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//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements a simple interprocedural pass which walks the
// call-graph, turning invoke instructions into calls, iff the callee cannot
// throw an exception, and marking functions 'nounwind' if they cannot throw.
// It implements this as a bottom-up traversal of the call-graph.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/CallGraphSCCPass.h"
#include "llvm/Analysis/EHPersonalities.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/IPO.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "prune-eh"
STATISTIC(NumRemoved, "Number of invokes removed");
STATISTIC(NumUnreach, "Number of noreturn calls optimized");
namespace {
struct PruneEH : public CallGraphSCCPass {
static char ID; // Pass identification, replacement for typeid
PruneEH() : CallGraphSCCPass(ID) {
initializePruneEHPass(*PassRegistry::getPassRegistry());
}
// runOnSCC - Analyze the SCC, performing the transformation if possible.
bool runOnSCC(CallGraphSCC &SCC) override;
};
}
static bool SimplifyFunction(Function *F, CallGraph &CG);
static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG);
char PruneEH::ID = 0;
INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
"Remove unused exception handling info", false, false)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
INITIALIZE_PASS_END(PruneEH, "prune-eh",
"Remove unused exception handling info", false, false)
Pass *llvm::createPruneEHPass() { return new PruneEH(); }
static bool runImpl(CallGraphSCC &SCC, CallGraph &CG) {
SmallPtrSet<CallGraphNode *, 8> SCCNodes;
bool MadeChange = false;
// Fill SCCNodes with the elements of the SCC. Used for quickly
// looking up whether a given CallGraphNode is in this SCC.
for (CallGraphNode *I : SCC)
SCCNodes.insert(I);
// First pass, scan all of the functions in the SCC, simplifying them
// according to what we know.
for (CallGraphNode *I : SCC)
if (Function *F = I->getFunction())
MadeChange |= SimplifyFunction(F, CG);
// Next, check to see if any callees might throw or if there are any external
// functions in this SCC: if so, we cannot prune any functions in this SCC.
// Definitions that are weak and not declared non-throwing might be
// overridden at linktime with something that throws, so assume that.
// If this SCC includes the unwind instruction, we KNOW it throws, so
// obviously the SCC might throw.
//
bool SCCMightUnwind = false, SCCMightReturn = false;
for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
(!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
Function *F = (*I)->getFunction();
if (!F) {
SCCMightUnwind = true;
SCCMightReturn = true;
} else if (!F->hasExactDefinition()) {
SCCMightUnwind |= !F->doesNotThrow();
SCCMightReturn |= !F->doesNotReturn();
} else {
bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
// Determine if we should scan for InlineAsm in a naked function as it
// is the only way to return without a ReturnInst. Only do this for
// no-inline functions as functions which may be inlined cannot
// meaningfully return via assembly.
bool CheckReturnViaAsm = CheckReturn &&
F->hasFnAttribute(Attribute::Naked) &&
F->hasFnAttribute(Attribute::NoInline);
if (!CheckUnwind && !CheckReturn)
continue;
for (const BasicBlock &BB : *F) {
const Instruction *TI = BB.getTerminator();
if (CheckUnwind && TI->mayThrow()) {
SCCMightUnwind = true;
} else if (CheckReturn && isa<ReturnInst>(TI)) {
SCCMightReturn = true;
}
for (const Instruction &I : BB) {
if ((!CheckUnwind || SCCMightUnwind) &&
(!CheckReturnViaAsm || SCCMightReturn))
break;
// Check to see if this function performs an unwind or calls an
// unwinding function.
if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
bool InstMightUnwind = true;
if (const auto *CI = dyn_cast<CallInst>(&I)) {
if (Function *Callee = CI->getCalledFunction()) {
CallGraphNode *CalleeNode = CG[Callee];
// If the callee is outside our current SCC then we may throw
// because it might. If it is inside, do nothing.
if (SCCNodes.count(CalleeNode) > 0)
InstMightUnwind = false;
}
}
SCCMightUnwind |= InstMightUnwind;
}
if (CheckReturnViaAsm && !SCCMightReturn)
if (auto ICS = ImmutableCallSite(&I))
if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
if (IA->hasSideEffects())
SCCMightReturn = true;
}
if (SCCMightUnwind && SCCMightReturn)
break;
}
}
}
// If the SCC doesn't unwind or doesn't throw, note this fact.
if (!SCCMightUnwind || !SCCMightReturn)
for (CallGraphNode *I : SCC) {
Function *F = I->getFunction();
if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
F->addFnAttr(Attribute::NoUnwind);
MadeChange = true;
}
if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
F->addFnAttr(Attribute::NoReturn);
MadeChange = true;
}
}
for (CallGraphNode *I : SCC) {
// Convert any invoke instructions to non-throwing functions in this node
// into call instructions with a branch. This makes the exception blocks
// dead.
if (Function *F = I->getFunction())
MadeChange |= SimplifyFunction(F, CG);
}
return MadeChange;
}
bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
if (skipSCC(SCC))
return false;
CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
return runImpl(SCC, CG);
}
// SimplifyFunction - Given information about callees, simplify the specified
// function if we have invokes to non-unwinding functions or code after calls to
// no-return functions.
static bool SimplifyFunction(Function *F, CallGraph &CG) {
bool MadeChange = false;
for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
BasicBlock *UnwindBlock = II->getUnwindDest();
removeUnwindEdge(&*BB);
// If the unwind block is now dead, nuke it.
if (pred_empty(UnwindBlock))
DeleteBasicBlock(UnwindBlock, CG); // Delete the new BB.
++NumRemoved;
MadeChange = true;
}
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
if (CallInst *CI = dyn_cast<CallInst>(I++))
if (CI->doesNotReturn() && !CI->isMustTailCall() &&
!isa<UnreachableInst>(I)) {
// This call calls a function that cannot return. Insert an
// unreachable instruction after it and simplify the code. Do this
// by splitting the BB, adding the unreachable, then deleting the
// new BB.
BasicBlock *New = BB->splitBasicBlock(I);
// Remove the uncond branch and add an unreachable.
BB->getInstList().pop_back();
new UnreachableInst(BB->getContext(), &*BB);
DeleteBasicBlock(New, CG); // Delete the new BB.
MadeChange = true;
++NumUnreach;
break;
}
}
return MadeChange;
}
/// DeleteBasicBlock - remove the specified basic block from the program,
/// updating the callgraph to reflect any now-obsolete edges due to calls that
/// exist in the BB.
static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG) {
assert(pred_empty(BB) && "BB is not dead!");
Instruction *TokenInst = nullptr;
CallGraphNode *CGN = CG[BB->getParent()];
for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
--I;
if (I->getType()->isTokenTy()) {
TokenInst = &*I;
break;
}
if (auto *Call = dyn_cast<CallBase>(&*I)) {
const Function *Callee = Call->getCalledFunction();
if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID()))
CGN->removeCallEdgeFor(*Call);
else if (!Callee->isIntrinsic())
CGN->removeCallEdgeFor(*Call);
}
if (!I->use_empty())
I->replaceAllUsesWith(UndefValue::get(I->getType()));
}
if (TokenInst) {
if (!TokenInst->isTerminator())
changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false);
} else {
// Get the list of successors of this block.
std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
for (unsigned i = 0, e = Succs.size(); i != e; ++i)
Succs[i]->removePredecessor(BB);
BB->eraseFromParent();
}
}