llvm-6502/lib/CodeGen/ShrinkWrap.cpp
Matthias Braun e4603f0daf MachineInstr: Change return value of getOpcode() to unsigned.
This was previously returning int. However there are no negative opcode
numbers and more importantly this was needlessly different from
MCInstrDesc::getOpcode() (which even is the value returned here) and
SDValue::getOpcode()/SDNode::getOpcode().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237611 91177308-0d34-0410-b5e6-96231b3b80d8
2015-05-18 20:27:55 +00:00

384 lines
13 KiB
C++

//===-- ShrinkWrap.cpp - Compute safe point for prolog/epilog insertion ---===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass looks for safe point where the prologue and epilogue can be
// inserted.
// The safe point for the prologue (resp. epilogue) is called Save
// (resp. Restore).
// A point is safe for prologue (resp. epilogue) if and only if
// it 1) dominates (resp. post-dominates) all the frame related operations and
// between 2) two executions of the Save (resp. Restore) point there is an
// execution of the Restore (resp. Save) point.
//
// For instance, the following points are safe:
// for (int i = 0; i < 10; ++i) {
// Save
// ...
// Restore
// }
// Indeed, the execution looks like Save -> Restore -> Save -> Restore ...
// And the following points are not:
// for (int i = 0; i < 10; ++i) {
// Save
// ...
// }
// for (int i = 0; i < 10; ++i) {
// ...
// Restore
// }
// Indeed, the execution looks like Save -> Save -> ... -> Restore -> Restore.
//
// This pass also ensures that the safe points are 3) cheaper than the regular
// entry and exits blocks.
//
// Property #1 is ensured via the use of MachineDominatorTree and
// MachinePostDominatorTree.
// Property #2 is ensured via property #1 and MachineLoopInfo, i.e., both
// points must be in the same loop.
// Property #3 is ensured via the MachineBlockFrequencyInfo.
//
// If this pass found points matching all this properties, then
// MachineFrameInfo is updated this that information.
//===----------------------------------------------------------------------===//
#include "llvm/ADT/Statistic.h"
// To check for profitability.
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
// For property #1 for Save.
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
// To record the result of the analysis.
#include "llvm/CodeGen/MachineFrameInfo.h"
// For property #2.
#include "llvm/CodeGen/MachineLoopInfo.h"
// For property #1 for Restore.
#include "llvm/CodeGen/MachinePostDominators.h"
#include "llvm/CodeGen/Passes.h"
// To know about callee-saved.
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/Support/Debug.h"
// To know about frame setup operation.
#include "llvm/Target/TargetInstrInfo.h"
// To access TargetInstrInfo.
#include "llvm/Target/TargetSubtargetInfo.h"
#define DEBUG_TYPE "shrink-wrap"
using namespace llvm;
STATISTIC(NumFunc, "Number of functions");
STATISTIC(NumCandidates, "Number of shrink-wrapping candidates");
STATISTIC(NumCandidatesDropped,
"Number of shrink-wrapping candidates dropped because of frequency");
namespace {
/// \brief Class to determine where the safe point to insert the
/// prologue and epilogue are.
/// Unlike the paper from Fred C. Chow, PLDI'88, that introduces the
/// shrink-wrapping term for prologue/epilogue placement, this pass
/// does not rely on expensive data-flow analysis. Instead we use the
/// dominance properties and loop information to decide which point
/// are safe for such insertion.
class ShrinkWrap : public MachineFunctionPass {
/// Hold callee-saved information.
RegisterClassInfo RCI;
MachineDominatorTree *MDT;
MachinePostDominatorTree *MPDT;
/// Current safe point found for the prologue.
/// The prologue will be inserted before the first instruction
/// in this basic block.
MachineBasicBlock *Save;
/// Current safe point found for the epilogue.
/// The epilogue will be inserted before the first terminator instruction
/// in this basic block.
MachineBasicBlock *Restore;
/// Hold the information of the basic block frequency.
/// Use to check the profitability of the new points.
MachineBlockFrequencyInfo *MBFI;
/// Hold the loop information. Used to determine if Save and Restore
/// are in the same loop.
