llvm-project/llvm/lib/CodeGen/MachineLICM.cpp

342 lines
12 KiB
C++

//===-- MachineLICM.cpp - Machine Loop Invariant Code Motion Pass ---------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs loop invariant code motion on machine instructions. We
// attempt to remove as much code from the body of a loop as possible.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "machine-licm"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
STATISTIC(NumHoisted, "Number of machine instructions hoisted out of loops");
namespace {
class VISIBILITY_HIDDEN MachineLICM : public MachineFunctionPass {
const TargetMachine *TM;
const TargetInstrInfo *TII;
// Various analyses that we use...
MachineLoopInfo *LI; // Current MachineLoopInfo
MachineDominatorTree *DT; // Machine dominator tree for the cur loop
MachineRegisterInfo *RegInfo; // Machine register information
// State that is updated as we process loops
bool Changed; // True if a loop is changed.
MachineLoop *CurLoop; // The current loop we are working on.
public:
static char ID; // Pass identification, replacement for typeid
MachineLICM() : MachineFunctionPass(&ID) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
// FIXME: Loop preheaders?
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<MachineLoopInfo>();
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineLoopInfo>();
AU.addPreserved<MachineDominatorTree>();
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
/// VisitAllLoops - Visit all of the loops in depth first order and try to
/// hoist invariant instructions from them.
///
void VisitAllLoops(MachineLoop *L) {
const std::vector<MachineLoop*> &SubLoops = L->getSubLoops();
for (MachineLoop::iterator
I = SubLoops.begin(), E = SubLoops.end(); I != E; ++I) {
MachineLoop *ML = *I;
// Traverse the body of the loop in depth first order on the dominator
// tree so that we are guaranteed to see definitions before we see uses.
VisitAllLoops(ML);
HoistRegion(DT->getNode(ML->getHeader()));
}
HoistRegion(DT->getNode(L->getHeader()));
}
/// IsInSubLoop - A little predicate that returns true if the specified
/// basic block is in a subloop of the current one, not the current one
/// itself.
///
bool IsInSubLoop(MachineBasicBlock *BB) {
assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
return LI->getLoopFor(BB) != CurLoop;
}
/// IsLoopInvariantInst - Returns true if the instruction is loop
/// invariant. I.e., all virtual register operands are defined outside of
/// the loop, physical registers aren't accessed (explicitly or implicitly),
/// and the instruction is hoistable.
///
bool IsLoopInvariantInst(MachineInstr &I);
/// FindPredecessors - Get all of the predecessors of the loop that are not
/// back-edges.
///
void FindPredecessors(std::vector<MachineBasicBlock*> &Preds) {
const MachineBasicBlock *Header = CurLoop->getHeader();
for (MachineBasicBlock::const_pred_iterator
I = Header->pred_begin(), E = Header->pred_end(); I != E; ++I)
if (!CurLoop->contains(*I))
Preds.push_back(*I);
}
/// MoveInstToEndOfBlock - Moves the machine instruction to the bottom of
/// the predecessor basic block (but before the terminator instructions).
///
void MoveInstToEndOfBlock(MachineBasicBlock *ToMBB,
MachineBasicBlock *FromMBB,
MachineInstr *MI);
/// HoistRegion - Walk the specified region of the CFG (defined by all
/// blocks dominated by the specified block, and that are in the current
/// loop) in depth first order w.r.t the DominatorTree. This allows us to
/// visit definitions before uses, allowing us to hoist a loop body in one
/// pass without iteration.
///
void HoistRegion(MachineDomTreeNode *N);
/// Hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void Hoist(MachineInstr &MI);
};
} // end anonymous namespace
char MachineLICM::ID = 0;
static RegisterPass<MachineLICM>
X("machinelicm", "Machine Loop Invariant Code Motion");
FunctionPass *llvm::createMachineLICMPass() { return new MachineLICM(); }
/// Hoist expressions out of the specified loop. Note, alias info for inner loop
/// is not preserved so it is not a good idea to run LICM multiple times on one
/// loop.
///
bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
DOUT << "******** Machine LICM ********\n";
Changed = false;
TM = &MF.getTarget();
TII = TM->getInstrInfo();
RegInfo = &MF.getRegInfo();
// Get our Loop information...
LI = &getAnalysis<MachineLoopInfo>();
DT = &getAnalysis<MachineDominatorTree>();
for (MachineLoopInfo::iterator
I = LI->begin(), E = LI->end(); I != E; ++I) {
CurLoop = *I;
// Visit all of the instructions of the loop. We want to visit the subloops
// first, though, so that we can hoist their invariants first into their
// containing loop before we process that loop.
VisitAllLoops(CurLoop);
}
return Changed;
}
/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree. This allows us to visit definitions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
///
void MachineLICM::HoistRegion(MachineDomTreeNode *N) {
assert(N != 0 && "Null dominator tree node?");
MachineBasicBlock *BB = N->getBlock();
// If this subregion is not in the top level loop at all, exit.
if (!CurLoop->contains(BB)) return;
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (!IsInSubLoop(BB))
for (MachineBasicBlock::iterator
I = BB->begin(), E = BB->end(); I != E; ) {
MachineInstr &MI = *I++;
// Try hoisting the instruction out of the loop. We can only do this if
// all of the operands of the instruction are loop invariant and if it is
// safe to hoist the instruction.
