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/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/Instructions.h"
29 #include "llvm/IR/IntrinsicInst.h"
30 #include "llvm/IR/LLVMContext.h"
31 #include <algorithm>
32 using namespace llvm;
33 
34 #define DEBUG_TYPE "prune-eh"
35 
36 STATISTIC(NumRemoved, "Number of invokes removed");
37 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
38 
39 namespace {
40   struct PruneEH : public CallGraphSCCPass {
41     static char ID; // Pass identification, replacement for typeid
PruneEH__anon32217f7e0111::PruneEH42     PruneEH() : CallGraphSCCPass(ID) {
43       initializePruneEHPass(*PassRegistry::getPassRegistry());
44     }
45 
46     // runOnSCC - Analyze the SCC, performing the transformation if possible.
47     bool runOnSCC(CallGraphSCC &SCC) override;
48 
49     bool SimplifyFunction(Function *F);
50     void DeleteBasicBlock(BasicBlock *BB);
51   };
52 }
53 
54 char PruneEH::ID = 0;
55 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
56                 "Remove unused exception handling info", false, false)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)57 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
58 INITIALIZE_PASS_END(PruneEH, "prune-eh",
59                 "Remove unused exception handling info", false, false)
60 
61 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
62 
63 
runOnSCC(CallGraphSCC & SCC)64 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
65   SmallPtrSet<CallGraphNode *, 8> SCCNodes;
66   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
67   bool MadeChange = false;
68 
69   // Fill SCCNodes with the elements of the SCC.  Used for quickly
70   // looking up whether a given CallGraphNode is in this SCC.
71   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
72     SCCNodes.insert(*I);
73 
74   // First pass, scan all of the functions in the SCC, simplifying them
75   // according to what we know.
76   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
77     if (Function *F = (*I)->getFunction())
78       MadeChange |= SimplifyFunction(F);
79 
80   // Next, check to see if any callees might throw or if there are any external
81   // functions in this SCC: if so, we cannot prune any functions in this SCC.
82   // Definitions that are weak and not declared non-throwing might be
83   // overridden at linktime with something that throws, so assume that.
84   // If this SCC includes the unwind instruction, we KNOW it throws, so
85   // obviously the SCC might throw.
86   //
87   bool SCCMightUnwind = false, SCCMightReturn = false;
88   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
89        (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
90     Function *F = (*I)->getFunction();
91     if (!F) {
92       SCCMightUnwind = true;
93       SCCMightReturn = true;
94     } else if (F->isDeclaration() || F->mayBeOverridden()) {
95       SCCMightUnwind |= !F->doesNotThrow();
96       SCCMightReturn |= !F->doesNotReturn();
97     } else {
98       bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
99       bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
100 
101       if (!CheckUnwind && !CheckReturn)
102         continue;
103 
104       // Check to see if this function performs an unwind or calls an
105       // unwinding function.
106       for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
107         if (CheckUnwind && isa<ResumeInst>(BB->getTerminator())) {
108           // Uses unwind / resume!
109           SCCMightUnwind = true;
110         } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
111           SCCMightReturn = true;
112         }
113 
114         // Invoke instructions don't allow unwinding to continue, so we are
115         // only interested in call instructions.
116         if (CheckUnwind && !SCCMightUnwind)
117           for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
118             if (CallInst *CI = dyn_cast<CallInst>(I)) {
119               if (CI->doesNotThrow()) {
120                 // This call cannot throw.
121               } else if (Function *Callee = CI->getCalledFunction()) {
122                 CallGraphNode *CalleeNode = CG[Callee];
123                 // If the callee is outside our current SCC then we may
124                 // throw because it might.
125                 if (!SCCNodes.count(CalleeNode)) {
126                   SCCMightUnwind = true;
127                   break;
128                 }
129               } else {
130                 // Indirect call, it might throw.
131                 SCCMightUnwind = true;
132                 break;
133               }
134             }
135         if (SCCMightUnwind && SCCMightReturn) break;
136       }
137     }
138   }
139 
140   // If the SCC doesn't unwind or doesn't throw, note this fact.
