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/SetVector.h"
17 #include "llvm/ADT/SmallPtrSet.h"
18 #include "llvm/ADT/Statistic.h"
19 #include "llvm/Analysis/CallGraph.h"
20 #include "llvm/Analysis/CallGraphSCCPass.h"
21 #include "llvm/Analysis/EHPersonalities.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/InitializePasses.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Transforms/IPO.h"
31 #include "llvm/Transforms/Utils/CallGraphUpdater.h"
32 #include "llvm/Transforms/Utils/Local.h"
33 #include <algorithm>
34
35 using namespace llvm;
36
37 #define DEBUG_TYPE "prune-eh"
38
39 STATISTIC(NumRemoved, "Number of invokes removed");
40 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
41
42 namespace {
43 struct PruneEH : public CallGraphSCCPass {
44 static char ID; // Pass identification, replacement for typeid
PruneEH__anonc5b6d3a70111::PruneEH45 PruneEH() : CallGraphSCCPass(ID) {
46 initializePruneEHPass(*PassRegistry::getPassRegistry());
47 }
48
49 // runOnSCC - Analyze the SCC, performing the transformation if possible.
50 bool runOnSCC(CallGraphSCC &SCC) override;
51 };
52 }
53 static bool SimplifyFunction(Function *F, CallGraphUpdater &CGU);
54 static void DeleteBasicBlock(BasicBlock *BB, CallGraphUpdater &CGU);
55
56 char PruneEH::ID = 0;
57 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
58 "Remove unused exception handling info", false, false)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)59 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
60 INITIALIZE_PASS_END(PruneEH, "prune-eh",
61 "Remove unused exception handling info", false, false)
62
63 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
64
runImpl(CallGraphUpdater & CGU,SetVector<Function * > & Functions)65 static bool runImpl(CallGraphUpdater &CGU, SetVector<Function *> &Functions) {
66 #ifndef NDEBUG
67 for (auto *F : Functions)
68 assert(F && "null Function");
69 #endif
70 bool MadeChange = false;
71
72 // First pass, scan all of the functions in the SCC, simplifying them
73 // according to what we know.
74 for (Function *F : Functions)
75 MadeChange |= SimplifyFunction(F, CGU);
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 (Function *F : Functions) {
86 if (!F->hasExactDefinition()) {
87 SCCMightUnwind |= !F->doesNotThrow();
88 SCCMightReturn |= !F->doesNotReturn();
89 } else {
90 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
91 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
92 // Determine if we should scan for InlineAsm in a naked function as it
93 // is the only way to return without a ReturnInst. Only do this for
94 // no-inline functions as functions which may be inlined cannot
95 // meaningfully return via assembly.
96 bool CheckReturnViaAsm = CheckReturn &&
97 F->hasFnAttribute(Attribute::Naked) &&
98 F->hasFnAttribute(Attribute::NoInline);
99
100 if (!CheckUnwind && !CheckReturn)
101 continue;
102
103 for (const BasicBlock &BB : *F) {
104 const Instruction *TI = BB.getTerminator();
105 if (CheckUnwind && TI->mayThrow()) {
106 SCCMightUnwind = true;
107 } else if (CheckReturn && isa<ReturnInst>(TI)) {
108 SCCMightReturn = true;
109 }
110
111 for (const Instruction &I : BB) {
112 if ((!CheckUnwind || SCCMightUnwind) &&
113 (!CheckReturnViaAsm || SCCMightReturn))
114 break;
115
116 // Check to see if this function performs an unwind or calls an
117 // unwinding function.
118 if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
119 bool InstMightUnwind = true;
120 if (const auto *CI = dyn_cast<CallInst>(&I)) {
121 if (Function *Callee = CI->getCalledFunction()) {
122 // If the callee is outside our current SCC then we may throw
123 // because it might. If it is inside, do nothing.
124 if (Functions.contains(Callee))
125 InstMightUnwind = false;
126 }
127 }
128 SCCMightUnwind |= InstMightUnwind;
129 }
130 if (CheckReturnViaAsm && !SCCMightReturn)
131 if (const auto *CB = dyn_cast<CallBase>(&I))
132 if (const auto *IA = dyn_cast<InlineAsm>(CB->getCalledOperand()))
133 if (IA->hasSideEffects())
134 SCCMightReturn = true;
135 }
136 }
137 if (SCCMightUnwind && SCCMightReturn)
138 break;
139 }
140 }
141
142 // If the SCC doesn't unwind or doesn't throw, note this fact.