Hoist(MI);
}
const std::vector<MachineDomTreeNode*> &Children = N->getChildren();
for (unsigned I = 0, E = Children.size(); I != E; ++I)
HoistRegion(Children[I]);
}
/// IsLoopInvariantInst - Returns true if the instruction is loop
/// invariant. I.e., all virtual register operands are defined outside of the
/// loop, physical registers aren't accessed explicitly, and there are no side
/// effects that aren't captured by the operands or other flags.
///
bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) {
const TargetInstrDesc &TID = I.getDesc();
// Ignore stuff that we obviously can't hoist.
if (TID.mayStore() || TID.isCall() || TID.isReturn() || TID.isBranch() ||
TID.hasUnmodeledSideEffects())
return false;
if (TID.mayLoad()) {
// Okay, this instruction does a load. As a refinement, we allow the target
// to decide whether the loaded value is actually a constant. If so, we can
// actually use it as a load.
if (!TII->isInvariantLoad(&I))
// FIXME: we should be able to sink loads with no other side effects if
// there is nothing that can change memory from here until the end of
// block. This is a trivial form of alias analysis.
return false;
}
DEBUG({
DOUT << "--- Checking if we can hoist " << I;
if (I.getDesc().getImplicitUses()) {
DOUT << " * Instruction has implicit uses:\n";
const TargetRegisterInfo *TRI = TM->getRegisterInfo();
for (const unsigned *ImpUses = I.getDesc().getImplicitUses();
*ImpUses; ++ImpUses)
DOUT << " -> " << TRI->getName(*ImpUses) << "\n";
}
if (I.getDesc().getImplicitDefs()) {
DOUT << " * Instruction has implicit defines:\n";
const TargetRegisterInfo *TRI = TM->getRegisterInfo();
for (const unsigned *ImpDefs = I.getDesc().getImplicitDefs();
*ImpDefs; ++ImpDefs)
DOUT << " -> " << TRI->getName(*ImpDefs) << "\n";
}
});
if (I.getDesc().getImplicitDefs() || I.getDesc().getImplicitUses()) {
DOUT << "Cannot hoist with implicit defines or uses\n";
return false;
}
// The instruction is loop invariant if all of its operands are.
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
const MachineOperand &MO = I.getOperand(i);
if (!MO.isRegister())
continue;
if (MO.isDef() && TargetRegisterInfo::isPhysicalRegister(MO.getReg()))
// Don't hoist an instruction that defines a physical register.
return false;
if (!MO.isUse())
continue;
unsigned Reg = MO.getReg();
if (Reg == 0) continue;
// Don't hoist instructions that access physical registers.
if (TargetRegisterInfo::isPhysicalRegister(Reg))
return false;
assert(RegInfo->getVRegDef(Reg) &&
"Machine instr not mapped for this vreg?!");
// If the loop contains the definition of an operand, then the instruction
// isn't loop invariant.
if (CurLoop->contains(RegInfo->getVRegDef(Reg)->getParent()))
return false;
}
// If we got this far, the instruction is loop invariant!
return true;
}
/// MoveInstToEndOfBlock - Moves the machine instruction to the bottom of the
/// predecessor basic block (but before the terminator instructions).
///
void MachineLICM::MoveInstToEndOfBlock(MachineBasicBlock *ToMBB,
MachineBasicBlock *FromMBB,
MachineInstr *MI) {
DEBUG({
DOUT << "Hoisting " << *MI;
if (ToMBB->getBasicBlock())
DOUT << " to MachineBasicBlock "
<< ToMBB->getBasicBlock()->getName();
if (FromMBB->getBasicBlock())
DOUT << " from MachineBasicBlock "
<< FromMBB->getBasicBlock()->getName();
DOUT << "\n";
});
MachineBasicBlock::iterator WhereIter = ToMBB->getFirstTerminator();
MachineBasicBlock::iterator To, From = FromMBB->begin();
while (&*From != MI)
++From;
assert(From != FromMBB->end() && "Didn't find instr in BB!");
To = From;
ToMBB->splice(WhereIter, FromMBB, From, ++To);
++NumHoisted;
}
/// Hoist - When an instruction is found to use only loop invariant operands
/// that are safe to hoist, this instruction is called to do the dirty work.
///
void MachineLICM::Hoist(MachineInstr &MI) {
if (!IsLoopInvariantInst(MI)) return;
std::vector<MachineBasicBlock*> Preds;
// Non-back-edge predecessors.
FindPredecessors(Preds);
// Either we don't have any predecessors(?!) or we have more than one, which
// is forbidden.
if (Preds.empty() || Preds.size() != 1) return;
// Check that the predecessor is qualified to take the hoisted instruction.
// I.e., there is only one edge from the predecessor, and it's to the loop
// header.
MachineBasicBlock *MBB = Preds.front();
// FIXME: We are assuming at first that the basic block coming into this loop
// has only one successor. This isn't the case in general because we haven't
// broken critical edges or added preheaders.
if (MBB->succ_size() != 1) return;
assert(*MBB->succ_begin() == CurLoop->getHeader() &&
"The predecessor doesn't feed directly into the loop header!");
// Now move the instructions to the predecessor.
MoveInstToEndOfBlock(MBB, MI.getParent(), &MI);
Changed = true;
}