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llvm-project/llvm/lib/Target/X86/X86CmovConversion.cpp

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//====- X86CmovConversion.cpp - Convert Cmov to Branch --------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
/// \file
/// This file implements a pass that converts X86 cmov instructions into
/// branches when profitable. This pass is conservative. It transforms if and
/// only if it can guarantee a gain with high confidence.
///
/// Thus, the optimization applies under the following conditions:
/// 1. Consider as candidates only CMOVs in innermost loops (assume that
/// most hotspots are represented by these loops).
/// 2. Given a group of CMOV instructions that are using the same EFLAGS def
/// instruction:
/// a. Consider them as candidates only if all have the same code condition
/// or the opposite one to prevent generating more than one conditional
/// jump per EFLAGS def instruction.
/// b. Consider them as candidates only if all are profitable to be
/// converted (assume that one bad conversion may cause a degradation).
/// 3. Apply conversion only for loops that are found profitable and only for
/// CMOV candidates that were found profitable.
/// a. A loop is considered profitable only if conversion will reduce its
/// depth cost by some threshold.
/// b. CMOV is considered profitable if the cost of its condition is higher
/// than the average cost of its true-value and false-value by 25% of
/// branch-misprediction-penalty. This assures no degradation even with
/// 25% branch misprediction.
///
/// Note: This pass is assumed to run on SSA machine code.
//
//===----------------------------------------------------------------------===//
//
// External interfaces:
// FunctionPass *llvm::createX86CmovConverterPass();
// bool X86CmovConverterPass::runOnMachineFunction(MachineFunction &MF);
//
//===----------------------------------------------------------------------===//
#include "X86.h"
#include "X86InstrInfo.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCSchedule.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "x86-cmov-conversion"
STATISTIC(NumOfSkippedCmovGroups, "Number of unsupported CMOV-groups");
STATISTIC(NumOfCmovGroupCandidate, "Number of CMOV-group candidates");
STATISTIC(NumOfLoopCandidate, "Number of CMOV-conversion profitable loops");
STATISTIC(NumOfOptimizedCmovGroups, "Number of optimized CMOV-groups");
// This internal switch can be used to turn off the cmov/branch optimization.
static cl::opt<bool>
EnableCmovConverter("x86-cmov-converter",
cl::desc("Enable the X86 cmov-to-branch optimization."),
cl::init(true), cl::Hidden);
static cl::opt<unsigned>
GainCycleThreshold("x86-cmov-converter-threshold",
cl::desc("Minimum gain per loop (in cycles) threshold."),
cl::init(4), cl::Hidden);
static cl::opt<bool> ForceMemOperand(
"x86-cmov-converter-force-mem-operand",
cl::desc("Convert cmovs to branches whenever they have memory operands."),
cl::init(true), cl::Hidden);
namespace {
/// Converts X86 cmov instructions into branches when profitable.
class X86CmovConverterPass : public MachineFunctionPass {
public:
X86CmovConverterPass() : MachineFunctionPass(ID) { }
StringRef getPassName() const override { return "X86 cmov Conversion"; }
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
/// Pass identification, replacement for typeid.
static char ID;
private:
MachineRegisterInfo *MRI = nullptr;
const TargetInstrInfo *TII = nullptr;
const TargetRegisterInfo *TRI = nullptr;
TargetSchedModel TSchedModel;
/// List of consecutive CMOV instructions.
using CmovGroup = SmallVector<MachineInstr *, 2>;
using CmovGroups = SmallVector<CmovGroup, 2>;
/// Collect all CMOV-group-candidates in \p CurrLoop and update \p
/// CmovInstGroups accordingly.
///
/// \param Blocks List of blocks to process.
/// \param CmovInstGroups List of consecutive CMOV instructions in CurrLoop.
/// \returns true iff it found any CMOV-group-candidate.
bool collectCmovCandidates(ArrayRef<MachineBasicBlock *> Blocks,
CmovGroups &CmovInstGroups,
bool IncludeLoads = false);
/// Check if it is profitable to transform each CMOV-group-candidates into
/// branch. Remove all groups that are not profitable from \p CmovInstGroups.
///
/// \param Blocks List of blocks to process.
/// \param CmovInstGroups List of consecutive CMOV instructions in CurrLoop.
