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llvm-project/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp

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//====- X86FlagsCopyLowering.cpp - Lowers COPY nodes of EFLAGS ------------===//
//
// 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
///
/// Lowers COPY nodes of EFLAGS by directly extracting and preserving individual
/// flag bits.
///
/// We have to do this by carefully analyzing and rewriting the usage of the
/// copied EFLAGS register because there is no general way to rematerialize the
/// entire EFLAGS register safely and efficiently. Using `popf` both forces
/// dynamic stack adjustment and can create correctness issues due to IF, TF,
/// and other non-status flags being overwritten. Using sequences involving
/// SAHF don't work on all x86 processors and are often quite slow compared to
/// directly testing a single status preserved in its own GPR.
///
//===----------------------------------------------------------------------===//
#include "X86.h"
#include "X86InstrBuilder.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SparseBitVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineSSAUpdater.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/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 PASS_KEY "x86-flags-copy-lowering"
#define DEBUG_TYPE PASS_KEY
STATISTIC(NumCopiesEliminated, "Number of copies of EFLAGS eliminated");
STATISTIC(NumSetCCsInserted, "Number of setCC instructions inserted");
STATISTIC(NumTestsInserted, "Number of test instructions inserted");
STATISTIC(NumAddsInserted, "Number of adds instructions inserted");
namespace {
// Convenient array type for storing registers associated with each condition.
using CondRegArray = std::array<unsigned, X86::LAST_VALID_COND + 1>;
class X86FlagsCopyLoweringPass : public MachineFunctionPass {
public:
X86FlagsCopyLoweringPass() : MachineFunctionPass(ID) { }
StringRef getPassName() const override { return "X86 EFLAGS copy lowering"; }
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
/// Pass identification, replacement for typeid.
static char ID;
private:
MachineRegisterInfo *MRI = nullptr;
const X86Subtarget *Subtarget = nullptr;
const X86InstrInfo *TII = nullptr;
const TargetRegisterInfo *TRI = nullptr;
const TargetRegisterClass *PromoteRC = nullptr;
MachineDominatorTree *MDT = nullptr;
CondRegArray collectCondsInRegs(MachineBasicBlock &MBB,
MachineBasicBlock::iterator CopyDefI);
unsigned promoteCondToReg(MachineBasicBlock &MBB,
MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc, X86::CondCode Cond);
std::pair<unsigned, bool>
getCondOrInverseInReg(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos, DebugLoc TestLoc,
X86::CondCode Cond, CondRegArray &CondRegs);
void insertTest(MachineBasicBlock &MBB, MachineBasicBlock::iterator Pos,
DebugLoc Loc, unsigned Reg);
void rewriteArithmetic(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos, DebugLoc TestLoc,
MachineInstr &MI, MachineOperand &FlagUse,
CondRegArray &CondRegs);
void rewriteCMov(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos, DebugLoc TestLoc,
MachineInstr &CMovI, MachineOperand &FlagUse,
CondRegArray &CondRegs);
void rewriteFCMov(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos, DebugLoc TestLoc,
MachineInstr &CMovI, MachineOperand &FlagUse,
CondRegArray &CondRegs);
void rewriteCondJmp(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos, DebugLoc TestLoc,
MachineInstr &JmpI, CondRegArray &CondRegs);
void rewriteCopy(MachineInstr &MI, MachineOperand &FlagUse,
MachineInstr &CopyDefI);
void rewriteSetCC(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos, DebugLoc TestLoc,
MachineInstr &SetCCI, MachineOperand &FlagUse,
CondRegArray &CondRegs);
};
} // end anonymous namespace
INITIALIZE_PASS_BEGIN(X86FlagsCopyLoweringPass, DEBUG_TYPE,
"X86 EFLAGS copy lowering", false, false)
INITIALIZE_PASS_END(X86FlagsCopyLoweringPass, DEBUG_TYPE,
"X86 EFLAGS copy lowering", false, false)
FunctionPass *llvm::createX86FlagsCopyLoweringPass() {
return new X86FlagsCopyLoweringPass();
}
char X86FlagsCopyLoweringPass::ID = 0;
void X86FlagsCopyLoweringPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineDominatorTree>();
MachineFunctionPass::getAnalysisUsage(AU);
}
namespace {
/// An enumeration of the arithmetic instruction mnemonics which have
/// interesting flag semantics.