MachineLoopInfo *MLI;
/// Frequency of the Entry block.
uint64_t EntryFreq;
/// Current opcode for frame setup.
unsigned FrameSetupOpcode;
/// Current opcode for frame destroy.
unsigned FrameDestroyOpcode;
/// Entry block.
const MachineBasicBlock *Entry;
/// \brief Check if \p MI uses or defines a callee-saved register or
/// a frame index. If this is the case, this means \p MI must happen
/// after Save and before Restore.
bool useOrDefCSROrFI(const MachineInstr &MI) const;
/// \brief Update the Save and Restore points such that \p MBB is in
/// the region that is dominated by Save and post-dominated by Restore
/// and Save and Restore still match the safe point definition.
/// Such point may not exist and Save and/or Restore may be null after
/// this call.
void updateSaveRestorePoints(MachineBasicBlock &MBB);
/// \brief Initialize the pass for \p MF.
void init(MachineFunction &MF) {
RCI.runOnMachineFunction(MF);
MDT = &getAnalysis<MachineDominatorTree>();
MPDT = &getAnalysis<MachinePostDominatorTree>();
Save = nullptr;
Restore = nullptr;
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MLI = &getAnalysis<MachineLoopInfo>();
EntryFreq = MBFI->getEntryFreq();
const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
FrameSetupOpcode = TII.getCallFrameSetupOpcode();
FrameDestroyOpcode = TII.getCallFrameDestroyOpcode();
Entry = &MF.front();
++NumFunc;
}
/// Check whether or not Save and Restore points are still interesting for
/// shrink-wrapping.
bool ArePointsInteresting() const { return Save != Entry && Save && Restore; }
public:
static char ID;
ShrinkWrap() : MachineFunctionPass(ID) {
initializeShrinkWrapPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<MachinePostDominatorTree>();
AU.addRequired<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
const char *getPassName() const override {
return "Shrink Wrapping analysis";
}
/// \brief Perform the shrink-wrapping analysis and update
/// the MachineFrameInfo attached to \p MF with the results.
bool runOnMachineFunction(MachineFunction &MF) override;
};
} // End anonymous namespace.
char ShrinkWrap::ID = 0;
char &llvm::ShrinkWrapID = ShrinkWrap::ID;
INITIALIZE_PASS_BEGIN(ShrinkWrap, "shrink-wrap", "Shrink Wrap Pass", false,
false)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_END(ShrinkWrap, "shrink-wrap", "Shrink Wrap Pass", false, false)
bool ShrinkWrap::useOrDefCSROrFI(const MachineInstr &MI) const {
if (MI.getOpcode() == FrameSetupOpcode ||
MI.getOpcode() == FrameDestroyOpcode) {
DEBUG(dbgs() << "Frame instruction: " << MI << '\n');
return true;
}
for (const MachineOperand &MO : MI.operands()) {
bool UseCSR = false;
if (MO.isReg()) {
unsigned PhysReg = MO.getReg();
if (!PhysReg)
continue;
assert(TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
"Unallocated register?!");
UseCSR = RCI.getLastCalleeSavedAlias(PhysReg);
}
// TODO: Handle regmask more accurately.
// For now, be conservative about them.
if (UseCSR || MO.isFI() || MO.isRegMask()) {
DEBUG(dbgs() << "Use or define CSR(" << UseCSR << ") or FI(" << MO.isFI()
<< "): " << MI << '\n');
return true;
}
}
return false;
}
/// \brief Helper function to find the immediate (post) dominator.
template <typename ListOfBBs, typename DominanceAnalysis>
MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
DominanceAnalysis &Dom) {
MachineBasicBlock *IDom = &Block;
for (MachineBasicBlock *BB : BBs) {
IDom = Dom.findNearestCommonDominator(IDom, BB);
if (!IDom)
break;
}
return IDom;
}
void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB) {
// Get rid of the easy cases first.
if (!Save)
Save = &MBB;
else
Save = MDT->findNearestCommonDominator(Save, &MBB);
if (!Save) {
DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
return;
}
if (!Restore)
Restore = &MBB;
else
Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
// Make sure we would be able to insert the restore code before the
// terminator.
if (Restore == &MBB) {
for (const MachineInstr &Terminator : MBB.terminators()) {
if (!useOrDefCSROrFI(Terminator))
continue;
// One of the terminator needs to happen before the restore point.
if (MBB.succ_empty()) {
Restore = nullptr;
break;
}
// Look for a restore point that post-dominates all the successors.
