EarlyIfConversion.cpp | Explore in Territory

//===-- EarlyIfConversion.cpp - If-conversion on SSA form machine code ----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Early if-conversion is for out-of-order CPUs that don't have a lot of
// predicable instructions. The goal is to eliminate conditional branches that
// may mispredict.
//
// Instructions from both sides of the branch are executed specutatively, and a
// cmov instruction selects the result.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SparseSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineTraceMetrics.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

usingnamespacellvm;

#define DEBUG_TYPE …

// Absolute maximum number of instructions allowed per speculated block.
// This bypasses all other heuristics, so it should be set fairly high.
static cl::opt<unsigned>
BlockInstrLimit("early-ifcvt-limit", cl::init(30), cl::Hidden,
  cl::desc("Maximum number of instructions per speculated block."));

// Stress testing mode - disable heuristics.
static cl::opt<bool> Stress("stress-early-ifcvt", cl::Hidden,
  cl::desc("Turn all knobs to 11"));

STATISTIC(NumDiamondsSeen,  "Number of diamonds");
STATISTIC(NumDiamondsConv,  "Number of diamonds converted");
STATISTIC(NumTrianglesSeen, "Number of triangles");
STATISTIC(NumTrianglesConv, "Number of triangles converted");

//===----------------------------------------------------------------------===//
//                                 SSAIfConv
//===----------------------------------------------------------------------===//
//
// The SSAIfConv class performs if-conversion on SSA form machine code after
// determining if it is possible. The class contains no heuristics; external
// code should be used to determine when if-conversion is a good idea.
//
// SSAIfConv can convert both triangles and diamonds:
//
//   Triangle: Head              Diamond: Head
//              | \                       /  \_
//              |  \                     /    |
//              |  [TF]BB              FBB    TBB
//              |  /                     \    /
//              | /                       \  /
//             Tail                       Tail
//
// Instructions in the conditional blocks TBB and/or FBB are spliced into the
// Head block, and phis in the Tail block are converted to select instructions.
//
namespace {
class SSAIfConv { … };
} // end anonymous namespace

/// canSpeculateInstrs - Returns true if all the instructions in MBB can safely
/// be speculated. The terminators are not considered.
///
/// If instructions use any values that are defined in the head basic block,
/// the defining instructions are added to InsertAfter.
///
/// Any clobbered regunits are added to ClobberedRegUnits.
///
bool SSAIfConv::canSpeculateInstrs(MachineBasicBlock *MBB) { … }

/// Check that there is no dependencies preventing if conversion.
///
/// If instruction uses any values that are defined in the head basic block,
/// the defining instructions are added to InsertAfter.
bool SSAIfConv::InstrDependenciesAllowIfConv(MachineInstr *I) { … }

/// canPredicateInstrs - Returns true if all the instructions in MBB can safely
/// be predicates. The terminators are not considered.
///
/// If instructions use any values that are defined in the head basic block,
/// the defining instructions are added to InsertAfter.
///
/// Any clobbered regunits are added to ClobberedRegUnits.
///
bool SSAIfConv::canPredicateInstrs(MachineBasicBlock *MBB) { … }

// Apply predicate to all instructions in the machine block.
void SSAIfConv::PredicateBlock(MachineBasicBlock *MBB, bool ReversePredicate) { … }

/// Find an insertion point in Head for the speculated instructions. The
/// insertion point must be:
///
/// 1. Before any terminators.
/// 2. After any instructions in InsertAfter.
/// 3. Not have any clobbered regunits live.
///
/// This function sets InsertionPoint and returns true when successful, it
/// returns false if no valid insertion point could be found.
///
bool SSAIfConv::findInsertionPoint() { … }



/// canConvertIf - analyze the sub-cfg rooted in MBB, and return true if it is
/// a potential candidate for if-conversion. Fill out the internal state.
///
bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB, bool Predicate) { … }

/// \return true iff the two registers are known to have the same value.
static bool hasSameValue(const MachineRegisterInfo &MRI,
                         const TargetInstrInfo *TII, Register TReg,
                         Register FReg) { … }