///
/// We can map instruction opcodes into these mnemonics to make it easy to
/// dispatch with specific functionality.
enum class FlagArithMnemonic {
ADC,
ADCX,
ADOX,
RCL,
RCR,
SBB,
SETB,
};
} // namespace
static FlagArithMnemonic getMnemonicFromOpcode(unsigned Opcode) {
switch (Opcode) {
default:
report_fatal_error("No support for lowering a copy into EFLAGS when used "
"by this instruction!");
#define LLVM_EXPAND_INSTR_SIZES(MNEMONIC, SUFFIX) \
case X86::MNEMONIC##8##SUFFIX: \
case X86::MNEMONIC##16##SUFFIX: \
case X86::MNEMONIC##32##SUFFIX: \
case X86::MNEMONIC##64##SUFFIX:
#define LLVM_EXPAND_ADC_SBB_INSTR(MNEMONIC) \
LLVM_EXPAND_INSTR_SIZES(MNEMONIC, rr) \
LLVM_EXPAND_INSTR_SIZES(MNEMONIC, rr_REV) \
LLVM_EXPAND_INSTR_SIZES(MNEMONIC, rm) \
LLVM_EXPAND_INSTR_SIZES(MNEMONIC, mr) \
case X86::MNEMONIC##8ri: \
case X86::MNEMONIC##16ri8: \
case X86::MNEMONIC##32ri8: \
case X86::MNEMONIC##64ri8: \
case X86::MNEMONIC##16ri: \
case X86::MNEMONIC##32ri: \
case X86::MNEMONIC##64ri32: \
case X86::MNEMONIC##8mi: \
case X86::MNEMONIC##16mi8: \
case X86::MNEMONIC##32mi8: \
case X86::MNEMONIC##64mi8: \
case X86::MNEMONIC##16mi: \
case X86::MNEMONIC##32mi: \
case X86::MNEMONIC##64mi32: \
case X86::MNEMONIC##8i8: \
case X86::MNEMONIC##16i16: \
case X86::MNEMONIC##32i32: \
case X86::MNEMONIC##64i32:
LLVM_EXPAND_ADC_SBB_INSTR(ADC)
return FlagArithMnemonic::ADC;
LLVM_EXPAND_ADC_SBB_INSTR(SBB)
return FlagArithMnemonic::SBB;
#undef LLVM_EXPAND_ADC_SBB_INSTR
LLVM_EXPAND_INSTR_SIZES(RCL, rCL)
LLVM_EXPAND_INSTR_SIZES(RCL, r1)
LLVM_EXPAND_INSTR_SIZES(RCL, ri)
return FlagArithMnemonic::RCL;
LLVM_EXPAND_INSTR_SIZES(RCR, rCL)
LLVM_EXPAND_INSTR_SIZES(RCR, r1)
LLVM_EXPAND_INSTR_SIZES(RCR, ri)
return FlagArithMnemonic::RCR;
#undef LLVM_EXPAND_INSTR_SIZES
case X86::ADCX32rr:
case X86::ADCX64rr:
case X86::ADCX32rm:
case X86::ADCX64rm:
return FlagArithMnemonic::ADCX;
case X86::ADOX32rr:
case X86::ADOX64rr:
case X86::ADOX32rm:
case X86::ADOX64rm:
return FlagArithMnemonic::ADOX;
case X86::SETB_C32r:
case X86::SETB_C64r:
return FlagArithMnemonic::SETB;
}
}
static MachineBasicBlock &splitBlock(MachineBasicBlock &MBB,
MachineInstr &SplitI,
const X86InstrInfo &TII) {
MachineFunction &MF = *MBB.getParent();
assert(SplitI.getParent() == &MBB &&
"Split instruction must be in the split block!");
assert(SplitI.isBranch() &&
"Only designed to split a tail of branch instructions!");
assert(X86::getCondFromBranch(SplitI) != X86::COND_INVALID &&
"Must split on an actual jCC instruction!");
// Dig out the previous instruction to the split point.
