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#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"
30#include <algorithm>
31using namespace llvm;
32
33STATISTIC(NumRemoved, "Number of invokes removed");
34STATISTIC(NumUnreach, "Number of noreturn calls optimized");
35
36namespace {
37  struct PruneEH : public CallGraphSCCPass {
38    static char ID; // Pass identification, replacement for typeid
39    PruneEH() : CallGraphSCCPass(ID) {
40      initializePruneEHPass(*PassRegistry::getPassRegistry());
41    }
42
43    // runOnSCC - Analyze the SCC, performing the transformation if possible.
44    bool runOnSCC(CallGraphSCC &SCC);
45
46    bool SimplifyFunction(Function *F);
47    void DeleteBasicBlock(BasicBlock *BB);
48  };
49}
50
51char PruneEH::ID = 0;
52INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
53                "Remove unused exception handling info", false, false)
54INITIALIZE_AG_DEPENDENCY(CallGraph)
55INITIALIZE_PASS_END(PruneEH, "prune-eh",
56                "Remove unused exception handling info", false, false)
57
58Pass *llvm::createPruneEHPass() { return new PruneEH(); }
59
60
61bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
62  SmallPtrSet<CallGraphNode *, 8> SCCNodes;
63  CallGraph &CG = getAnalysis<CallGraph>();
64  bool MadeChange = false;
65
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)
69    SCCNodes.insert(*I);
70
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);
76
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.
83  //
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();
88    if (F == 0) {
89      SCCMightUnwind = true;
90      SCCMightReturn = true;
91    } else if (F->isDeclaration() || F->mayBeOverridden()) {
92      SCCMightUnwind |= !F->doesNotThrow();
93      SCCMightReturn |= !F->doesNotReturn();
94    } else {
95      bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
96      bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
97
98      if (!CheckUnwind && !CheckReturn)
99        continue;
100
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;
109        }
110
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;
124                  break;
125                }
126              } else {
127                // Indirect call, it might throw.
128                SCCMightUnwind = true;
129                break;
130              }
131            }
132        if (SCCMightUnwind && SCCMightReturn) break;
133      }
134    }
135  }
136
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      Attributes NewAttributes = Attribute::None;
141
142      if (!SCCMightUnwind)
143        NewAttributes |= Attribute::NoUnwind;
144      if (!SCCMightReturn)
145        NewAttributes |= Attribute::NoReturn;
146
147      Function *F = (*I)->getFunction();
148      const AttrListPtr &PAL = F->getAttributes();
149      const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
150      if (PAL != NPAL) {
151        MadeChange = true;
152        F->setAttributes(NPAL);
153      }
154    }
155
156  for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
157    // Convert any invoke instructions to non-throwing functions in this node
158    // into call instructions with a branch.  This makes the exception blocks
159    // dead.
160    if (Function *F = (*I)->getFunction())
161      MadeChange |= SimplifyFunction(F);
162  }
163
164  return MadeChange;
165}
166
167
168// SimplifyFunction - Given information about callees, simplify the specified
169// function if we have invokes to non-unwinding functions or code after calls to
170// no-return functions.
171bool PruneEH::SimplifyFunction(Function *F) {
172  bool MadeChange = false;
173  for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
174    if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
175      if (II->doesNotThrow()) {
176        SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
177        // Insert a call instruction before the invoke.
178        CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
179        Call->takeName(II);
180        Call->setCallingConv(II->getCallingConv());
181        Call->setAttributes(II->getAttributes());
182        Call->setDebugLoc(II->getDebugLoc());
183
184        // Anything that used the value produced by the invoke instruction
185        // now uses the value produced by the call instruction.  Note that we
186        // do this even for void functions and calls with no uses so that the
187        // callgraph edge is updated.
188        II->replaceAllUsesWith(Call);
189        BasicBlock *UnwindBlock = II->getUnwindDest();
190        UnwindBlock->removePredecessor(II->getParent());
191
192        // Insert a branch to the normal destination right before the
193        // invoke.
194        BranchInst::Create(II->getNormalDest(), II);
195
196        // Finally, delete the invoke instruction!
197        BB->getInstList().pop_back();
198
199        // If the unwind block is now dead, nuke it.
200        if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
201          DeleteBasicBlock(UnwindBlock);  // Delete the new BB.
202
203        ++NumRemoved;
204        MadeChange = true;
205      }
206
207    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
208      if (CallInst *CI = dyn_cast<CallInst>(I++))
209        if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
210          // This call calls a function that cannot return.  Insert an
211          // unreachable instruction after it and simplify the code.  Do this
212          // by splitting the BB, adding the unreachable, then deleting the
213          // new BB.
214          BasicBlock *New = BB->splitBasicBlock(I);
215
216          // Remove the uncond branch and add an unreachable.
217          BB->getInstList().pop_back();
218          new UnreachableInst(BB->getContext(), BB);
219
220          DeleteBasicBlock(New);  // Delete the new BB.
221          MadeChange = true;
222          ++NumUnreach;
223          break;
224        }
225  }
226
227  return MadeChange;
228}
229
230/// DeleteBasicBlock - remove the specified basic block from the program,
231/// updating the callgraph to reflect any now-obsolete edges due to calls that
232/// exist in the BB.
233void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
234  assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
235  CallGraph &CG = getAnalysis<CallGraph>();
236
237  CallGraphNode *CGN = CG[BB->getParent()];
238  for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
239    --I;
240    if (CallInst *CI = dyn_cast<CallInst>(I)) {
241      if (!isa<IntrinsicInst>(I))
242        CGN->removeCallEdgeFor(CI);
243    } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
244      CGN->removeCallEdgeFor(II);
245    if (!I->use_empty())
246      I->replaceAllUsesWith(UndefValue::get(I->getType()));
247  }
248
249  // Get the list of successors of this block.
250  std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
251
252  for (unsigned i = 0, e = Succs.size(); i != e; ++i)
253    Succs[i]->removePredecessor(BB);
254
255  BB->eraseFromParent();
256}
257