I)->getFunction();
144 const AttrListPtr &PAL = F->getAttributes();
145 const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
148 F->setAttributes(NPAL);
152 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
153 // Convert any invoke instructions to non-throwing functions in this node
154 // into call instructions with a branch. This makes the exception blocks
156 if (Function *F = (*I)->getFunction())
157 MadeChange |= SimplifyFunction(F);
164 // SimplifyFunction - Given information about callees, simplify the specified
165 // function if we have invokes to non-unwinding functions or code after calls to
166 // no-return functions.
167 bool PruneEH::SimplifyFunction(Function *F) {
168 bool MadeChange = false;
169 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
170 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
171 if (II->doesNotThrow()) {
172 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
173 // Insert a call instruction before the invoke.
174 CallInst *Call = CallInst::Create(II->getCalledValue(),
175 Args.begin(), Args.end(), "", II);
177 Call->setCallingConv(II->getCallingConv());
178 Call->setAttributes(II->getAttributes());
180 // Anything that used the value produced by the invoke instruction
181 // now uses the value produced by the call instruction. Note that we
182 // do this even for void functions and calls with no uses so that the
183 // callgraph edge is updated.
184 II->replaceAllUsesWith(Call);
185 BasicBlock *UnwindBlock = II->getUnwindDest();
186 UnwindBlock->removePredecessor(II->getParent());
188 // Insert a branch to the normal destination right before the
190 BranchInst::Create(II->getNormalDest(), II);
192 // Finally, delete the invoke instruction!
193 BB->getInstList().pop_back();
195 // If the unwind block is now dead, nuke it.
196 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
197 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
203 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
204 if (CallInst *CI = dyn_cast<CallInst>(I++))
205 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
206 // This call calls a function that cannot return. Insert an
207 // unreachable instruction after it and simplify the code. Do this
208 // by splitting the BB, adding the unreachable, then deleting the
210 BasicBlock *New = BB->splitBasicBlock(I);
212 // Remove the uncond branch and add an unreachable.
213 BB->getInstList().pop_back();
214 new UnreachableInst(BB->getContext(), BB);
216 DeleteBasicBlock(New); // Delete the new BB.
226 /// DeleteBasicBlock - remove the specified basic block from the program,
227 /// updating the callgraph to reflect any now-obsolete edges due to calls that
229 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
230 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
231 CallGraph &CG = getAnalysis<CallGraph>();
233 CallGraphNode *CGN = CG[BB->getParent()];
234 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
236 if (CallInst *CI = dyn_cast<CallInst>(I)) {
237 if (!isa<DbgInfoIntrinsic>(I))
238 CGN->removeCallEdgeFor(CI);
239 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
240 CGN->removeCallEdgeFor(II);
242 I->replaceAllUsesWith(UndefValue::get(I->getType()));
245 // Get the list of successors of this block.
246 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
248 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
249 Succs[i]->removePredecessor(BB);
251 BB->eraseFromParent();