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