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) {
41 initializePruneEHPass(*PassRegistry::getPassRegistry());
44 // runOnSCC - Analyze the SCC, performing the transformation if possible.
45 bool runOnSCC(CallGraphSCC &SCC);
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_AG_DEPENDENCY(CallGraph)
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<CallGraph>();
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<UnwindInst>(BB->getTerminator())) {
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 Attributes NewAttributes = Attribute::None;
144 NewAttributes |= Attribute::NoUnwind;
146 NewAttributes |= Attribute::NoReturn;
148 Function *F = (*I)->getFunction();
149 const AttrListPtr &PAL = F->getAttributes();
150 const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
153 F->setAttributes(NPAL);
157 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
158 // Convert any invoke instructions to non-throwing functions in this node
159 // into call instructions with a branch. This makes the exception blocks
161 if (Function *F = (*I)->getFunction())
162 MadeChange |= SimplifyFunction(F);
169 // SimplifyFunction - Given information about callees, simplify the specified
170 // function if we have invokes to non-unwinding functions or code after calls to
171 // no-return functions.
172 bool PruneEH::SimplifyFunction(Function *F) {
173 bool MadeChange = false;
174 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
175 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
176 if (II->doesNotThrow()) {
177 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
178 // Insert a call instruction before the invoke.
179 CallInst *Call = CallInst::Create(II->getCalledValue(),
180 Args.begin(), Args.end(), "", II);
182 Call->setCallingConv(II->getCallingConv());
183 Call->setAttributes(II->getAttributes());
185 // Anything that used the value produced by the invoke instruction
186 // now uses the value produced by the call instruction. Note that we
187 // do this even for void functions and calls with no uses so that the
188 // callgraph edge is updated.
189 II->replaceAllUsesWith(Call);
190 BasicBlock *UnwindBlock = II->getUnwindDest();
191 UnwindBlock->removePredecessor(II->getParent());
193 // Insert a branch to the normal destination right before the
195 BranchInst::Create(II->getNormalDest(), II);
197 // Finally, delete the invoke instruction!
198 BB->getInstList().pop_back();
200 // If the unwind block is now dead, nuke it.
201 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
202 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
208 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
209 if (CallInst *CI = dyn_cast<CallInst>(I++))
210 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
211 // This call calls a function that cannot return. Insert an
212 // unreachable instruction after it and simplify the code. Do this
213 // by splitting the BB, adding the unreachable, then deleting the
215 BasicBlock *New = BB->splitBasicBlock(I);
217 // Remove the uncond branch and add an unreachable.
218 BB->getInstList().pop_back();
219 new UnreachableInst(BB->getContext(), BB);
221 DeleteBasicBlock(New); // Delete the new BB.
231 /// DeleteBasicBlock - remove the specified basic block from the program,
232 /// updating the callgraph to reflect any now-obsolete edges due to calls that
234 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
235 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
236 CallGraph &CG = getAnalysis<CallGraph>();
238 CallGraphNode *CGN = CG[BB->getParent()];
239 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
241 if (CallInst *CI = dyn_cast<CallInst>(I)) {
242 if (!isa<DbgInfoIntrinsic>(I))
243 CGN->removeCallEdgeFor(CI);
244 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
245 CGN->removeCallEdgeFor(II);
247 I->replaceAllUsesWith(UndefValue::get(I->getType()));
250 // Get the list of successors of this block.
251 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
253 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
254 Succs[i]->removePredecessor(BB);
256 BB->eraseFromParent();