llvm-6502/lib/Transforms/Scalar/TailDuplication.cpp

249 lines
9.9 KiB
C++

//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs a limited form of tail duplication, intended to simplify
// CFGs by removing some unconditional branches. This pass is necessary to
// straighten out loops created by the C front-end, but also is capable of
// making other code nicer. After this pass is run, the CFG simplify pass
// should be run to clean up the mess.
//
// This pass could be enhanced in the future to use profile information to be
// more aggressive.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constant.h"
#include "llvm/Function.h"
#include "llvm/iPHINode.h"
#include "llvm/iTerminators.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/ValueHolder.h"
#include "llvm/Transforms/Utils/Local.h"
#include "Support/CommandLine.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
using namespace llvm;
namespace {
cl::opt<unsigned>
Threshold("taildup-threshold", cl::desc("Max block size to tail duplicate"),
cl::init(6), cl::Hidden);
Statistic<> NumEliminated("tailduplicate",
"Number of unconditional branches eliminated");
Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");
class TailDup : public FunctionPass {
bool runOnFunction(Function &F);
private:
inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
inline void eliminateUnconditionalBranch(BranchInst *BI);
};
RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
}
// Public interface to the Tail Duplication pass
Pass *llvm::createTailDuplicationPass() { return new TailDup(); }
/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
/// the function, eliminating it if it looks attractive enough.
///
bool TailDup::runOnFunction(Function &F) {
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; )
if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
Changed = true;
} else {
++I;
}
return Changed;
}
/// shouldEliminateUnconditionalBranch - Return true if this branch looks
/// attractive to eliminate. We eliminate the branch if the destination basic
/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
/// since one of these is a terminator instruction, this means that we will add
/// up to 4 instructions to the new block.
///
/// We don't count PHI nodes in the count since they will be removed when the
/// contents of the block are copied over.
///
bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
BranchInst *BI = dyn_cast<BranchInst>(TI);
if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch!
BasicBlock *Dest = BI->getSuccessor(0);
if (Dest == BI->getParent()) return false; // Do not loop infinitely!
// Do not inline a block if we will just get another branch to the same block!
TerminatorInst *DTI = Dest->getTerminator();
if (BranchInst *DBI = dyn_cast<BranchInst>(DTI))
if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
return false; // Do not loop infinitely!
// FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack,
// because doing so would require breaking critical edges. This should be
// fixed eventually.
if (!DTI->use_empty())
return false;
// Do not bother working on dead blocks...
pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
if (PI == PE && Dest != Dest->getParent()->begin())
return false; // It's just a dead block, ignore it...
// Also, do not bother with blocks with only a single predecessor: simplify
// CFG will fold these two blocks together!
++PI;
if (PI == PE) return false; // Exactly one predecessor!
BasicBlock::iterator I = Dest->begin();
while (isa<PHINode>(*I)) ++I;
for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
if (Size == Threshold) return false; // The block is too large...
// Do not tail duplicate a block that has thousands of successors into a block
// with a single successor if the block has many other predecessors. This can
// cause an N^2 explosion in CFG edges (and PHI node entries), as seen in
// cases that have a large number of indirect gotos.
if (DTI->getNumSuccessors() > 8)
if (std::distance(PI, PE) * DTI->getNumSuccessors() > 128)
return false;
return true;
}
/// eliminateUnconditionalBranch - Clone the instructions from the destination
/// block into the source block, eliminating the specified unconditional branch.
/// If the destination block defines values used by successors of the dest
/// block, we may need to insert PHI nodes.
///
void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
BasicBlock *SourceBlock = Branch->getParent();
BasicBlock *DestBlock = Branch->getSuccessor(0);
assert(SourceBlock != DestBlock && "Our predicate is broken!");
DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
<< "]: Eliminating branch: " << *Branch);
// Tail duplication can not update SSA properties correctly if the values
// defined in the duplicated tail are used outside of the tail itself. For
// this reason, we spill all values that are used outside of the tail to the
// stack.
for (BasicBlock::iterator I = DestBlock->begin(); I != DestBlock->end(); ++I)
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
++UI) {
bool ShouldDemote = false;
if (cast<Instruction>(*UI)->getParent() != DestBlock) {
// We must allow our successors to use tail values in their PHI nodes
// (if the incoming value corresponds to the tail block).
if (PHINode *PN = dyn_cast<PHINode>(*UI)) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == I &&
PN->getIncomingBlock(i) != DestBlock) {
ShouldDemote = true;
break;
}
} else {
ShouldDemote = true;
}
} else if (PHINode *PN = dyn_cast<PHINode>(cast<Instruction>(*UI))) {
// If the user of this instruction is a PHI node in the current block,
// which has an entry from another block using the value, spill it.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == I &&
PN->getIncomingBlock(i) != DestBlock) {
ShouldDemote = true;
break;
}
}
if (ShouldDemote) {
// We found a use outside of the tail. Create a new stack slot to
// break this inter-block usage pattern.
DemoteRegToStack(*I);
break;
}
}
// We are going to have to map operands from the original block B to the new
// copy of the block B'. If there are PHI nodes in the DestBlock, these PHI
// nodes also define part of this mapping. Loop over these PHI nodes, adding
// them to our mapping.
//
std::map<Value*, Value*> ValueMapping;
BasicBlock::iterator BI = DestBlock->begin();
bool HadPHINodes = isa<PHINode>(BI);
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);
// Clone the non-phi instructions of the dest block into the source block,
// keeping track of the mapping...
//
for (; BI != DestBlock->end(); ++BI) {
Instruction *New = BI->clone();
New->setName(BI->getName());
SourceBlock->getInstList().push_back(New);
ValueMapping[BI] = New;
}
// Now that we have built the mapping information and cloned all of the
// instructions (giving us a new terminator, among other things), walk the new
// instructions, rewriting references of old instructions to use new
// instructions.
//
BI = Branch; ++BI; // Get an iterator to the first new instruction
for (; BI != SourceBlock->end(); ++BI)
for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
if (Value *Remapped = ValueMapping[BI->getOperand(i)])
BI->setOperand(i, Remapped);
// Next we check to see if any of the successors of DestBlock had PHI nodes.
// If so, we need to add entries to the PHI nodes for SourceBlock now.
for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
SI != SE; ++SI) {
BasicBlock *Succ = *SI;
for (BasicBlock::iterator PNI = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
// Ok, we have a PHI node. Figure out what the incoming value was for the
// DestBlock.
Value *IV = PN->getIncomingValueForBlock(DestBlock);
// Remap the value if necessary...
if (Value *MappedIV = ValueMapping[IV])
IV = MappedIV;
PN->addIncoming(IV, SourceBlock);
}
}
// Next, remove the old branch instruction, and any PHI node entries that we
// had.
BI = Branch; ++BI; // Get an iterator to the first new instruction
DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...
// Final step: now that we have finished everything up, walk the cloned
// instructions one last time, constant propagating and DCE'ing them, because
// they may not be needed anymore.
//
if (HadPHINodes)
while (BI != SourceBlock->end())
if (!dceInstruction(BI) && !doConstantPropagation(BI))
++BI;
++NumEliminated; // We just killed a branch!
}