PruneEH.cpp revision 360660 
1//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements a simple interprocedural pass which walks the
10// call-graph, turning invoke instructions into calls, iff the callee cannot
11// throw an exception, and marking functions 'nounwind' if they cannot throw.
12// It implements this as a bottom-up traversal of the call-graph.
13//
14//===----------------------------------------------------------------------===//
15
16#include "llvm/ADT/SmallPtrSet.h"
17#include "llvm/ADT/Statistic.h"
18#include "llvm/Analysis/CallGraph.h"
19#include "llvm/Analysis/CallGraphSCCPass.h"
20#include "llvm/Analysis/EHPersonalities.h"
21#include "llvm/Transforms/Utils/Local.h"
22#include "llvm/IR/CFG.h"
23#include "llvm/IR/Constants.h"
24#include "llvm/IR/Function.h"
25#include "llvm/IR/InlineAsm.h"
26#include "llvm/IR/Instructions.h"
27#include "llvm/IR/LLVMContext.h"
28#include "llvm/Support/raw_ostream.h"
29#include "llvm/Transforms/IPO.h"
30#include <algorithm>
31using namespace llvm;
32
33#define DEBUG_TYPE "prune-eh"
34
35STATISTIC(NumRemoved, "Number of invokes removed");
36STATISTIC(NumUnreach, "Number of noreturn calls optimized");
37
38namespace {
39  struct PruneEH : public CallGraphSCCPass {
40    static char ID; // Pass identification, replacement for typeid
41    PruneEH() : CallGraphSCCPass(ID) {
42      initializePruneEHPass(*PassRegistry::getPassRegistry());
43    }
44
45    // runOnSCC - Analyze the SCC, performing the transformation if possible.
46    bool runOnSCC(CallGraphSCC &SCC) override;
47
48  };
49}
50static bool SimplifyFunction(Function *F, CallGraph &CG);
51static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG);
52
53char PruneEH::ID = 0;
54INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
55                "Remove unused exception handling info", false, false)
56INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
57INITIALIZE_PASS_END(PruneEH, "prune-eh",
58                "Remove unused exception handling info", false, false)
59
60Pass *llvm::createPruneEHPass() { return new PruneEH(); }
61
62static bool runImpl(CallGraphSCC &SCC, CallGraph &CG) {
63  SmallPtrSet<CallGraphNode *, 8> SCCNodes;
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 (CallGraphNode *I : SCC)
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 (CallGraphNode *I : SCC)
74    if (Function *F = I->getFunction())
75      MadeChange |= SimplifyFunction(F, CG);
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) {
89      SCCMightUnwind = true;
90      SCCMightReturn = true;
91    } else if (!F->hasExactDefinition()) {
92      SCCMightUnwind |= !F->doesNotThrow();
93      SCCMightReturn |= !F->doesNotReturn();
94    } else {
95      bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
96      bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
97      // Determine if we should scan for InlineAsm in a naked function as it
98      // is the only way to return without a ReturnInst.  Only do this for
99      // no-inline functions as functions which may be inlined cannot
100      // meaningfully return via assembly.
101      bool CheckReturnViaAsm = CheckReturn &&
102                               F->hasFnAttribute(Attribute::Naked) &&
103                               F->hasFnAttribute(Attribute::NoInline);
104
105      if (!CheckUnwind && !CheckReturn)
106        continue;
107
108      for (const BasicBlock &BB : *F) {
109        const Instruction *TI = BB.getTerminator();
110        if (CheckUnwind && TI->mayThrow()) {
111          SCCMightUnwind = true;
112        } else if (CheckReturn && isa<ReturnInst>(TI)) {
113          SCCMightReturn = true;
114        }
115
116        for (const Instruction &I : BB) {
117          if ((!CheckUnwind || SCCMightUnwind) &&
118              (!CheckReturnViaAsm || SCCMightReturn))
119            break;
120
121          // Check to see if this function performs an unwind or calls an
122          // unwinding function.
123          if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
124            bool InstMightUnwind = true;
125            if (const auto *CI = dyn_cast<CallInst>(&I)) {
126              if (Function *Callee = CI->getCalledFunction()) {
127                CallGraphNode *CalleeNode = CG[Callee];
128                // If the callee is outside our current SCC then we may throw
129                // because it might.  If it is inside, do nothing.