/// \returns true iff any CMOV-group-candidate remain.
bool checkForProfitableCmovCandidates(ArrayRef<MachineBasicBlock *> Blocks,
CmovGroups &CmovInstGroups);
/// Convert the given list of consecutive CMOV instructions into a branch.
///
/// \param Group Consecutive CMOV instructions to be converted into branch.
void convertCmovInstsToBranches(SmallVectorImpl<MachineInstr *> &Group) const;
};
} // end anonymous namespace
char X86CmovConverterPass::ID = 0;
void X86CmovConverterPass::getAnalysisUsage(AnalysisUsage &AU) const {
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<MachineLoopInfo>();
}
bool X86CmovConverterPass::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
if (!EnableCmovConverter)
return false;
LLVM_DEBUG(dbgs() << "********** " << getPassName() << " : " << MF.getName()
<< "**********\n");
bool Changed = false;
MachineLoopInfo &MLI = getAnalysis<MachineLoopInfo>();
const TargetSubtargetInfo &STI = MF.getSubtarget();
MRI = &MF.getRegInfo();
TII = STI.getInstrInfo();
TRI = STI.getRegisterInfo();
TSchedModel.init(&STI);
// Before we handle the more subtle cases of register-register CMOVs inside
// of potentially hot loops, we want to quickly remove all CMOVs with
// a memory operand. The CMOV will risk a stall waiting for the load to
// complete that speculative execution behind a branch is better suited to
// handle on modern x86 chips.
if (ForceMemOperand) {
CmovGroups AllCmovGroups;
SmallVector<MachineBasicBlock *, 4> Blocks;
for (auto &MBB : MF)
Blocks.push_back(&MBB);
if (collectCmovCandidates(Blocks, AllCmovGroups, /*IncludeLoads*/ true)) {
for (auto &Group : AllCmovGroups) {
// Skip any group that doesn't do at least one memory operand cmov.
if (!llvm::any_of(Group, [&](MachineInstr *I) { return I->mayLoad(); }))
continue;
// For CMOV groups which we can rewrite and which contain a memory load,
// always rewrite them. On x86, a CMOV will dramatically amplify any
// memory latency by blocking speculative execution.
Changed = true;
convertCmovInstsToBranches(Group);
}
}
}
//===--------------------------------------------------------------------===//
// Register-operand Conversion Algorithm
// ---------
// For each inner most loop
// collectCmovCandidates() {
// Find all CMOV-group-candidates.
// }
//
// checkForProfitableCmovCandidates() {
// * Calculate both loop-depth and optimized-loop-depth.
// * Use these depth to check for loop transformation profitability.
// * Check for CMOV-group-candidate transformation profitability.
// }
//
// For each profitable CMOV-group-candidate
// convertCmovInstsToBranches() {
// * Create FalseBB, SinkBB, Conditional branch to SinkBB.
// * Replace each CMOV instruction with a PHI instruction in SinkBB.
// }
//
// Note: For more details, see each function description.
//===--------------------------------------------------------------------===//
// Build up the loops in pre-order.
SmallVector<MachineLoop *, 4> Loops(MLI.begin(), MLI.end());
// Note that we need to check size on each iteration as we accumulate child
// loops.
for (int i = 0; i < (int)Loops.size(); ++i)
for (MachineLoop *Child : Loops[i]->getSubLoops())
Loops.push_back(Child);
for (MachineLoop *CurrLoop : Loops) {
// Optimize only inner most loops.
if (!CurrLoop->getSubLoops().empty())
continue;
// List of consecutive CMOV instructions to be processed.
CmovGroups CmovInstGroups;
if (!collectCmovCandidates(CurrLoop->getBlocks(), CmovInstGroups))
continue;
if (!checkForProfitableCmovCandidates(CurrLoop->getBlocks(),
CmovInstGroups))
continue;
Changed = true;
for (auto &Group : CmovInstGroups)
convertCmovInstsToBranches(Group);
}
return Changed;
}
bool X86CmovConverterPass::collectCmovCandidates(
ArrayRef<MachineBasicBlock *> Blocks, CmovGroups &CmovInstGroups,
bool IncludeLoads) {
//===--------------------------------------------------------------------===//
// Collect all CMOV-group-candidates and add them into CmovInstGroups.
