llvm-project/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

317 lines
11 KiB
C++

//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements dead code elimination and basic block merging, along
// with a collection of other peephole control flow optimizations. For example:
//
// * Removes basic blocks with no predecessors.
// * Merges a basic block into its predecessor if there is only one and the
// predecessor only has one successor.
// * Eliminates PHI nodes for basic blocks with a single predecessor.
// * Eliminates a basic block that only contains an unconditional branch.
// * Changes invoke instructions to nounwind functions to be calls.
// * Change things like "if (x) if (y)" into "if (x&y)".
// * etc..
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "simplifycfg"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Module.h"
#include "llvm/Attributes.h"
#include "llvm/Support/CFG.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumSimpl, "Number of blocks simplified");
namespace {
struct CFGSimplifyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
CFGSimplifyPass() : FunctionPass(&ID) {}
virtual bool runOnFunction(Function &F);
};
}
char CFGSimplifyPass::ID = 0;
static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
// Public interface to the CFGSimplification pass
FunctionPass *llvm::createCFGSimplificationPass() {
return new CFGSimplifyPass();
}
/// ChangeToUnreachable - Insert an unreachable instruction before the specified
/// instruction, making it and the rest of the code in the block dead.
static void ChangeToUnreachable(Instruction *I) {
BasicBlock *BB = I->getParent();
// Loop over all of the successors, removing BB's entry from any PHI
// nodes.
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
(*SI)->removePredecessor(BB);
new UnreachableInst(I->getContext(), I);
// All instructions after this are dead.
BasicBlock::iterator BBI = I, BBE = BB->end();
while (BBI != BBE) {
if (!BBI->use_empty())
BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
BB->getInstList().erase(BBI++);
}
}
/// ChangeToCall - Convert the specified invoke into a normal call.
static void ChangeToCall(InvokeInst *II) {
BasicBlock *BB = II->getParent();
SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
Args.end(), "", II);
NewCall->takeName(II);
NewCall->setCallingConv(II->getCallingConv());
NewCall->setAttributes(II->getAttributes());
II->replaceAllUsesWith(NewCall);
// Follow the call by a branch to the normal destination.
BranchInst::Create(II->getNormalDest(), II);
// Update PHI nodes in the unwind destination
II->getUnwindDest()->removePredecessor(BB);
BB->getInstList().erase(II);
}
static bool MarkAliveBlocks(BasicBlock *BB,
SmallPtrSet<BasicBlock*, 128> &Reachable) {
SmallVector<BasicBlock*, 128> Worklist;
Worklist.push_back(BB);
bool Changed = false;
do {
BB = Worklist.pop_back_val();
if (!Reachable.insert(BB))
continue;
// Do a quick scan of the basic block, turning any obviously unreachable
// instructions into LLVM unreachable insts. The instruction combining pass
// canonicalizes unreachable insts into stores to null or undef.
for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
if (CI->doesNotReturn()) {
// If we found a call to a no-return function, insert an unreachable
// instruction after it. Make sure there isn't *already* one there
// though.
++BBI;
if (!isa<UnreachableInst>(BBI)) {
ChangeToUnreachable(BBI);
Changed = true;
}
break;
}
}
// Store to undef and store to null are undefined and used to signal that
// they should be changed to unreachable by passes that can't modify the
// CFG.
if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
Value *Ptr = SI->getOperand(1);
if (isa<UndefValue>(Ptr) ||
(isa<ConstantPointerNull>(Ptr) &&
SI->getPointerAddressSpace() == 0)) {
ChangeToUnreachable(SI);
Changed = true;
break;
}
}
}
// Turn invokes that call 'nounwind' functions into ordinary calls.
if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
if (II->doesNotThrow()) {
ChangeToCall(II);
Changed = true;
}
Changed |= ConstantFoldTerminator(BB);
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
Worklist.push_back(*SI);
} while (!Worklist.empty());
return Changed;
}
/// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
/// if they are in a dead cycle. Return true if a change was made, false
/// otherwise.