// The immediate post-dominator is what we are looking for.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
break;
}
}
if (!Restore) {
DEBUG(dbgs() << "Restore point needs to be spanned on several blocks\n");
return;
}
// Make sure Save and Restore are suitable for shrink-wrapping:
// 1. all path from Save needs to lead to Restore before exiting.
// 2. all path to Restore needs to go through Save from Entry.
// We achieve that by making sure that:
// A. Save dominates Restore.
// B. Restore post-dominates Save.
// C. Save and Restore are in the same loop.
bool SaveDominatesRestore = false;
bool RestorePostDominatesSave = false;
while (Save && Restore &&
(!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
!(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
MLI->getLoopFor(Save) != MLI->getLoopFor(Restore))) {
// Fix (A).
if (!SaveDominatesRestore) {
Save = MDT->findNearestCommonDominator(Save, Restore);
continue;
}
// Fix (B).
if (!RestorePostDominatesSave)
Restore = MPDT->findNearestCommonDominator(Restore, Save);
// Fix (C).
if (Save && Restore && Save != Restore &&
MLI->getLoopFor(Save) != MLI->getLoopFor(Restore)) {
if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore))
// Push Save outside of this loop.
Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
else
// Push Restore outside of this loop.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
}
}
}
bool ShrinkWrap::runOnMachineFunction(MachineFunction &MF) {
if (MF.empty())
return false;
DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
init(MF);
for (MachineBasicBlock &MBB : MF) {
DEBUG(dbgs() << "Look into: " << MBB.getNumber() << ' ' << MBB.getName()
<< '\n');
for (const MachineInstr &MI : MBB) {
if (!useOrDefCSROrFI(MI))
continue;
// Save (resp. restore) point must dominate (resp. post dominate)
// MI. Look for the proper basic block for those.
updateSaveRestorePoints(MBB);
// If we are at a point where we cannot improve the placement of
// save/restore instructions, just give up.
if (!ArePointsInteresting()) {
DEBUG(dbgs() << "No Shrink wrap candidate found\n");
return false;
}
// No need to look for other instructions, this basic block
// will already be part of the handled region.
break;
}
}
if (!ArePointsInteresting()) {
// If the points are not interesting at this point, then they must be null
// because it means we did not encounter any frame/CSR related code.
// Otherwise, we would have returned from the previous loop.
assert(!Save && !Restore && "We miss a shrink-wrap opportunity?!");
DEBUG(dbgs() << "Nothing to shrink-wrap\n");
return false;
}
DEBUG(dbgs() << "\n ** Results **\nFrequency of the Entry: " << EntryFreq
<< '\n');
do {
DEBUG(dbgs() << "Shrink wrap candidates (#, Name, Freq):\nSave: "
<< Save->getNumber() << ' ' << Save->getName() << ' '
<< MBFI->getBlockFreq(Save).getFrequency() << "\nRestore: "
<< Restore->getNumber() << ' ' << Restore->getName() << ' '
<< MBFI->getBlockFreq(Restore).getFrequency() << '\n');
bool IsSaveCheap;
if ((IsSaveCheap = EntryFreq >= MBFI->getBlockFreq(Save).getFrequency()) &&
EntryFreq >= MBFI->getBlockFreq(Restore).getFrequency())
break;
DEBUG(dbgs() << "New points are too expensive\n");
MachineBasicBlock *NewBB;
if (!IsSaveCheap) {
Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
if (!Save)
break;
NewBB = Save;
} else {
// Restore is expensive.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
if (!Restore)
break;
NewBB = Restore;
}
updateSaveRestorePoints(*NewBB);
} while (Save && Restore);
if (!ArePointsInteresting()) {
++NumCandidatesDropped;
return false;
}
DEBUG(dbgs() << "Final shrink wrap candidates:\nSave: " << Save->getNumber()
<< ' ' << Save->getName() << "\nRestore: "
<< Restore->getNumber() << ' ' << Restore->getName() << '\n');
MachineFrameInfo *MFI = MF.getFrameInfo();
MFI->setSavePoint(Save);
MFI->setRestorePoint(Restore);
++NumCandidates;
return false;
}