//
// CMOV-group:
// CMOV instructions, in same MBB, that uses same EFLAGS def instruction.
//
// CMOV-group-candidate:
// CMOV-group where all the CMOV instructions are
// 1. consecutive.
// 2. have same condition code or opposite one.
// 3. have only operand registers (X86::CMOVrr).
//===--------------------------------------------------------------------===//
// List of possible improvement (TODO's):
// --------------------------------------
// TODO: Add support for X86::CMOVrm instructions.
// TODO: Add support for X86::SETcc instructions.
// TODO: Add support for CMOV-groups with non consecutive CMOV instructions.
//===--------------------------------------------------------------------===//
// Current processed CMOV-Group.
CmovGroup Group;
for (auto *MBB : Blocks) {
Group.clear();
// Condition code of first CMOV instruction current processed range and its
// opposite condition code.
X86::CondCode FirstCC = X86::COND_INVALID, FirstOppCC = X86::COND_INVALID,
MemOpCC = X86::COND_INVALID;
// Indicator of a non CMOVrr instruction in the current processed range.
bool FoundNonCMOVInst = false;
// Indicator for current processed CMOV-group if it should be skipped.
bool SkipGroup = false;
for (auto &I : *MBB) {
// Skip debug instructions.
if (I.isDebugInstr())
continue;
X86::CondCode CC = X86::getCondFromCMov(I);
// Check if we found a X86::CMOVrr instruction.
if (CC != X86::COND_INVALID && (IncludeLoads || !I.mayLoad())) {
if (Group.empty()) {
// We found first CMOV in the range, reset flags.
FirstCC = CC;
FirstOppCC = X86::GetOppositeBranchCondition(CC);
// Clear out the prior group's memory operand CC.
MemOpCC = X86::COND_INVALID;
FoundNonCMOVInst = false;
SkipGroup = false;
}
Group.push_back(&I);
// Check if it is a non-consecutive CMOV instruction or it has different
// condition code than FirstCC or FirstOppCC.
if (FoundNonCMOVInst || (CC != FirstCC && CC != FirstOppCC))
// Mark the SKipGroup indicator to skip current processed CMOV-Group.
SkipGroup = true;
if (I.mayLoad()) {
if (MemOpCC == X86::COND_INVALID)
// The first memory operand CMOV.
MemOpCC = CC;
else if (CC != MemOpCC)
// Can't handle mixed conditions with memory operands.
SkipGroup = true;
}
// Check if we were relying on zero-extending behavior of the CMOV.
if (!SkipGroup &&
llvm::any_of(
MRI->use_nodbg_instructions(I.defs().begin()->getReg()),
[&](MachineInstr &UseI) {
return UseI.getOpcode() == X86::SUBREG_TO_REG;
}))
// FIXME: We should model the cost of using an explicit MOV to handle
// the zero-extension rather than just refusing to handle this.
SkipGroup = true;
continue;
}
// If Group is empty, keep looking for first CMOV in the range.
if (Group.empty())
continue;
// We found a non X86::CMOVrr instruction.
FoundNonCMOVInst = true;
// Check if this instruction define EFLAGS, to determine end of processed
// range, as there would be no more instructions using current EFLAGS def.
if (I.definesRegister(X86::EFLAGS)) {
// Check if current processed CMOV-group should not be skipped and add
// it as a CMOV-group-candidate.
if (!SkipGroup)
CmovInstGroups.push_back(Group);
else
++NumOfSkippedCmovGroups;
Group.clear();
}
}
// End of basic block is considered end of range, check if current processed
// CMOV-group should not be skipped and add it as a CMOV-group-candidate.
if (Group.empty())
continue;
if (!SkipGroup)
CmovInstGroups.push_back(Group);
else
++NumOfSkippedCmovGroups;
}
NumOfCmovGroupCandidate += CmovInstGroups.size();
return !CmovInstGroups.empty();
}
/// \returns Depth of CMOV instruction as if it was converted into branch.
/// \param TrueOpDepth depth cost of CMOV true value operand.
/// \param FalseOpDepth depth cost of CMOV false value operand.
static unsigned getDepthOfOptCmov(unsigned TrueOpDepth, unsigned FalseOpDepth) {
// The depth of the result after branch conversion is
// TrueOpDepth * TrueOpProbability + FalseOpDepth * FalseOpProbability.
