PruneEH.cpp revision 360784 
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/IR/CFG.h"
22#include "llvm/IR/Constants.h"
23#include "llvm/IR/Function.h"
24#include "llvm/IR/InlineAsm.h"
25#include "llvm/IR/Instructions.h"
26#include "llvm/IR/LLVMContext.h"
27#include "llvm/InitializePasses.h"
28#include "llvm/Support/raw_ostream.h"
29#include "llvm/Transforms/IPO.h"
30#include "llvm/Transforms/Utils/Local.h"
31#include <algorithm>
32using namespace llvm;
33
34#define DEBUG_TYPE "prune-eh"
35
36STATISTIC(NumRemoved, "Number of invokes removed");
37STATISTIC(NumUnreach, "Number of noreturn calls optimized");
38
39namespace {
40  struct PruneEH : public CallGraphSCCPass {
41    static char ID; // Pass identification, replacement for typeid
42    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  };
50}
51static bool SimplifyFunction(Function *F, CallGraph &CG);
52static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG);
53
54char PruneEH::ID = 0;
55INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
56                "Remove unused exception handling info", false, false)
57INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
58INITIALIZE_PASS_END(PruneEH, "prune-eh",
59                "Remove unused exception handling info", false, false)
60
61Pass *llvm::createPruneEHPass() { return new PruneEH(); }
62
63static bool runImpl(CallGraphSCC &SCC, CallGraph &CG) {
64  SmallPtrSet<CallGraphNode *, 8> SCCNodes;
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 (CallGraphNode *I : SCC)
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 (CallGraphNode *I : SCC)
75    if (Function *F = I->getFunction())
76      MadeChange |= SimplifyFunction(F, CG);
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->hasExactDefinition()) {
93      SCCMightUnwind |= !F->doesNotThrow();
94      SCCMightReturn |= !F->doesNotReturn();
95    } else {
96      bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
97      bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
98      // Determine if we should scan for InlineAsm in a naked function as it
99      // is the only way to return without a ReturnInst.  Only do this for
100      // no-inline functions as functions which may be inlined cannot
101      // meaningfully return via assembly.
102      bool CheckReturnViaAsm = CheckReturn &&
103                               F->hasFnAttribute(Attribute::Naked) &&
104                               F->hasFnAttribute(Attribute::NoInline);
105
106      if (!CheckUnwind && !CheckReturn)
107        continue;
108
109      for (const BasicBlock &BB : *F) {
110        const Instruction *TI = BB.getTerminator();
111        if (CheckUnwind && TI->mayThrow()) {
112          SCCMightUnwind = true;
113        } else if (CheckReturn && isa<ReturnInst>(TI)) {
114          SCCMightReturn = true;
115        }
116
117        for (const Instruction &I : BB) {
118          if ((!CheckUnwind || SCCMightUnwind) &&
119              (!CheckReturnViaAsm || SCCMightReturn))
120            break;
121
122          // Check to see if this function performs an unwind or calls an
123          // unwinding function.
124          if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
125            bool InstMightUnwind = true;
126            if (const auto *CI = dyn_cast<CallInst>(&I)) {
127              if (Function *Callee = CI->getCalledFunction()) {
128                CallGraphNode *CalleeNode = CG[Callee];
129                // If the callee is outside our current SCC then we may throw
130                // because it might.  If it is inside, do nothing.
131                if (SCCNodes.count(CalleeNode) > 0)
132                  InstMightUnwind = false;
133              }
134            }
135            SCCMightUnwind |= InstMightUnwind;
136          }
137          if (CheckReturnViaAsm && !SCCMightReturn)
138            if (auto ICS = ImmutableCallSite(&I))
139              if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
140                if (IA->hasSideEffects())
141                  SCCMightReturn = true;
142        }
143
144        if (SCCMightUnwind && SCCMightReturn)
145          break;
146      }
147    }
148  }
149
150  // If the SCC doesn't unwind or doesn't throw, note this fact.
151  if (!SCCMightUnwind || !SCCMightReturn)
152    for (CallGraphNode *I : SCC) {
153      Function *F = I->getFunction();
154
155      if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
156        F->addFnAttr(Attribute::NoUnwind);
157        MadeChange = true;
158      }
159
160      if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
161        F->addFnAttr(Attribute::NoReturn);
162        MadeChange = true;
163      }
164    }
165
166  for (CallGraphNode *I : SCC) {
167    // Convert any invoke instructions to non-throwing functions in this node
168    // into call instructions with a branch.  This makes the exception blocks
169    // dead.
170    if (Function *F = I->getFunction())
171      MadeChange |= SimplifyFunction(F, CG);
172  }
173
174  return MadeChange;
175}
176
177
178bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
179  if (skipSCC(SCC))
180    return false;
181  CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
182  return runImpl(SCC, CG);
183}
184
185
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.
189static bool SimplifyFunction(Function *F, CallGraph &CG) {
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        BasicBlock *UnwindBlock = II->getUnwindDest();
195        removeUnwindEdge(&*BB);
196
197        // If the unwind block is now dead, nuke it.
198        if (pred_empty(UnwindBlock))
199          DeleteBasicBlock(UnwindBlock, CG);  // Delete the new BB.
200
201        ++NumRemoved;
202        MadeChange = true;
203      }
204
205    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
206      if (CallInst *CI = dyn_cast<CallInst>(I++))
207        if (CI->doesNotReturn() && !CI->isMustTailCall() &&
208            !isa<UnreachableInst>(I)) {
209          // This call calls a function that cannot return.  Insert an
210          // unreachable instruction after it and simplify the code.  Do this
211          // by splitting the BB, adding the unreachable, then deleting the
212          // new BB.
213          BasicBlock *New = BB->splitBasicBlock(I);
214
215          // Remove the uncond branch and add an unreachable.
216          BB->getInstList().pop_back();
217          new UnreachableInst(BB->getContext(), &*BB);
218
219          DeleteBasicBlock(New, CG);  // Delete the new BB.
220          MadeChange = true;
221          ++NumUnreach;
222          break;
223        }
224  }
225
226  return MadeChange;
227}
228
229/// DeleteBasicBlock - remove the specified basic block from the program,
230/// updating the callgraph to reflect any now-obsolete edges due to calls that
231/// exist in the BB.
232static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG) {
233  assert(pred_empty(BB) && "BB is not dead!");
234
235  Instruction *TokenInst = nullptr;
236
237  CallGraphNode *CGN = CG[BB->getParent()];
238  for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
239    --I;
240
241    if (I->getType()->isTokenTy()) {
242      TokenInst = &*I;
243      break;
244    }
245
246    if (auto *Call = dyn_cast<CallBase>(&*I)) {
247      const Function *Callee = Call->getCalledFunction();
248      if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID()))
249        CGN->removeCallEdgeFor(*Call);
250      else if (!Callee->isIntrinsic())
251        CGN->removeCallEdgeFor(*Call);
252    }
253
254    if (!I->use_empty())
255      I->replaceAllUsesWith(UndefValue::get(I->getType()));
256  }
257
258  if (TokenInst) {
259    if (!TokenInst->isTerminator())
260      changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false);
261  } else {
262    // Get the list of successors of this block.
263    std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
264
265    for (unsigned i = 0, e = Succs.size(); i != e; ++i)
266      Succs[i]->removePredecessor(BB);
267
268    BB->eraseFromParent();
269  }
270}
271