MachineInstr &PrevI = *std::prev(SplitI.getIterator());
assert(PrevI.isBranch() && "Must split after a branch!");
assert(X86::getCondFromBranch(PrevI) != X86::COND_INVALID &&
"Must split after an actual jCC instruction!");
assert(!std::prev(PrevI.getIterator())->isTerminator() &&
"Must only have this one terminator prior to the split!");
// Grab the one successor edge that will stay in `MBB`.
MachineBasicBlock &UnsplitSucc = *PrevI.getOperand(0).getMBB();
// Analyze the original block to see if we are actually splitting an edge
// into two edges. This can happen when we have multiple conditional jumps to
// the same successor.
bool IsEdgeSplit =
std::any_of(SplitI.getIterator(), MBB.instr_end(),
[&](MachineInstr &MI) {
assert(MI.isTerminator() &&
"Should only have spliced terminators!");
return llvm::any_of(
MI.operands(), [&](MachineOperand &MOp) {
return MOp.isMBB() && MOp.getMBB() == &UnsplitSucc;
});
}) ||
MBB.getFallThrough() == &UnsplitSucc;
MachineBasicBlock &NewMBB = *MF.CreateMachineBasicBlock();
// Insert the new block immediately after the current one. Any existing
// fallthrough will be sunk into this new block anyways.
MF.insert(std::next(MachineFunction::iterator(&MBB)), &NewMBB);
// Splice the tail of instructions into the new block.
NewMBB.splice(NewMBB.end(), &MBB, SplitI.getIterator(), MBB.end());
// Copy the necessary succesors (and their probability info) into the new
// block.
for (auto SI = MBB.succ_begin(), SE = MBB.succ_end(); SI != SE; ++SI)
if (IsEdgeSplit || *SI != &UnsplitSucc)
NewMBB.copySuccessor(&MBB, SI);
// Normalize the probabilities if we didn't end up splitting the edge.
if (!IsEdgeSplit)
NewMBB.normalizeSuccProbs();
// Now replace all of the moved successors in the original block with the new
// block. This will merge their probabilities.
for (MachineBasicBlock *Succ : NewMBB.successors())
if (Succ != &UnsplitSucc)
MBB.replaceSuccessor(Succ, &NewMBB);
// We should always end up replacing at least one successor.
assert(MBB.isSuccessor(&NewMBB) &&
"Failed to make the new block a successor!");
// Now update all the PHIs.
for (MachineBasicBlock *Succ : NewMBB.successors()) {
for (MachineInstr &MI : *Succ) {
if (!MI.isPHI())
break;
for (int OpIdx = 1, NumOps = MI.getNumOperands(); OpIdx < NumOps;
OpIdx += 2) {
MachineOperand &OpV = MI.getOperand(OpIdx);
MachineOperand &OpMBB = MI.getOperand(OpIdx + 1);
assert(OpMBB.isMBB() && "Block operand to a PHI is not a block!");
if (OpMBB.getMBB() != &MBB)
continue;
// Replace the operand for unsplit successors
if (!IsEdgeSplit || Succ != &UnsplitSucc) {
OpMBB.setMBB(&NewMBB);
// We have to continue scanning as there may be multiple entries in
// the PHI.
continue;
}
// When we have split the edge append a new successor.