141   if (!SCCMightUnwind || !SCCMightReturn)
142     for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
143       AttrBuilder NewAttributes;
144 
145       if (!SCCMightUnwind)
146         NewAttributes.addAttribute(Attribute::NoUnwind);
147       if (!SCCMightReturn)
148         NewAttributes.addAttribute(Attribute::NoReturn);
149 
150       Function *F = (*I)->getFunction();
151       const AttributeSet &PAL = F->getAttributes().getFnAttributes();
152       const AttributeSet &NPAL = AttributeSet::get(
153           F->getContext(), AttributeSet::FunctionIndex, NewAttributes);
154 
155       if (PAL != NPAL) {
156         MadeChange = true;
157         F->addAttributes(AttributeSet::FunctionIndex, NPAL);
158       }
159     }
160 
161   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
162     // Convert any invoke instructions to non-throwing functions in this node
163     // into call instructions with a branch.  This makes the exception blocks
164     // dead.
165     if (Function *F = (*I)->getFunction())
166       MadeChange |= SimplifyFunction(F);
167   }
168 
169   return MadeChange;
170 }
171 
172 
173 // SimplifyFunction - Given information about callees, simplify the specified
174 // function if we have invokes to non-unwinding functions or code after calls to
175 // no-return functions.
SimplifyFunction(Function * F)176 bool PruneEH::SimplifyFunction(Function *F) {
177   bool MadeChange = false;
178   for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
179     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
180       if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(II)) {
181         SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
182         // Insert a call instruction before the invoke.
183         CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
184         Call->takeName(II);
185         Call->setCallingConv(II->getCallingConv());
186         Call->setAttributes(II->getAttributes());
187         Call->setDebugLoc(II->getDebugLoc());
188 
189         // Anything that used the value produced by the invoke instruction
190         // now uses the value produced by the call instruction.  Note that we
191         // do this even for void functions and calls with no uses so that the
192         // callgraph edge is updated.
193         II->replaceAllUsesWith(Call);
194         BasicBlock *UnwindBlock = II->getUnwindDest();
195         UnwindBlock->removePredecessor(II->getParent());
196 
197         // Insert a branch to the normal destination right before the
198         // invoke.
199         BranchInst::Create(II->getNormalDest(), II);
200 
201         // Finally, delete the invoke instruction!
202         BB->getInstList().pop_back();
203 
204         // If the unwind block is now dead, nuke it.
205         if (pred_empty(UnwindBlock))
206           DeleteBasicBlock(UnwindBlock);  // Delete the new BB.
207 
208         ++NumRemoved;
209         MadeChange = true;
210       }
211 
212     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
213       if (CallInst *CI = dyn_cast<CallInst>(I++))
214         if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
215           // This call calls a function that cannot return.  Insert an
216           // unreachable instruction after it and simplify the code.  Do this
217           // by splitting the BB, adding the unreachable, then deleting the
218           // new BB.
219           BasicBlock *New = BB->splitBasicBlock(I);
220 
221           // Remove the uncond branch and add an unreachable.
222           BB->getInstList().pop_back();
223           new UnreachableInst(BB->getContext(), BB);
224 
225           DeleteBasicBlock(New);  // Delete the new BB.
226           MadeChange = true;
227           ++NumUnreach;
228           break;
229         }
230   }
231 
232   return MadeChange;
233 }
234 
235 /// DeleteBasicBlock - remove the specified basic block from the program,
236 /// updating the callgraph to reflect any now-obsolete edges due to calls that
237 /// exist in the BB.
DeleteBasicBlock(BasicBlock * BB)238 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
239   assert(pred_empty(BB) && "BB is not dead!");
240   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
241 
242   CallGraphNode *CGN = CG[BB->getParent()];
243   for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
244     --I;
245     if (CallInst *CI = dyn_cast<CallInst>(I)) {
246       if (!isa<IntrinsicInst>(I))
247         CGN->removeCallEdgeFor(CI);
248     } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
249       CGN->removeCallEdgeFor(II);
250     if (!I->use_empty())
251       I->replaceAllUsesWith(UndefValue::get(I->getType()));
252   }
253 
254   // Get the list of successors of this block.
255   std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
256 
257   for (unsigned i = 0, e = Succs.size(); i != e; ++i)
258     Succs[i]->removePredecessor(BB);
259 
260   BB->eraseFromParent();
261 }
262