static bool RemoveUnreachableBlocksFromFn(Function &F) {
SmallPtrSet<BasicBlock*, 128> Reachable;
bool Changed = MarkAliveBlocks(F.begin(), Reachable);
// If there are unreachable blocks in the CFG...
if (Reachable.size() == F.size())
return Changed;
assert(Reachable.size() < F.size());
NumSimpl += F.size()-Reachable.size();
// Loop over all of the basic blocks that are not reachable, dropping all of
// their internal references...
for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
if (Reachable.count(BB))
continue;
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
if (Reachable.count(*SI))
(*SI)->removePredecessor(BB);
BB->dropAllReferences();
}
for (Function::iterator I = ++F.begin(); I != F.end();)
if (!Reachable.count(I))
I = F.getBasicBlockList().erase(I);
else
++I;
return true;
}
/// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
/// node) return blocks, merge them together to promote recursive block merging.
static bool MergeEmptyReturnBlocks(Function &F) {
bool Changed = false;
BasicBlock *RetBlock = 0;
// Scan all the blocks in the function, looking for empty return blocks.
for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
BasicBlock &BB = *BBI++;
// Only look at return blocks.
ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
if (Ret == 0) continue;
// Only look at the block if it is empty or the only other thing in it is a
// single PHI node that is the operand to the return.
if (Ret != &BB.front()) {
// Check for something else in the block.
BasicBlock::iterator I = Ret;
--I;
// Skip over debug info.
while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
--I;
if (!isa<DbgInfoIntrinsic>(I) &&
(!isa<PHINode>(I) || I != BB.begin() ||
Ret->getNumOperands() == 0 ||
Ret->getOperand(0) != I))
continue;
}
// If this is the first returning block, remember it and keep going.
if (RetBlock == 0) {
RetBlock = &BB;
continue;
}
// Otherwise, we found a duplicate return block. Merge the two.
Changed = true;
// Case when there is no input to the return or when the returned values
// agree is trivial. Note that they can't agree if there are phis in the
// blocks.
if (Ret->getNumOperands() == 0 ||
Ret->getOperand(0) ==
cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
BB.replaceAllUsesWith(RetBlock);
BB.eraseFromParent();
continue;
}
// If the canonical return block has no PHI node, create one now.
PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
if (RetBlockPHI == 0) {
Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
&RetBlock->front());
for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
PI != E; ++PI)
RetBlockPHI->addIncoming(InVal, *PI);
RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
}
// Turn BB into a block that just unconditionally branches to the return
// block. This handles the case when the two return blocks have a common
// predecessor but that return different things.
RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
BB.getTerminator()->eraseFromParent();
BranchInst::Create(RetBlock, &BB);
}
return Changed;
}
/// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
/// iterating until no more changes are made.
static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
bool Changed = false;
bool LocalChange = true;
while (LocalChange) {
LocalChange = false;
// Loop over all of the basic blocks (except the first one) and remove them
// if they are unneeded...
//
for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
if (SimplifyCFG(BBIt++, TD)) {
LocalChange = true;
++NumSimpl;
}
}
Changed |= LocalChange;
}
return Changed;
}
// It is possible that we may require multiple passes over the code to fully
// simplify the CFG.
//
bool CFGSimplifyPass::runOnFunction(Function &F) {
const TargetData *TD = getAnalysisIfAvailable<TargetData>();
bool EverChanged = RemoveUnreachableBlocksFromFn(F);
EverChanged |= MergeEmptyReturnBlocks(F);
EverChanged |= IterativeSimplifyCFG(F, TD);
// If neither pass changed anything, we're done.
if (!EverChanged) return false;
// IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
// RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
// iterate between the two optimizations. We structure the code like this to
// avoid reruning IterativeSimplifyCFG if the second pass of
// RemoveUnreachableBlocksFromFn doesn't do anything.
if (!RemoveUnreachableBlocksFromFn(F))
return true;
do {
EverChanged = IterativeSimplifyCFG(F, TD);
EverChanged |= RemoveUnreachableBlocksFromFn(F);
} while (EverChanged);
return true;
}