// As we have no info about branch weight, we assume 75% for one and 25% for
// the other, and pick the result with the largest resulting depth.
return std::max(
divideCeil(TrueOpDepth * 3 + FalseOpDepth, 4),
divideCeil(FalseOpDepth * 3 + TrueOpDepth, 4));
}
bool X86CmovConverterPass::checkForProfitableCmovCandidates(
ArrayRef<MachineBasicBlock *> Blocks, CmovGroups &CmovInstGroups) {
struct DepthInfo {
/// Depth of original loop.
unsigned Depth;
/// Depth of optimized loop.
unsigned OptDepth;
};
/// Number of loop iterations to calculate depth for ?!
static const unsigned LoopIterations = 2;
DenseMap<MachineInstr *, DepthInfo> DepthMap;
DepthInfo LoopDepth[LoopIterations] = {{0, 0}, {0, 0}};
enum { PhyRegType = 0, VirRegType = 1, RegTypeNum = 2 };
/// For each register type maps the register to its last def instruction.
DenseMap<unsigned, MachineInstr *> RegDefMaps[RegTypeNum];
/// Maps register operand to its def instruction, which can be nullptr if it
/// is unknown (e.g., operand is defined outside the loop).
DenseMap<MachineOperand *, MachineInstr *> OperandToDefMap;
// Set depth of unknown instruction (i.e., nullptr) to zero.
DepthMap[nullptr] = {0, 0};
SmallPtrSet<MachineInstr *, 4> CmovInstructions;
for (auto &Group : CmovInstGroups)
CmovInstructions.insert(Group.begin(), Group.end());
//===--------------------------------------------------------------------===//
// Step 1: Calculate instruction depth and loop depth.
// Optimized-Loop:
// loop with CMOV-group-candidates converted into branches.
//
// Instruction-Depth:
// instruction latency + max operand depth.
// * For CMOV instruction in optimized loop the depth is calculated as:
// CMOV latency + getDepthOfOptCmov(True-Op-Depth, False-Op-depth)
// TODO: Find a better way to estimate the latency of the branch instruction
// rather than using the CMOV latency.
//
// Loop-Depth:
// max instruction depth of all instructions in the loop.
// Note: instruction with max depth represents the critical-path in the loop.
//
// Loop-Depth[i]:
// Loop-Depth calculated for first `i` iterations.
// Note: it is enough to calculate depth for up to two iterations.
//
// Depth-Diff[i]:
// Number of cycles saved in first 'i` iterations by optimizing the loop.
//===--------------------------------------------------------------------===//
for (unsigned I = 0; I < LoopIterations; ++I) {
DepthInfo &MaxDepth = LoopDepth[I];
for (auto *MBB : Blocks) {
// Clear physical registers Def map.
RegDefMaps[PhyRegType].clear();
for (MachineInstr &MI : *MBB) {
// Skip debug instructions.
if (MI.isDebugInstr())
continue;
unsigned MIDepth = 0;
unsigned MIDepthOpt = 0;
bool IsCMOV = CmovInstructions.count(&MI);
for (auto &MO : MI.uses()) {
// Checks for "isUse()" as "uses()" returns also implicit definitions.
if (!MO.isReg() || !MO.isUse())
continue;
Register Reg = MO.getReg();
auto &RDM = RegDefMaps[Reg.isVirtual()];
if (MachineInstr *DefMI = RDM.lookup(Reg)) {
OperandToDefMap[&MO] = DefMI;
DepthInfo Info = DepthMap.lookup(DefMI);
MIDepth = std::max(MIDepth, Info.Depth);
if (!IsCMOV)
MIDepthOpt = std::max(MIDepthOpt, Info.OptDepth);
}
}
if (IsCMOV)
MIDepthOpt = getDepthOfOptCmov(
DepthMap[OperandToDefMap.lookup(&MI.getOperand(1))].OptDepth,
DepthMap[OperandToDefMap.lookup(&MI.getOperand(2))].OptDepth);
// Iterates over all operands to handle implicit definitions as well.
