1 //===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
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.
14 //
15 //===----------------------------------------------------------------------===//
16
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/Analysis/CallGraph.h"
22 #include "llvm/Analysis/CallGraphSCCPass.h"
23 #include "llvm/IR/CFG.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 <algorithm>
30 using namespace llvm;
31
32 #define DEBUG_TYPE "prune-eh"
33
34 STATISTIC(NumRemoved, "Number of invokes removed");
35 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
36
37 namespace {
38 struct PruneEH : public CallGraphSCCPass {
39 static char ID; // Pass identification, replacement for typeid
PruneEH__anon67afa5d50111::PruneEH40 PruneEH() : CallGraphSCCPass(ID) {
41 initializePruneEHPass(*PassRegistry::getPassRegistry());
42 }
43
44 // runOnSCC - Analyze the SCC, performing the transformation if possible.
45 bool runOnSCC(CallGraphSCC &SCC) override;
46
47 bool SimplifyFunction(Function *F);
48 void DeleteBasicBlock(BasicBlock *BB);
49 };
50 }
51
52 char PruneEH::ID = 0;
53 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
54 "Remove unused exception handling info", false, false)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)55 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
56 INITIALIZE_PASS_END(PruneEH, "prune-eh",
57 "Remove unused exception handling info", false, false)
58
59 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
60
61
runOnSCC(CallGraphSCC & SCC)62 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
63 SmallPtrSet<CallGraphNode *, 8> SCCNodes;
64 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
65 bool MadeChange = false;
66
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)
70 SCCNodes.insert(*I);
71
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);
77
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.
84 //
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();
89 if (!F) {
90 SCCMightUnwind = true;
91 SCCMightReturn = true;
92 } else if (F->isDeclaration() || F->mayBeOverridden()) {
93 SCCMightUnwind |= !F->doesNotThrow();
94 SCCMightReturn |= !F->doesNotReturn();
95 } else {
96 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
97 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
98
99 if (!CheckUnwind && !CheckReturn)
100 continue;
101
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<ResumeInst>(BB->getTerminator())) {
106 // Uses unwind / resume!
107 SCCMightUnwind = true;
108 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
109 SCCMightReturn = true;
110 }
111
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;
125 break;
126 }
127 } else {
128 // Indirect call, it might throw.
129 SCCMightUnwind = true;
130 break;
131 }
132 }
133 if (SCCMightUnwind && SCCMightReturn) break;
134 }
135 }
136 }
137
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 AttrBuilder NewAttributes;
142
143 if (!SCCMightUnwind)
144 NewAttributes.addAttribute(Attribute::NoUnwind);
145 if (!SCCMightReturn)
146 NewAttributes.addAttribute(Attribute::NoReturn);
147
148 Function *F = (*I)->getFunction();
149 const AttributeSet &PAL = F->getAttributes().getFnAttributes();
150 const AttributeSet &NPAL = AttributeSet::get(
151 F->getContext(), AttributeSet::FunctionIndex, NewAttributes);
152
153 if (PAL != NPAL) {
154 MadeChange = true;
155 F->addAttributes(AttributeSet::FunctionIndex, NPAL);
156 }
157 }
158
159 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
160 // Convert any invoke instructions to non-throwing functions in this node
161 // into call instructions with a branch. This makes the exception blocks
162 // dead.
163 if (Function *F = (*I)->getFunction())
164 MadeChange |= SimplifyFunction(F);
165 }
166
167 return MadeChange;
168 }
169
170
171 // SimplifyFunction - Given information about callees, simplify the specified
172 // function if we have invokes to non-unwinding functions or code after calls to
173 // no-return functions.
SimplifyFunction(Function * F)174 bool PruneEH::SimplifyFunction(Function *F) {
175 bool MadeChange = false;
176 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
177 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
178 if (II->doesNotThrow()) {
179 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
180 // Insert a call instruction before the invoke.
181 CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
182 Call->takeName(II);
183 Call->setCallingConv(II->getCallingConv());
184 Call->setAttributes(II->getAttributes());
185 Call->setDebugLoc(II->getDebugLoc());
186
187 // Anything that used the value produced by the invoke instruction
188 // now uses the value produced by the call instruction. Note that we
189 // do this even for void functions and calls with no uses so that the
190 // callgraph edge is updated.
191 II->replaceAllUsesWith(Call);
192 BasicBlock *UnwindBlock = II->getUnwindDest();
193 UnwindBlock->removePredecessor(II->getParent());
194
195 // Insert a branch to the normal destination right before the
196 // invoke.
197 BranchInst::Create(II->getNormalDest(), II);
198
199 // Finally, delete the invoke instruction!
200 BB->getInstList().pop_back();
201
202 // If the unwind block is now dead, nuke it.
203 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
204 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
205
206 ++NumRemoved;
207 MadeChange = true;
208 }
209
210 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
211 if (CallInst *CI = dyn_cast<CallInst>(I++))
212 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
213 // This call calls a function that cannot return. Insert an
214 // unreachable instruction after it and simplify the code. Do this
215 // by splitting the BB, adding the unreachable, then deleting the
216 // new BB.
217 BasicBlock *New = BB->splitBasicBlock(I);
218
219 // Remove the uncond branch and add an unreachable.
220 BB->getInstList().pop_back();
221 new UnreachableInst(BB->getContext(), BB);
222
223 DeleteBasicBlock(New); // Delete the new BB.
224 MadeChange = true;
225 ++NumUnreach;
226 break;
227 }
228 }
229
230 return MadeChange;
231 }
232
233 /// DeleteBasicBlock - remove the specified basic block from the program,
234 /// updating the callgraph to reflect any now-obsolete edges due to calls that
235 /// exist in the BB.
DeleteBasicBlock(BasicBlock * BB)236 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
237 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
238 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
239
240 CallGraphNode *CGN = CG[BB->getParent()];
241 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
242 --I;
243 if (CallInst *CI = dyn_cast<CallInst>(I)) {
244 if (!isa<IntrinsicInst>(I))
245 CGN->removeCallEdgeFor(CI);
246 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
247 CGN->removeCallEdgeFor(II);
248 if (!I->use_empty())
249 I->replaceAllUsesWith(UndefValue::get(I->getType()));
250 }
251
252 // Get the list of successors of this block.
253 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
254
255 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
256 Succs[i]->removePredecessor(BB);
257
258 BB->eraseFromParent();
259 }
260