MI.addOperand(MF, OpV);
MI.addOperand(MF, MachineOperand::CreateMBB(&NewMBB));
break;
}
}
}
return NewMBB;
}
static X86::CondCode getCondFromFCMOV(unsigned Opcode) {
switch (Opcode) {
default: return X86::COND_INVALID;
case X86::CMOVBE_Fp32: case X86::CMOVBE_Fp64: case X86::CMOVBE_Fp80:
return X86::COND_BE;
case X86::CMOVB_Fp32: case X86::CMOVB_Fp64: case X86::CMOVB_Fp80:
return X86::COND_B;
case X86::CMOVE_Fp32: case X86::CMOVE_Fp64: case X86::CMOVE_Fp80:
return X86::COND_E;
case X86::CMOVNBE_Fp32: case X86::CMOVNBE_Fp64: case X86::CMOVNBE_Fp80:
return X86::COND_A;
case X86::CMOVNB_Fp32: case X86::CMOVNB_Fp64: case X86::CMOVNB_Fp80:
return X86::COND_AE;
case X86::CMOVNE_Fp32: case X86::CMOVNE_Fp64: case X86::CMOVNE_Fp80:
return X86::COND_NE;
case X86::CMOVNP_Fp32: case X86::CMOVNP_Fp64: case X86::CMOVNP_Fp80:
return X86::COND_NP;
case X86::CMOVP_Fp32: case X86::CMOVP_Fp64: case X86::CMOVP_Fp80:
return X86::COND_P;
}
}
bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** " << getPassName() << " : " << MF.getName()
<< " **********\n");
Subtarget = &MF.getSubtarget<X86Subtarget>();
MRI = &MF.getRegInfo();
TII = Subtarget->getInstrInfo();
TRI = Subtarget->getRegisterInfo();
MDT = &getAnalysis<MachineDominatorTree>();
PromoteRC = &X86::GR8RegClass;
if (MF.begin() == MF.end())
// Nothing to do for a degenerate empty function...
return false;
// Collect the copies in RPO so that when there are chains where a copy is in
// turn copied again we visit the first one first. This ensures we can find
// viable locations for testing the original EFLAGS that dominate all the
// uses across complex CFGs.
SmallVector<MachineInstr *, 4> Copies;
ReversePostOrderTraversal<MachineFunction *> RPOT(&MF);
for (MachineBasicBlock *MBB : RPOT)
for (MachineInstr &MI : *MBB)
if (MI.getOpcode() == TargetOpcode::COPY &&
MI.getOperand(0).getReg() == X86::EFLAGS)
Copies.push_back(&MI);
for (MachineInstr *CopyI : Copies) {
MachineBasicBlock &MBB = *CopyI->getParent();
MachineOperand &VOp = CopyI->getOperand(1);
assert(VOp.isReg() &&
"The input to the copy for EFLAGS should always be a register!");
MachineInstr &CopyDefI = *MRI->getVRegDef(VOp.getReg());
if (CopyDefI.getOpcode() != TargetOpcode::COPY) {
// FIXME: The big likely candidate here are PHI nodes. We could in theory
// handle PHI nodes, but it gets really, really hard. Insanely hard. Hard
// enough that it is probably better to change every other part of LLVM
// to avoid creating them. The issue is that once we have PHIs we won't
// know which original EFLAGS value we need to capture with our setCCs
// below. The end result will be computing a complete set of setCCs that
// we *might* want, computing them in every place where we copy *out* of
// EFLAGS and then doing SSA formation on all of them to insert necessary
// PHI nodes and consume those here. Then hoping that somehow we DCE the
// unnecessary ones. This DCE seems very unlikely to be successful and so
// we will almost certainly end up with a glut of dead setCC
// instructions. Until we have a motivating test case and fail to avoid
// it by changing other parts of LLVM's lowering, we refuse to handle
// this complex case here.
LLVM_DEBUG(
dbgs() << "ERROR: Encountered unexpected def of an eflags copy: ";
CopyDefI.dump());
report_fatal_error(
"Cannot lower EFLAGS copy unless it is defined in turn by a copy!");
}
auto Cleanup = make_scope_exit([&] {
// All uses of the EFLAGS copy are now rewritten, kill the copy into
// eflags and if dead the copy from.