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !MO.isDef())
continue;
Register Reg = MO.getReg();
RegDefMaps[Reg.isVirtual()][Reg] = &MI;
}
unsigned Latency = TSchedModel.computeInstrLatency(&MI);
DepthMap[&MI] = {MIDepth += Latency, MIDepthOpt += Latency};
MaxDepth.Depth = std::max(MaxDepth.Depth, MIDepth);
MaxDepth.OptDepth = std::max(MaxDepth.OptDepth, MIDepthOpt);
}
}
}
unsigned Diff[LoopIterations] = {LoopDepth[0].Depth - LoopDepth[0].OptDepth,
LoopDepth[1].Depth - LoopDepth[1].OptDepth};
//===--------------------------------------------------------------------===//
// Step 2: Check if Loop worth to be optimized.
// Worth-Optimize-Loop:
// case 1: Diff[1] == Diff[0]
// Critical-path is iteration independent - there is no dependency
// of critical-path instructions on critical-path instructions of
// previous iteration.
// Thus, it is enough to check gain percent of 1st iteration -
// To be conservative, the optimized loop need to have a depth of
// 12.5% cycles less than original loop, per iteration.
//
// case 2: Diff[1] > Diff[0]
// Critical-path is iteration dependent - there is dependency of
// critical-path instructions on critical-path instructions of
// previous iteration.
// Thus, check the gain percent of the 2nd iteration (similar to the
// previous case), but it is also required to check the gradient of
// the gain - the change in Depth-Diff compared to the change in
// Loop-Depth between 1st and 2nd iterations.
// To be conservative, the gradient need to be at least 50%.
//
// In addition, In order not to optimize loops with very small gain, the
// gain (in cycles) after 2nd iteration should not be less than a given
// threshold. Thus, the check (Diff[1] >= GainCycleThreshold) must apply.
//
// If loop is not worth optimizing, remove all CMOV-group-candidates.
//===--------------------------------------------------------------------===//
if (Diff[1] < GainCycleThreshold)
return false;
bool WorthOptLoop = false;
if (Diff[1] == Diff[0])
WorthOptLoop = Diff[0] * 8 >= LoopDepth[0].Depth;
else if (Diff[1] > Diff[0])
WorthOptLoop =
(Diff[1] - Diff[0]) * 2 >= (LoopDepth[1].Depth - LoopDepth[0].Depth) &&
(Diff[1] * 8 >= LoopDepth[1].Depth);
if (!WorthOptLoop)
return false;
++NumOfLoopCandidate;
//===--------------------------------------------------------------------===//
// Step 3: Check for each CMOV-group-candidate if it worth to be optimized.
// Worth-Optimize-Group:
// Iff it worths to optimize all CMOV instructions in the group.
//
// Worth-Optimize-CMOV:
// Predicted branch is faster than CMOV by the difference between depth of
// condition operand and depth of taken (predicted) value operand.
// To be conservative, the gain of such CMOV transformation should cover at
// at least 25% of branch-misprediction-penalty.
//===--------------------------------------------------------------------===//
unsigned MispredictPenalty = TSchedModel.getMCSchedModel()->MispredictPenalty;
CmovGroups TempGroups;
std::swap(TempGroups, CmovInstGroups);
for (auto &Group : TempGroups) {
bool WorthOpGroup = true;
for (auto *MI : Group) {
// Avoid CMOV instruction which value is used as a pointer to load from.
// This is another conservative check to avoid converting CMOV instruction
// used with tree-search like algorithm, where the branch is unpredicted.
auto UIs = MRI->use_instructions(MI->defs().begin()->getReg());
if (!UIs.empty() && ++UIs.begin() == UIs.end()) {
unsigned Op = UIs.begin()->getOpcode();
if (Op == X86::MOV64rm || Op == X86::MOV32rm) {
WorthOpGroup = false;
break;
}
}
unsigned CondCost =
DepthMap[OperandToDefMap.lookup(&MI->getOperand(4))].Depth;
unsigned ValCost = getDepthOfOptCmov(
DepthMap[OperandToDefMap.lookup(&MI->getOperand(1))].Depth,
DepthMap[OperandToDefMap.lookup(&MI->getOperand(2))].Depth);
if (ValCost > CondCost || (CondCost - ValCost) * 4 < MispredictPenalty) {
WorthOpGroup = false;
break;
}
}
if (WorthOpGroup)
CmovInstGroups.push_back(Group);
}
return !CmovInstGroups.empty();
}
static bool checkEFLAGSLive(MachineInstr *MI) {
if (MI->killsRegister(X86::EFLAGS))
return false;
// The EFLAGS operand of MI might be missing a kill marker.
