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by inserting unreachable after no-return calls. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44099 91177308-0d34-0410-b5e6-96231b3b80d8
220 lines
7.2 KiB
C++
220 lines
7.2 KiB
C++
//===- SimplifyCFG.cpp - CFG Simplification Pass --------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source 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 dead code elimination and basic block merging.
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// Specifically:
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//
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// * Removes basic blocks with no predecessors.
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// * Merges a basic block into its predecessor if there is only one and the
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// predecessor only has one successor.
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// * Eliminates PHI nodes for basic blocks with a single predecessor.
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// * Eliminates a basic block that only contains an unconditional branch.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "simplifycfg"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Module.h"
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#include "llvm/ParameterAttributes.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Pass.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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using namespace llvm;
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STATISTIC(NumSimpl, "Number of blocks simplified");
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namespace {
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struct VISIBILITY_HIDDEN CFGSimplifyPass : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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CFGSimplifyPass() : FunctionPass((intptr_t)&ID) {}
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virtual bool runOnFunction(Function &F);
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};
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char CFGSimplifyPass::ID = 0;
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RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
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}
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// Public interface to the CFGSimplification pass
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FunctionPass *llvm::createCFGSimplificationPass() {
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return new CFGSimplifyPass();
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}
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/// ChangeToUnreachable - Insert an unreachable instruction before the specified
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/// instruction, making it and the rest of the code in the block dead.
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static void ChangeToUnreachable(Instruction *I) {
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BasicBlock *BB = I->getParent();
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// Loop over all of the successors, removing BB's entry from any PHI
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// nodes.
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
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(*SI)->removePredecessor(BB);
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new UnreachableInst(I);
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// All instructions after this are dead.
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BasicBlock::iterator BBI = I, BBE = BB->end();
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while (BBI != BBE) {
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if (!BBI->use_empty())
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BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
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BB->getInstList().erase(BBI++);
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}
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}
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/// IsNoReturn - Return true if the specified call is to a no-return function.
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static bool IsNoReturn(const CallInst *CI) {
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if (const ParamAttrsList *Attrs = CI->getParamAttrs())
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if (Attrs->paramHasAttr(0, ParamAttr::NoReturn))
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return true;
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if (const Function *Callee = CI->getCalledFunction()) {
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if (const ParamAttrsList *Attrs = Callee->getParamAttrs())
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if (Attrs->paramHasAttr(0, ParamAttr::NoReturn))
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return true;
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const FunctionType *FT = Callee->getFunctionType();
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if (const ParamAttrsList *Attrs = FT->getParamAttrs())
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if (Attrs->paramHasAttr(0, ParamAttr::NoReturn))
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return true;
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}
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return false;
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}
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static bool MarkAliveBlocks(BasicBlock *BB,
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SmallPtrSet<BasicBlock*, 128> &Reachable) {
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SmallVector<BasicBlock*, 128> Worklist;
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Worklist.push_back(BB);
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bool Changed = false;
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while (!Worklist.empty()) {
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BB = Worklist.back();
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Worklist.pop_back();
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if (!Reachable.insert(BB))
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continue;
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// Do a quick scan of the basic block, turning any obviously unreachable
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// instructions into LLVM unreachable insts. The instruction combining pass
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// canonnicalizes unreachable insts into stores to null or undef.
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for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
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if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
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if (IsNoReturn(CI)) {
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// If we found a call to a no-return function, insert an unreachable
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// instruction after it. Make sure there isn't *already* one there
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// though.
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++BBI;
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if (!isa<UnreachableInst>(BBI)) {
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ChangeToUnreachable(BBI);
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Changed = true;
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}
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break;
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}
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}
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if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
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if (isa<ConstantPointerNull>(SI->getOperand(1)) ||
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isa<UndefValue>(SI->getOperand(1))) {
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ChangeToUnreachable(SI);
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Changed = true;
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break;
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}
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}
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Changed |= ConstantFoldTerminator(BB);
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
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Worklist.push_back(*SI);
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}
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return Changed;
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}
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/// RemoveUnreachableBlocks - Remove blocks that are not reachable, even if they
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/// are in a dead cycle. Return true if a change was made, false otherwise.
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static bool RemoveUnreachableBlocks(Function &F) {
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SmallPtrSet<BasicBlock*, 128> Reachable;
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bool Changed = MarkAliveBlocks(F.begin(), Reachable);
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// If there are unreachable blocks in the CFG...
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if (Reachable.size() == F.size())
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return Changed;
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assert(Reachable.size() < F.size());
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NumSimpl += F.size()-Reachable.size();
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// Loop over all of the basic blocks that are not reachable, dropping all of
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// their internal references...
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for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB)
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if (!Reachable.count(BB)) {
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI!=SE; ++SI)
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if (Reachable.count(*SI))
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(*SI)->removePredecessor(BB);
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BB->dropAllReferences();
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}
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for (Function::iterator I = ++F.begin(); I != F.end();)
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if (!Reachable.count(I))
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I = F.getBasicBlockList().erase(I);
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else
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++I;
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return true;
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}
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/// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
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/// iterating until no more changes are made.
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static bool IterativeSimplifyCFG(Function &F) {
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bool Changed = false;
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bool LocalChange = true;
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while (LocalChange) {
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LocalChange = false;
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// Loop over all of the basic blocks (except the first one) and remove them
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// if they are unneeded...
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//
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for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
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if (SimplifyCFG(BBIt++)) {
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LocalChange = true;
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++NumSimpl;
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}
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}
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Changed |= LocalChange;
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}
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return Changed;
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}
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// It is possible that we may require multiple passes over the code to fully
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// simplify the CFG.
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//
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bool CFGSimplifyPass::runOnFunction(Function &F) {
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bool EverChanged = RemoveUnreachableBlocks(F);
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EverChanged |= IterativeSimplifyCFG(F);
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// If neither pass changed anything, we're done.
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if (!EverChanged) return false;
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// IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
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// RemoveUnreachableBlocks is needed to nuke them, which means we should
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// iterate between the two optimizations. We structure the code like this to
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// avoid reruning IterativeSimplifyCFG if the second pass of
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// RemoveUnreachableBlocks doesn't do anything.
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if (!RemoveUnreachableBlocks(F))
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return true;
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do {
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EverChanged = IterativeSimplifyCFG(F);
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EverChanged |= RemoveUnreachableBlocks(F);
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} while (EverChanged);
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return true;
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}
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