CopyI->eraseFromParent();
if (MRI->use_empty(CopyDefI.getOperand(0).getReg()))
CopyDefI.eraseFromParent();
++NumCopiesEliminated;
});
MachineOperand &DOp = CopyI->getOperand(0);
assert(DOp.isDef() && "Expected register def!");
assert(DOp.getReg() == X86::EFLAGS && "Unexpected copy def register!");
if (DOp.isDead())
continue;
MachineBasicBlock *TestMBB = CopyDefI.getParent();
auto TestPos = CopyDefI.getIterator();
DebugLoc TestLoc = CopyDefI.getDebugLoc();
LLVM_DEBUG(dbgs() << "Rewriting copy: "; CopyI->dump());
// Walk up across live-in EFLAGS to find where they were actually def'ed.
//
// This copy's def may just be part of a region of blocks covered by
// a single def of EFLAGS and we want to find the top of that region where
// possible.
//
// This is essentially a search for a *candidate* reaching definition
// location. We don't need to ever find the actual reaching definition here,
// but we want to walk up the dominator tree to find the highest point which
// would be viable for such a definition.
auto HasEFLAGSClobber = [&](MachineBasicBlock::iterator Begin,
MachineBasicBlock::iterator End) {
// Scan backwards as we expect these to be relatively short and often find
// a clobber near the end.
return llvm::any_of(
llvm::reverse(llvm::make_range(Begin, End)), [&](MachineInstr &MI) {
// Flag any instruction (other than the copy we are
// currently rewriting) that defs EFLAGS.
return &MI != CopyI && MI.findRegisterDefOperand(X86::EFLAGS);
});
};
auto HasEFLAGSClobberPath = [&](MachineBasicBlock *BeginMBB,
MachineBasicBlock *EndMBB) {
assert(MDT->dominates(BeginMBB, EndMBB) &&
"Only support paths down the dominator tree!");
SmallPtrSet<MachineBasicBlock *, 4> Visited;
SmallVector<MachineBasicBlock *, 4> Worklist;
// We terminate at the beginning. No need to scan it.
Visited.insert(BeginMBB);
Worklist.push_back(EndMBB);
do {
auto *MBB = Worklist.pop_back_val();
for (auto *PredMBB : MBB->predecessors()) {
if (!Visited.insert(PredMBB).second)
continue;
if (HasEFLAGSClobber(PredMBB->begin(), PredMBB->end()))
return true;
// Enqueue this block to walk its predecessors.
Worklist.push_back(PredMBB);
}
} while (!Worklist.empty());
// No clobber found along a path from the begin to end.
return false;
};
while (TestMBB->isLiveIn(X86::EFLAGS) && !TestMBB->pred_empty() &&
!HasEFLAGSClobber(TestMBB->begin(), TestPos)) {
// Find the nearest common dominator of the predecessors, as
// that will be the best candidate to hoist into.
MachineBasicBlock *HoistMBB =
std::accumulate(std::next(TestMBB->pred_begin()), TestMBB->pred_end(),
*TestMBB->pred_begin(),
[&](MachineBasicBlock *LHS, MachineBasicBlock *RHS) {
return MDT->findNearestCommonDominator(LHS, RHS);
});
// Now we need to scan all predecessors that may be reached along paths to
// the hoist block. A clobber anywhere in any of these blocks the hoist.
// Note that this even handles loops because we require *no* clobbers.
if (HasEFLAGSClobberPath(HoistMBB, TestMBB))
break;
// We also need the terminators to not sneakily clobber flags.
if (HasEFLAGSClobber(HoistMBB->getFirstTerminator()->getIterator(),
HoistMBB->instr_end()))
break;
// We found a viable location, hoist our test position to it.
TestMBB = HoistMBB;
TestPos = TestMBB->getFirstTerminator()->getIterator();
// Clear the debug location as it would just be confusing after hoisting.