// Figure out whether EFLAGS operand should LIVE after MI instruction.
MachineBasicBlock *BB = MI->getParent();
MachineBasicBlock::iterator ItrMI = MI;
// Scan forward through BB for a use/def of EFLAGS.
for (auto I = std::next(ItrMI), E = BB->end(); I != E; ++I) {
if (I->readsRegister(X86::EFLAGS))
return true;
if (I->definesRegister(X86::EFLAGS))
return false;
}
// We hit the end of the block, check whether EFLAGS is live into a successor.
for (auto I = BB->succ_begin(), E = BB->succ_end(); I != E; ++I) {
if ((*I)->isLiveIn(X86::EFLAGS))
return true;
}
return false;
}
/// Given /p First CMOV instruction and /p Last CMOV instruction representing a
/// group of CMOV instructions, which may contain debug instructions in between,
/// move all debug instructions to after the last CMOV instruction, making the
/// CMOV group consecutive.
static void packCmovGroup(MachineInstr *First, MachineInstr *Last) {
assert(X86::getCondFromCMov(*Last) != X86::COND_INVALID &&
"Last instruction in a CMOV group must be a CMOV instruction");
SmallVector<MachineInstr *, 2> DBGInstructions;
for (auto I = First->getIterator(), E = Last->getIterator(); I != E; I++) {
if (I->isDebugInstr())
DBGInstructions.push_back(&*I);
}
// Splice the debug instruction after the cmov group.
MachineBasicBlock *MBB = First->getParent();
for (auto *MI : DBGInstructions)
MBB->insertAfter(Last, MI->removeFromParent());
}
void X86CmovConverterPass::convertCmovInstsToBranches(
SmallVectorImpl<MachineInstr *> &Group) const {
assert(!Group.empty() && "No CMOV instructions to convert");
++NumOfOptimizedCmovGroups;
// If the CMOV group is not packed, e.g., there are debug instructions between
// first CMOV and last CMOV, then pack the group and make the CMOV instruction
// consecutive by moving the debug instructions to after the last CMOV.
packCmovGroup(Group.front(), Group.back());
// To convert a CMOVcc instruction, we actually have to insert the diamond
// control-flow pattern. The incoming instruction knows the destination vreg
// to set, the condition code register to branch on, the true/false values to
// select between, and a branch opcode to use.
// Before
// -----
// MBB:
// cond = cmp ...
// v1 = CMOVge t1, f1, cond
// v2 = CMOVlt t2, f2, cond
// v3 = CMOVge v1, f3, cond
//
// After
// -----
// MBB:
// cond = cmp ...
// jge %SinkMBB
//
// FalseMBB:
// jmp %SinkMBB
//
// SinkMBB:
// %v1 = phi[%f1, %FalseMBB], [%t1, %MBB]
// %v2 = phi[%t2, %FalseMBB], [%f2, %MBB] ; For CMOV with OppCC switch
// ; true-value with false-value
// %v3 = phi[%f3, %FalseMBB], [%t1, %MBB] ; Phi instruction cannot use
// ; previous Phi instruction result
MachineInstr &MI = *Group.front();
MachineInstr *LastCMOV = Group.back();
DebugLoc DL = MI.getDebugLoc();
X86::CondCode CC = X86::CondCode(X86::getCondFromCMov(MI));
X86::CondCode OppCC = X86::GetOppositeBranchCondition(CC);
// Potentially swap the condition codes so that any memory operand to a CMOV
// is in the *false* position instead of the *true* position. We can invert
// any non-memory operand CMOV instructions to cope with this and we ensure
// memory operand CMOVs are only included with a single condition code.
