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/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/SmallVector.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/Analysis/CallGraph.h"
23 #include "llvm/Analysis/CallGraphSCCPass.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/Function.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/IntrinsicInst.h"
28 #include "llvm/IR/LLVMContext.h"
29 #include "llvm/Support/CFG.h"
33 STATISTIC(NumRemoved, "Number of invokes removed");
34 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
37 struct PruneEH : public CallGraphSCCPass {
38 static char ID; // Pass identification, replacement for typeid
39 PruneEH() : CallGraphSCCPass(ID) {
40 initializePruneEHPass(*PassRegistry::getPassRegistry());
43 // runOnSCC - Analyze the SCC, performing the transformation if possible.
44 bool runOnSCC(CallGraphSCC &SCC);
46 bool SimplifyFunction(Function *F);
47 void DeleteBasicBlock(BasicBlock *BB);
52 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
53 "Remove unused exception handling info", false, false)
54 INITIALIZE_AG_DEPENDENCY(CallGraph)
55 INITIALIZE_PASS_END(PruneEH, "prune-eh",
56 "Remove unused exception handling info", false, false)
58 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
61 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
62 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
63 CallGraph &CG = getAnalysis<CallGraph>();
64 bool MadeChange = false;
66 // Fill SCCNodes with the elements of the SCC. Used for quickly
67 // looking up whether a given CallGraphNode is in this SCC.
68 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
71 // First pass, scan all of the functions in the SCC, simplifying them
72 // according to what we know.
73 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
74 if (Function *F = (*I)->getFunction())
75 MadeChange |= SimplifyFunction(F);
77 // Next, check to see if any callees might throw or if there are any external
78 // functions in this SCC: if so, we cannot prune any functions in this SCC.
79 // Definitions that are weak and not declared non-throwing might be
80 // overridden at linktime with something that throws, so assume that.
81 // If this SCC includes the unwind instruction, we KNOW it throws, so
82 // obviously the SCC might throw.
84 bool SCCMightUnwind = false, SCCMightReturn = false;
85 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
86 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
87 Function *F = (*I)->getFunction();
89 SCCMightUnwind = true;
90 SCCMightReturn = true;
91 } else if (F->isDeclaration() || F->mayBeOverridden()) {
92 SCCMightUnwind |= !F->doesNotThrow();
93 SCCMightReturn |= !F->doesNotReturn();
95 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
96 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
98 if (!CheckUnwind && !CheckReturn)
101 // Check to see if this function performs an unwind or calls an
102 // unwinding function.
103 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
104 if (CheckUnwind && isa<ResumeInst>(BB->getTerminator())) {
105 // Uses unwind / resume!
106 SCCMightUnwind = true;
107 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
108 SCCMightReturn = true;
111 // Invoke instructions don't allow unwinding to continue, so we are
112 // only interested in call instructions.
113 if (CheckUnwind && !SCCMightUnwind)
114 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
115 if (CallInst *CI = dyn_cast<CallInst>(I)) {
116 if (CI->doesNotThrow()) {
117 // This call cannot throw.
118 } else if (Function *Callee = CI->getCalledFunction()) {
119 CallGraphNode *CalleeNode = CG[Callee];
120 // If the callee is outside our current SCC then we may
121 // throw because it might.
122 if (!SCCNodes.count(CalleeNode)) {
123 SCCMightUnwind = true;
127 // Indirect call, it might throw.
128 SCCMightUnwind = true;
132 if (SCCMightUnwind && SCCMightReturn) break;
137 // If the SCC doesn't unwind or doesn't throw, note this fact.
138 if (!SCCMightUnwind || !SCCMightReturn)
139 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
140 AttrBuilder NewAttributes;
143 NewAttributes.addAttribute(Attribute::NoUnwind);
145 NewAttributes.addAttribute(Attribute::NoReturn);
147 Function *F = (*I)->getFunction();
148 const AttributeSet &PAL = F->getAttributes().getFnAttributes();
149 const AttributeSet &NPAL = AttributeSet::get(
150 F->getContext(), AttributeSet::FunctionIndex, NewAttributes);
154 F->addAttributes(AttributeSet::FunctionIndex, NPAL);
158 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
159 // Convert any invoke instructions to non-throwing functions in this node
160 // into call instructions with a branch. This makes the exception blocks
162 if (Function *F = (*I)->getFunction())
163 MadeChange |= SimplifyFunction(F);
170 // SimplifyFunction - Given information about callees, simplify the specified
171 // function if we have invokes to non-unwinding functions or code after calls to
172 // no-return functions.
173 bool PruneEH::SimplifyFunction(Function *F) {
174 bool MadeChange = false;
175 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
176 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
177 if (II->doesNotThrow()) {
178 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
179 // Insert a call instruction before the invoke.
180 CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
182 Call->setCallingConv(II->getCallingConv());
183 Call->setAttributes(II->getAttributes());
184 Call->setDebugLoc(II->getDebugLoc());
186 // Anything that used the value produced by the invoke instruction
187 // now uses the value produced by the call instruction. Note that we
188 // do this even for void functions and calls with no uses so that the
189 // callgraph edge is updated.
190 II->replaceAllUsesWith(Call);
191 BasicBlock *UnwindBlock = II->getUnwindDest();
192 UnwindBlock->removePredecessor(II->getParent());
194 // Insert a branch to the normal destination right before the
196 BranchInst::Create(II->getNormalDest(), II);
198 // Finally, delete the invoke instruction!
199 BB->getInstList().pop_back();
201 // If the unwind block is now dead, nuke it.
202 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
203 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
209 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
210 if (CallInst *CI = dyn_cast<CallInst>(I++))
211 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
212 // This call calls a function that cannot return. Insert an
213 // unreachable instruction after it and simplify the code. Do this
214 // by splitting the BB, adding the unreachable, then deleting the
216 BasicBlock *New = BB->splitBasicBlock(I);
218 // Remove the uncond branch and add an unreachable.
219 BB->getInstList().pop_back();
220 new UnreachableInst(BB->getContext(), BB);
222 DeleteBasicBlock(New); // Delete the new BB.
232 /// DeleteBasicBlock - remove the specified basic block from the program,
233 /// updating the callgraph to reflect any now-obsolete edges due to calls that
235 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
236 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
237 CallGraph &CG = getAnalysis<CallGraph>();
239 CallGraphNode *CGN = CG[BB->getParent()];
240 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
242 if (CallInst *CI = dyn_cast<CallInst>(I)) {
243 if (!isa<IntrinsicInst>(I))
244 CGN->removeCallEdgeFor(CI);
245 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
246 CGN->removeCallEdgeFor(II);
248 I->replaceAllUsesWith(UndefValue::get(I->getType()));
251 // Get the list of successors of this block.
252 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
254 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
255 Succs[i]->removePredecessor(BB);
257 BB->eraseFromParent();