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 #include "llvm/Transforms/IPO.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Support/raw_ostream.h"
22 #include "llvm/Analysis/CallGraph.h"
23 #include "llvm/Analysis/CallGraphSCCPass.h"
24 #include "llvm/Analysis/LibCallSemantics.h"
25 #include "llvm/IR/CFG.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/IR/InlineAsm.h"
29 #include "llvm/IR/Instructions.h"
30 #include "llvm/IR/IntrinsicInst.h"
31 #include "llvm/IR/LLVMContext.h"
35 #define DEBUG_TYPE "prune-eh"
37 STATISTIC(NumRemoved, "Number of invokes removed");
38 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
41 struct PruneEH : public CallGraphSCCPass {
42 static char ID; // Pass identification, replacement for typeid
43 PruneEH() : CallGraphSCCPass(ID) {
44 initializePruneEHPass(*PassRegistry::getPassRegistry());
47 // runOnSCC - Analyze the SCC, performing the transformation if possible.
48 bool runOnSCC(CallGraphSCC &SCC) override;
50 bool SimplifyFunction(Function *F);
51 void DeleteBasicBlock(BasicBlock *BB);
56 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
57 "Remove unused exception handling info", false, false)
58 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
59 INITIALIZE_PASS_END(PruneEH, "prune-eh",
60 "Remove unused exception handling info", false, false)
62 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
65 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
66 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
67 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
68 bool MadeChange = false;
70 // Fill SCCNodes with the elements of the SCC. Used for quickly
71 // looking up whether a given CallGraphNode is in this SCC.
72 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
75 // First pass, scan all of the functions in the SCC, simplifying them
76 // according to what we know.
77 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
78 if (Function *F = (*I)->getFunction())
79 MadeChange |= SimplifyFunction(F);
81 // Next, check to see if any callees might throw or if there are any external
82 // functions in this SCC: if so, we cannot prune any functions in this SCC.
83 // Definitions that are weak and not declared non-throwing might be
84 // overridden at linktime with something that throws, so assume that.
85 // If this SCC includes the unwind instruction, we KNOW it throws, so
86 // obviously the SCC might throw.
88 bool SCCMightUnwind = false, SCCMightReturn = false;
89 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
90 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
91 Function *F = (*I)->getFunction();
93 SCCMightUnwind = true;
94 SCCMightReturn = true;
95 } else if (F->isDeclaration() || F->mayBeOverridden()) {
96 SCCMightUnwind |= !F->doesNotThrow();
97 SCCMightReturn |= !F->doesNotReturn();
99 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
100 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
101 // Determine if we should scan for InlineAsm in a naked function as it
102 // is the only way to return without a ReturnInst. Only do this for
103 // no-inline functions as functions which may be inlined cannot
104 // meaningfully return via assembly.
105 bool CheckReturnViaAsm = CheckReturn &&
106 F->hasFnAttribute(Attribute::Naked) &&
107 F->hasFnAttribute(Attribute::NoInline);
109 if (!CheckUnwind && !CheckReturn)
112 for (const BasicBlock &BB : *F) {
113 const TerminatorInst *TI = BB.getTerminator();
114 if (CheckUnwind && TI->mayThrow()) {
115 SCCMightUnwind = true;
116 } else if (CheckReturn && isa<ReturnInst>(TI)) {
117 SCCMightReturn = true;
120 for (const Instruction &I : BB) {
121 if ((!CheckUnwind || SCCMightUnwind) &&
122 (!CheckReturnViaAsm || SCCMightReturn))
125 // Check to see if this function performs an unwind or calls an
126 // unwinding function.
127 if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
128 bool InstMightUnwind = true;
129 if (const auto *CI = dyn_cast<CallInst>(&I)) {
130 if (Function *Callee = CI->getCalledFunction()) {
131 CallGraphNode *CalleeNode = CG[Callee];
132 // If the callee is outside our current SCC then we may throw
133 // because it might. If it is inside, do nothing.