143 if (!SCCMightUnwind || !SCCMightReturn)
144 for (Function *F : Functions) {
145 if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
146 F->addFnAttr(Attribute::NoUnwind);
147 MadeChange = true;
148 }
149
150 if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
151 F->addFnAttr(Attribute::NoReturn);
152 MadeChange = true;
153 }
154 }
155
156 for (Function *F : Functions) {
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 MadeChange |= SimplifyFunction(F, CGU);
161 }
162
163 return MadeChange;
164 }
165
runOnSCC(CallGraphSCC & SCC)166 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
167 if (skipSCC(SCC))
168 return false;
169 SetVector<Function *> Functions;
170 for (auto &N : SCC) {
171 if (auto *F = N->getFunction())
172 Functions.insert(F);
173 }
174 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
175 CallGraphUpdater CGU;
176 CGU.initialize(CG, SCC);
177 return runImpl(CGU, Functions);
178 }
179
180
181 // SimplifyFunction - Given information about callees, simplify the specified
182 // function if we have invokes to non-unwinding functions or code after calls to
183 // no-return functions.
SimplifyFunction(Function * F,CallGraphUpdater & CGU)184 static bool SimplifyFunction(Function *F, CallGraphUpdater &CGU) {
185 bool MadeChange = false;
186 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
187 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
188 if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
189 BasicBlock *UnwindBlock = II->getUnwindDest();
190 removeUnwindEdge(&*BB);
191
192 // If the unwind block is now dead, nuke it.
193 if (pred_empty(UnwindBlock))
194 DeleteBasicBlock(UnwindBlock, CGU); // Delete the new BB.
195
196 ++NumRemoved;
197 MadeChange = true;
198 }
199
200 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
201 if (CallInst *CI = dyn_cast<CallInst>(I++))
202 if (CI->doesNotReturn() && !CI->isMustTailCall() &&
203 !isa<UnreachableInst>(I)) {
204 // This call calls a function that cannot return. Insert an
205 // unreachable instruction after it and simplify the code. Do this
206 // by splitting the BB, adding the unreachable, then deleting the
207 // new BB.
208 BasicBlock *New = BB->splitBasicBlock(I);
209
210 // Remove the uncond branch and add an unreachable.
211 BB->getInstList().pop_back();
212 new UnreachableInst(BB->getContext(), &*BB);
213
214 DeleteBasicBlock(New, CGU); // Delete the new BB.
215 MadeChange = true;
216 ++NumUnreach;
217 break;
218 }
219 }
220
221 return MadeChange;
222 }
223
224 /// DeleteBasicBlock - remove the specified basic block from the program,
225 /// updating the callgraph to reflect any now-obsolete edges due to calls that
226 /// exist in the BB.
DeleteBasicBlock(BasicBlock * BB,CallGraphUpdater & CGU)227 static void DeleteBasicBlock(BasicBlock *BB, CallGraphUpdater &CGU) {
228 assert(pred_empty(BB) && "BB is not dead!");
229
230 Instruction *TokenInst = nullptr;
231
232 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
233 --I;
234
235 if (I->getType()->isTokenTy()) {
236 TokenInst = &*I;
237 break;
238 }
239
240 if (auto *Call = dyn_cast<CallBase>(&*I)) {
241 const Function *Callee = Call->getCalledFunction();
242 if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID()))
243 CGU.removeCallSite(*Call);
244 else if (!Callee->isIntrinsic())
245 CGU.removeCallSite(*Call);
246 }
247
248 if (!I->use_empty())
249 I->replaceAllUsesWith(UndefValue::get(I->getType()));
250 }
251
252 if (TokenInst) {
253 if (!TokenInst->isTerminator())
254 changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false);
255 } else {
256 // Get the list of successors of this block.
257 std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
258
259 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
260 Succs[i]->removePredecessor(BB);
261
262 BB->eraseFromParent();
263 }
264 }
265