130                if (SCCNodes.count(CalleeNode) > 0)
131                  InstMightUnwind = false;
132              }
133            }
134            SCCMightUnwind |= InstMightUnwind;
135          }
136          if (CheckReturnViaAsm && !SCCMightReturn)
137            if (auto ICS = ImmutableCallSite(&I))
138              if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
139                if (IA->hasSideEffects())
140                  SCCMightReturn = true;
141        }
142
143        if (SCCMightUnwind && SCCMightReturn)
144          break;
145      }
146    }
147  }
148
149  // If the SCC doesn't unwind or doesn't throw, note this fact.
150  if (!SCCMightUnwind || !SCCMightReturn)
151    for (CallGraphNode *I : SCC) {
152      Function *F = I->getFunction();
153
154      if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
155        F->addFnAttr(Attribute::NoUnwind);
156        MadeChange = true;
157      }
158
159      if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
160        F->addFnAttr(Attribute::NoReturn);
161        MadeChange = true;
162      }
163    }
164
165  for (CallGraphNode *I : SCC) {
166    // Convert any invoke instructions to non-throwing functions in this node
167    // into call instructions with a branch.  This makes the exception blocks
168    // dead.
169    if (Function *F = I->getFunction())
170      MadeChange |= SimplifyFunction(F, CG);
171  }
172
173  return MadeChange;
174}
175
176
177bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
178  if (skipSCC(SCC))
179    return false;
180  CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
181  return runImpl(SCC, CG);
182}
183
184
185// SimplifyFunction - Given information about callees, simplify the specified
186// function if we have invokes to non-unwinding functions or code after calls to
187// no-return functions.
188static bool SimplifyFunction(Function *F, CallGraph &CG) {
189  bool MadeChange = false;
190  for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
191    if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
192      if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
193        BasicBlock *UnwindBlock = II->getUnwindDest();
194        removeUnwindEdge(&*BB);
195
196        // If the unwind block is now dead, nuke it.
197        if (pred_empty(UnwindBlock))
198          DeleteBasicBlock(UnwindBlock, CG);  // Delete the new BB.
199
200        ++NumRemoved;
201        MadeChange = true;
202      }
203
204    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
205      if (CallInst *CI = dyn_cast<CallInst>(I++))
206        if (CI->doesNotReturn() && !CI->isMustTailCall() &&
207            !isa<UnreachableInst>(I)) {
208          // This call calls a function that cannot return.  Insert an
209          // unreachable instruction after it and simplify the code.  Do this
210          // by splitting the BB, adding the unreachable, then deleting the
211          // new BB.
212          BasicBlock *New = BB->splitBasicBlock(I);
213
214          // Remove the uncond branch and add an unreachable.
215          BB->getInstList().pop_back();
216          new UnreachableInst(BB->getContext(), &*BB);
217
218          DeleteBasicBlock(New, CG);  // Delete the new BB.
219          MadeChange = true;
220          ++NumUnreach;
221          break;
222        }
223  }
224
225  return MadeChange;
226}
227
228/// DeleteBasicBlock - remove the specified basic block from the program,
229/// updating the callgraph to reflect any now-obsolete edges due to calls that
230/// exist in the BB.
231static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG) {
232  assert(pred_empty(BB) && "BB is not dead!");
233
234  Instruction *TokenInst = nullptr;
235
236  CallGraphNode *CGN = CG[BB->getParent()];
237  for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
238    --I;
239
240    if (I->getType()->isTokenTy()) {
241      TokenInst = &*I;
242      break;
243    }
244
245    if (auto *Call = dyn_cast<CallBase>(&*I)) {
246      const Function *Callee = Call->getCalledFunction();
247      if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID()))
248        CGN->removeCallEdgeFor(*Call);
249      else if (!Callee->isIntrinsic())
250        CGN->removeCallEdgeFor(*Call);
251    }
252
253    if (!I->use_empty())
254      I->replaceAllUsesWith(UndefValue::get(I->getType()));
255  }
256
257  if (TokenInst) {
258    if (!TokenInst->isTerminator())
259      changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false);
260  } else {
261    // Get the list of successors of this block.
262    std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
263
264    for (unsigned i = 0, e = Succs.size(); i != e; ++i)
265      Succs[i]->removePredecessor(BB);
266
267    BB->eraseFromParent();
268  }
269}
270