134 if (SCCNodes.count(CalleeNode) > 0)
135 InstMightUnwind = false;
138 SCCMightUnwind |= InstMightUnwind;
140 if (CheckReturnViaAsm && !SCCMightReturn)
141 if (auto ICS = ImmutableCallSite(&I))
142 if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
143 if (IA->hasSideEffects())
144 SCCMightReturn = true;
147 if (SCCMightUnwind && SCCMightReturn)
153 // If the SCC doesn't unwind or doesn't throw, note this fact.
154 if (!SCCMightUnwind || !SCCMightReturn)
155 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
156 AttrBuilder NewAttributes;
159 NewAttributes.addAttribute(Attribute::NoUnwind);
161 NewAttributes.addAttribute(Attribute::NoReturn);
163 Function *F = (*I)->getFunction();
164 const AttributeSet &PAL = F->getAttributes().getFnAttributes();
165 const AttributeSet &NPAL = AttributeSet::get(
166 F->getContext(), AttributeSet::FunctionIndex, NewAttributes);
170 F->addAttributes(AttributeSet::FunctionIndex, NPAL);
174 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
175 // Convert any invoke instructions to non-throwing functions in this node
176 // into call instructions with a branch. This makes the exception blocks
178 if (Function *F = (*I)->getFunction())
179 MadeChange |= SimplifyFunction(F);
186 // SimplifyFunction - Given information about callees, simplify the specified
187 // function if we have invokes to non-unwinding functions or code after calls to
188 // no-return functions.
189 bool PruneEH::SimplifyFunction(Function *F) {
190 bool MadeChange = false;
191 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
192 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
193 if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
194 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
195 // Insert a call instruction before the invoke.
196 CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
198 Call->setCallingConv(II->getCallingConv());
199 Call->setAttributes(II->getAttributes());
200 Call->setDebugLoc(II->getDebugLoc());
202 // Anything that used the value produced by the invoke instruction
203 // now uses the value produced by the call instruction. Note that we
204 // do this even for void functions and calls with no uses so that the
205 // callgraph edge is updated.
206 II->replaceAllUsesWith(Call);
207 BasicBlock *UnwindBlock = II->getUnwindDest();
208 UnwindBlock->removePredecessor(II->getParent());
210 // Insert a branch to the normal destination right before the
212 BranchInst::Create(II->getNormalDest(), II);
214 // Finally, delete the invoke instruction!
215 BB->getInstList().pop_back();
217 // If the unwind block is now dead, nuke it.
218 if (pred_empty(UnwindBlock))
219 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
225 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
226 if (CallInst *CI = dyn_cast<CallInst>(I++))
227 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
228 // This call calls a function that cannot return. Insert an
229 // unreachable instruction after it and simplify the code. Do this
230 // by splitting the BB, adding the unreachable, then deleting the
232 BasicBlock *New = BB->splitBasicBlock(I);
234 // Remove the uncond branch and add an unreachable.
235 BB->getInstList().pop_back();
236 new UnreachableInst(BB->getContext(), BB);
238 DeleteBasicBlock(New); // Delete the new BB.
248 /// DeleteBasicBlock - remove the specified basic block from the program,
249 /// updating the callgraph to reflect any now-obsolete edges due to calls that
251 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
252 assert(pred_empty(BB) && "BB is not dead!");
253 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
255 CallGraphNode *CGN = CG[BB->getParent()];
256 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
258 if (CallInst *CI = dyn_cast<CallInst>(I)) {
259 if (!isa<IntrinsicInst>(I))
260 CGN->removeCallEdgeFor(CI);
261 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
262 CGN->removeCallEdgeFor(II);
264 I->replaceAllUsesWith(UndefValue::get(I->getType()));
267 // Get the list of successors of this block.
268 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
270 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
271 Succs[i]->removePredecessor(BB);
273 BB->eraseFromParent();