if (llvm::any_of(Group, [&](MachineInstr *I) {
return I->mayLoad() && X86::getCondFromCMov(*I) == CC;
}))
std::swap(CC, OppCC);
MachineBasicBlock *MBB = MI.getParent();
MachineFunction::iterator It = ++MBB->getIterator();
MachineFunction *F = MBB->getParent();
const BasicBlock *BB = MBB->getBasicBlock();
MachineBasicBlock *FalseMBB = F->CreateMachineBasicBlock(BB);
MachineBasicBlock *SinkMBB = F->CreateMachineBasicBlock(BB);
F->insert(It, FalseMBB);
F->insert(It, SinkMBB);
// If the EFLAGS register isn't dead in the terminator, then claim that it's
// live into the sink and copy blocks.
if (checkEFLAGSLive(LastCMOV)) {
FalseMBB->addLiveIn(X86::EFLAGS);
SinkMBB->addLiveIn(X86::EFLAGS);
}
// Transfer the remainder of BB and its successor edges to SinkMBB.
SinkMBB->splice(SinkMBB->begin(), MBB,
std::next(MachineBasicBlock::iterator(LastCMOV)), MBB->end());
SinkMBB->transferSuccessorsAndUpdatePHIs(MBB);
// Add the false and sink blocks as its successors.
MBB->addSuccessor(FalseMBB);
MBB->addSuccessor(SinkMBB);
// Create the conditional branch instruction.
BuildMI(MBB, DL, TII->get(X86::JCC_1)).addMBB(SinkMBB).addImm(CC);
// Add the sink block to the false block successors.
FalseMBB->addSuccessor(SinkMBB);
MachineInstrBuilder MIB;
MachineBasicBlock::iterator MIItBegin = MachineBasicBlock::iterator(MI);
MachineBasicBlock::iterator MIItEnd =
std::next(MachineBasicBlock::iterator(LastCMOV));
MachineBasicBlock::iterator FalseInsertionPoint = FalseMBB->begin();
MachineBasicBlock::iterator SinkInsertionPoint = SinkMBB->begin();
// First we need to insert an explicit load on the false path for any memory
// operand. We also need to potentially do register rewriting here, but it is
// simpler as the memory operands are always on the false path so we can
// simply take that input, whatever it is.
DenseMap<unsigned, unsigned> FalseBBRegRewriteTable;
for (MachineBasicBlock::iterator MIIt = MIItBegin; MIIt != MIItEnd;) {
auto &MI = *MIIt++;
// Skip any CMOVs in this group which don't load from memory.
if (!MI.mayLoad()) {
// Remember the false-side register input.
Register FalseReg =
MI.getOperand(X86::getCondFromCMov(MI) == CC ? 1 : 2).getReg();
// Walk back through any intermediate cmovs referenced.
while (true) {
auto FRIt = FalseBBRegRewriteTable.find(FalseReg);
if (FRIt == FalseBBRegRewriteTable.end())
break;
FalseReg = FRIt->second;
}
FalseBBRegRewriteTable[MI.getOperand(0).getReg()] = FalseReg;
continue;
}
// The condition must be the *opposite* of the one we've decided to branch
// on as the branch will go *around* the load and the load should happen
// when the CMOV condition is false.
assert(X86::getCondFromCMov(MI) == OppCC &&
"Can only handle memory-operand cmov instructions with a condition "
"opposite to the selected branch direction.");
// The goal is to rewrite the cmov from:
//
// MBB:
// %A = CMOVcc %B (tied), (mem)
//
// to
//
// MBB:
// %A = CMOVcc %B (tied), %C
// FalseMBB:
// %C = MOV (mem)
//
// Which will allow the next loop to rewrite the CMOV in terms of a PHI:
//
// MBB:
// JMP!cc SinkMBB
// FalseMBB:
// %C = MOV (mem)
// SinkMBB:
// %A = PHI [ %C, FalseMBB ], [ %B, MBB]
// Get a fresh register to use as the destination of the MOV.
const TargetRegisterClass *RC = MRI->getRegClass(MI.getOperand(0).getReg());
Register TmpReg = MRI->createVirtualRegister(RC);
SmallVector<MachineInstr *, 4> NewMIs;
bool Unfolded = TII->unfoldMemoryOperand(*MBB->getParent(), MI, TmpReg,
/*UnfoldLoad*/ true,
/*UnfoldStore*/ false, NewMIs);
(void)Unfolded;
assert(Unfolded && "Should never fail to unfold a loading cmov!");
// Move the new CMOV to just before the old one and reset any impacted
// iterator.
