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>
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "prune-eh"
34 
35 STATISTIC(NumRemoved, "Number of invokes removed");
36 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
37 
38 namespace {
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 }
50 static bool SimplifyFunction(Function *F, CallGraph &CG);
51 static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG);
52 
53 char PruneEH::ID = 0;
54 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
55                 "Remove unused exception handling info", false, false)
56 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
57 INITIALIZE_PASS_END(PruneEH, "prune-eh",
58                 "Remove unused exception handling info", false, false)
59 
60 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
61 
62 static 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 
177 bool 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.
188 static 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.
231 static 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