/// replacePHIInstrs - Completely replace PHI instructions with selects.
/// This is possible when the only Tail predecessors are the if-converted
/// blocks.
void SSAIfConv::replacePHIInstrs() { … }

/// rewritePHIOperands - When there are additional Tail predecessors, insert
/// select instructions in Head and rewrite PHI operands to use the selects.
/// Keep the PHI instructions in Tail to handle the other predecessors.
void SSAIfConv::rewritePHIOperands() { … }

/// convertIf - Execute the if conversion after canConvertIf has determined the
/// feasibility.
///
/// Any basic blocks that need to be erased will be added to RemoveBlocks.
///
void SSAIfConv::convertIf(SmallVectorImpl<MachineBasicBlock *> &RemoveBlocks,
                          bool Predicate) { … }

//===----------------------------------------------------------------------===//
//                           EarlyIfConverter Pass
//===----------------------------------------------------------------------===//

namespace {
class EarlyIfConverter : public MachineFunctionPass { … };
} // end anonymous namespace

char EarlyIfConverter::ID = …;
char &llvm::EarlyIfConverterID = …;

INITIALIZE_PASS_BEGIN(EarlyIfConverter, DEBUG_TYPE,
                      "Early If Converter", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MachineTraceMetrics)
INITIALIZE_PASS_END(EarlyIfConverter, DEBUG_TYPE,
                    "Early If Converter", false, false)

void EarlyIfConverter::getAnalysisUsage(AnalysisUsage &AU) const { … }

namespace {
/// Update the dominator tree after if-conversion erased some blocks.
void updateDomTree(MachineDominatorTree *DomTree, const SSAIfConv &IfConv,
                   ArrayRef<MachineBasicBlock *> Removed) { … }

/// Update LoopInfo after if-conversion.
void updateLoops(MachineLoopInfo *Loops,
                 ArrayRef<MachineBasicBlock *> Removed) { … }
} // namespace

/// Invalidate MachineTraceMetrics before if-conversion.
void EarlyIfConverter::invalidateTraces() { … }

// Adjust cycles with downward saturation.
static unsigned adjCycles(unsigned Cyc, int Delta) { … }

namespace {
/// Helper class to simplify emission of cycle counts into optimization remarks.
struct Cycles { … };
template <typename Remark> Remark &operator<<(Remark &R, Cycles C) { … }
} // anonymous namespace

/// Apply cost model and heuristics to the if-conversion in IfConv.
/// Return true if the conversion is a good idea.
///
bool EarlyIfConverter::shouldConvertIf() { … }

/// Attempt repeated if-conversion on MBB, return true if successful.
///
bool EarlyIfConverter::tryConvertIf(MachineBasicBlock *MBB) { … }

bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) { … }

//===----------------------------------------------------------------------===//
//                           EarlyIfPredicator Pass
//===----------------------------------------------------------------------===//

namespace {
class EarlyIfPredicator : public MachineFunctionPass { … };
} // end anonymous namespace

#undef DEBUG_TYPE
#define DEBUG_TYPE …

char EarlyIfPredicator::ID = …;
char &llvm::EarlyIfPredicatorID = …;

INITIALIZE_PASS_BEGIN(EarlyIfPredicator, DEBUG_TYPE, "Early If Predicator",
                      false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfoWrapperPass)
INITIALIZE_PASS_END(EarlyIfPredicator, DEBUG_TYPE, "Early If Predicator", false,
                    false)

void EarlyIfPredicator::getAnalysisUsage(AnalysisUsage &AU) const { … }

/// Apply the target heuristic to decide if the transformation is profitable.
bool EarlyIfPredicator::shouldConvertIf() { … }

/// Attempt repeated if-conversion on MBB, return true if successful.
///
bool EarlyIfPredicator::tryConvertIf(MachineBasicBlock *MBB) { … }

bool EarlyIfPredicator::runOnMachineFunction(MachineFunction &MF) { … }
llvm/llvm/lib/CodeGen/EarlyIfConversion.cpp