auto *NewCMOV = NewMIs.pop_back_val();
assert(X86::getCondFromCMov(*NewCMOV) == OppCC &&
"Last new instruction isn't the expected CMOV!");
LLVM_DEBUG(dbgs() << "\tRewritten cmov: "; NewCMOV->dump());
MBB->insert(MachineBasicBlock::iterator(MI), NewCMOV);
if (&*MIItBegin == &MI)
MIItBegin = MachineBasicBlock::iterator(NewCMOV);
// Sink whatever instructions were needed to produce the unfolded operand
// into the false block.
for (auto *NewMI : NewMIs) {
LLVM_DEBUG(dbgs() << "\tRewritten load instr: "; NewMI->dump());
FalseMBB->insert(FalseInsertionPoint, NewMI);
// Re-map any operands that are from other cmovs to the inputs for this block.
for (auto &MOp : NewMI->uses()) {
if (!MOp.isReg())
continue;
auto It = FalseBBRegRewriteTable.find(MOp.getReg());
if (It == FalseBBRegRewriteTable.end())
continue;
MOp.setReg(It->second);
// This might have been a kill when it referenced the cmov result, but
// it won't necessarily be once rewritten.
// FIXME: We could potentially improve this by tracking whether the
// operand to the cmov was also a kill, and then skipping the PHI node
// construction below.
MOp.setIsKill(false);
}
}
MBB->erase(MachineBasicBlock::iterator(MI),
std::next(MachineBasicBlock::iterator(MI)));
// Add this PHI to the rewrite table.
FalseBBRegRewriteTable[NewCMOV->getOperand(0).getReg()] = TmpReg;
}
// As we are creating the PHIs, we have to be careful if there is more than
// one. Later CMOVs may reference the results of earlier CMOVs, but later
// PHIs have to reference the individual true/false inputs from earlier PHIs.
// That also means that PHI construction must work forward from earlier to
// later, and that the code must maintain a mapping from earlier PHI's
// destination registers, and the registers that went into the PHI.
DenseMap<unsigned, std::pair<unsigned, unsigned>> RegRewriteTable;
for (MachineBasicBlock::iterator MIIt = MIItBegin; MIIt != MIItEnd; ++MIIt) {
Register DestReg = MIIt->getOperand(0).getReg();
Register Op1Reg = MIIt->getOperand(1).getReg();
Register Op2Reg = MIIt->getOperand(2).getReg();
// If this CMOV we are processing is the opposite condition from the jump we
// generated, then we have to swap the operands for the PHI that is going to
// be generated.
if (X86::getCondFromCMov(*MIIt) == OppCC)
std::swap(Op1Reg, Op2Reg);
auto Op1Itr = RegRewriteTable.find(Op1Reg);
if (Op1Itr != RegRewriteTable.end())
Op1Reg = Op1Itr->second.first;
auto Op2Itr = RegRewriteTable.find(Op2Reg);
if (Op2Itr != RegRewriteTable.end())
Op2Reg = Op2Itr->second.second;
// SinkMBB:
// %Result = phi [ %FalseValue, FalseMBB ], [ %TrueValue, MBB ]
// ...
MIB = BuildMI(*SinkMBB, SinkInsertionPoint, DL, TII->get(X86::PHI), DestReg)
.addReg(Op1Reg)
.addMBB(FalseMBB)
.addReg(Op2Reg)
.addMBB(MBB);
(void)MIB;
LLVM_DEBUG(dbgs() << "\tFrom: "; MIIt->dump());
LLVM_DEBUG(dbgs() << "\tTo: "; MIB->dump());
// Add this PHI to the rewrite table.
RegRewriteTable[DestReg] = std::make_pair(Op1Reg, Op2Reg);
}
// Now remove the CMOV(s).
MBB->erase(MIItBegin, MIItEnd);
}
INITIALIZE_PASS_BEGIN(X86CmovConverterPass, DEBUG_TYPE, "X86 cmov Conversion",
false, false)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_END(X86CmovConverterPass, DEBUG_TYPE, "X86 cmov Conversion",
false, false)
FunctionPass *llvm::createX86CmovConverterPass() {
return new X86CmovConverterPass();
}
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