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//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
//
// 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 forwards branches to unconditional branches to make them branch
// directly to the target block. This pass often results in dead MBB's, which
// it then removes.
//
// Note that this pass must be run after register allocation, it cannot handle
// SSA form.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/STLExtras.h"
using namespace llvm;
namespace {
struct BranchFolder : public MachineFunctionPass {
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual const char *getPassName() const { return "Branch Folder"; }
private:
bool OptimizeBlock(MachineFunction::iterator MBB,
const TargetInstrInfo &TII);
bool isUncondBranch(const MachineInstr *MI, const TargetInstrInfo &TII) {
return TII.isBarrier(MI->getOpcode()) && TII.isBranch(MI->getOpcode());
}
bool isCondBranch(const MachineInstr *MI, const TargetInstrInfo &TII) {
return TII.isBranch(MI->getOpcode()) && !TII.isBarrier(MI->getOpcode());
}
};
}
FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); }
bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
bool EverMadeChange = false;
bool MadeChange = true;
const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
while (MadeChange) {
MadeChange = false;
for (MachineFunction::iterator MBB = ++MF.begin(), E = MF.end(); MBB != E;
++MBB)
MadeChange |= OptimizeBlock(MBB, TII);
// If branches were folded away somehow, do a quick scan and delete any dead
// blocks.
if (MadeChange) {
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
MachineBasicBlock *MBB = I++;
// Is it dead?
if (MBB->pred_empty()) {
// drop all successors.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end()-1);
MF.getBasicBlockList().erase(MBB);
}
}
}
EverMadeChange |= MadeChange;
}
return EverMadeChange;
}
/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
/// 'Old', change the code and CFG so that it branches to 'New' instead.
static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
MachineBasicBlock *Old,
MachineBasicBlock *New,
const TargetInstrInfo &TII) {
assert(Old != New && "Cannot replace self with self!");
MachineBasicBlock::iterator I = BB->end();
while (I != BB->begin()) {
--I;
if (!TII.isTerminatorInstr(I->getOpcode())) break;
// Scan the operands of this machine instruction, replacing any uses of Old
// with New.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (I->getOperand(i).isMachineBasicBlock() &&
I->getOperand(i).getMachineBasicBlock() == Old)
I->getOperand(i).setMachineBasicBlock(New);
}
// If BB falls through into Old, insert an unconditional branch to New.
MachineFunction::iterator BBSucc = BB; ++BBSucc;
if (BBSucc != BB->getParent()->end() && &*BBSucc == Old)
TII.insertGoto(*BB, *New);
std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end());
for (int i = Succs.size()-1; i >= 0; --i)
if (Succs[i] == Old) {
BB->removeSuccessor(Old);
BB->addSuccessor(New);
}
}
bool BranchFolder::OptimizeBlock(MachineFunction::iterator MBB,
const TargetInstrInfo &TII) {
// If this block is empty, make everyone use it's fall-through, not the block
// explicitly.
if (MBB->empty()) {
if (MBB->pred_empty()) return false;
MachineFunction::iterator FallThrough =next(MBB);
assert(FallThrough != MBB->getParent()->end() &&
"Fell off the end of the function!");
while (!MBB->pred_empty()) {
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII);
}
return true;
}
if (MBB->pred_size() == 1) {
// If this block has a single predecessor, and if that block has a single
// successor, merge this block into that block.
MachineBasicBlock *Pred = *MBB->pred_begin();
if (Pred->succ_size() == 1) {
// Delete all of the terminators from end of the pred block. NOTE, this
// assumes that terminators do not have side effects!
while (!Pred->empty() && TII.isTerminatorInstr(Pred->back().getOpcode()))
Pred->pop_back();
// Splice the instructions over.
Pred->splice(Pred->end(), MBB, MBB->begin(), MBB->end());
// If MBB does not end with a barrier, add a goto instruction to the end.
if (Pred->empty() || !TII.isBarrier(Pred->back().getOpcode()))
TII.insertGoto(*Pred, *next(MBB));
// Update the CFG now.
Pred->removeSuccessor(Pred->succ_begin());
while (!MBB->succ_empty()) {
Pred->addSuccessor(*(MBB->succ_end()-1));
MBB->removeSuccessor(MBB->succ_end()-1);
}
return true;
}
}
// If the first instruction in this block is an unconditional branch, and if
// there are predecessors, fold the branch into the predecessors.
if (!MBB->pred_empty() && isUncondBranch(MBB->begin(), TII)) {
MachineInstr *Br = MBB->begin();
assert(Br->getNumOperands() == 1 && Br->getOperand(0).isMachineBasicBlock()
&& "Uncond branch should take one MBB argument!");
MachineBasicBlock *Dest = Br->getOperand(0).getMachineBasicBlock();
while (!MBB->pred_empty()) {
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
ReplaceUsesOfBlockWith(Pred, MBB, Dest, TII);
}
return true;
}
// If the last instruction is an unconditional branch and the fall through
// block is the destination, just delete the branch.
if (isUncondBranch(--MBB->end(), TII)) {
MachineBasicBlock::iterator MI = --MBB->end();
MachineInstr *UncondBr = MI;
MachineFunction::iterator FallThrough = next(MBB);
MachineFunction::iterator UncondDest =
MI->getOperand(0).getMachineBasicBlock();
if (UncondDest == FallThrough) {
// Just delete the branch. This does not effect the CFG.
MBB->erase(UncondBr);
return true;
}
// Okay, so we don't have a fall-through. Check to see if we have an
// conditional branch that would be a fall through if we reversed it. If
// so, invert the condition and delete the uncond branch.
if (MI != MBB->begin() && isCondBranch(--MI, TII)) {
// We assume that conditional branches always have the branch dest as the
// last operand. This could be generalized in the future if needed.
unsigned LastOpnd = MI->getNumOperands()-1;
if (MachineFunction::iterator(
MI->getOperand(LastOpnd).getMachineBasicBlock()) == FallThrough) {
// Change the cond branch to go to the uncond dest, nuke the uncond,
// then reverse the condition.
MI->getOperand(LastOpnd).setMachineBasicBlock(UncondDest);
MBB->erase(UncondBr);
TII.reverseBranchCondition(MI);
return true;
}
}
}
return false;
}
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