TestLoc = DebugLoc();
}
LLVM_DEBUG({
auto DefIt = llvm::find_if(
llvm::reverse(llvm::make_range(TestMBB->instr_begin(), TestPos)),
[&](MachineInstr &MI) {
return MI.findRegisterDefOperand(X86::EFLAGS);
});
if (DefIt.base() != TestMBB->instr_begin()) {
dbgs() << " Using EFLAGS defined by: ";
DefIt->dump();
} else {
dbgs() << " Using live-in flags for BB:\n";
TestMBB->dump();
}
});
// While rewriting uses, we buffer jumps and rewrite them in a second pass
// because doing so will perturb the CFG that we are walking to find the
// uses in the first place.
SmallVector<MachineInstr *, 4> JmpIs;
// Gather the condition flags that have already been preserved in
// registers. We do this from scratch each time as we expect there to be
// very few of them and we expect to not revisit the same copy definition
// many times. If either of those change sufficiently we could build a map
// of these up front instead.
CondRegArray CondRegs = collectCondsInRegs(*TestMBB, TestPos);
// Collect the basic blocks we need to scan. Typically this will just be
// a single basic block but we may have to scan multiple blocks if the
// EFLAGS copy lives into successors.
SmallVector<MachineBasicBlock *, 2> Blocks;
SmallPtrSet<MachineBasicBlock *, 2> VisitedBlocks;
Blocks.push_back(&MBB);
do {
MachineBasicBlock &UseMBB = *Blocks.pop_back_val();
// Track when if/when we find a kill of the flags in this block.
bool FlagsKilled = false;
// In most cases, we walk from the beginning to the end of the block. But
// when the block is the same block as the copy is from, we will visit it
// twice. The first time we start from the copy and go to the end. The
// second time we start from the beginning and go to the copy. This lets
// us handle copies inside of cycles.
// FIXME: This loop is *super* confusing. This is at least in part
// a symptom of all of this routine needing to be refactored into
// documentable components. Once done, there may be a better way to write
// this loop.
for (auto MII = (&UseMBB == &MBB && !VisitedBlocks.count(&UseMBB))
? std::next(CopyI->getIterator())
: UseMBB.instr_begin(),
MIE = UseMBB.instr_end();
MII != MIE;) {
MachineInstr &MI = *MII++;
// If we are in the original copy block and encounter either the copy
// def or the copy itself, break so that we don't re-process any part of
// the block or process the instructions in the range that was copied
// over.
if (&MI == CopyI || &MI == &CopyDefI) {
assert(&UseMBB == &MBB && VisitedBlocks.count(&MBB) &&
"Should only encounter these on the second pass over the "
"original block.");
break;
}
MachineOperand *FlagUse = MI.findRegisterUseOperand(X86::EFLAGS);
if (!FlagUse) {
if (MI.findRegisterDefOperand(X86::EFLAGS)) {
// If EFLAGS are defined, it's as-if they were killed. We can stop
// scanning here.
//
// NB!!! Many instructions only modify some flags. LLVM currently
// models this as clobbering all flags, but if that ever changes
// this will need to be carefully updated to handle that more
// complex logic.
FlagsKilled = true;
break;
}
continue;
}
LLVM_DEBUG(dbgs() << " Rewriting use: "; MI.dump());
// Check the kill flag before we rewrite as that may change it.
if (FlagUse->isKill())
FlagsKilled = true;
// Once we encounter a branch, the rest of the instructions must also be
// branches. We can't rewrite in place here, so we handle them below.
//
// Note that we don't have to handle tail calls here, even conditional
// tail calls, as those are not introduced into the X86 MI until post-RA
// branch folding or black placement. As a consequence, we get to deal
// with the simpler formulation of conditional branches followed by tail
// calls.
if (X86::getCondFromBranch(MI) != X86::COND_INVALID) {
auto JmpIt = MI.getIterator();
do {
JmpIs.push_back(&*JmpIt);
++JmpIt;
} while (JmpIt != UseMBB.instr_end() &&
X86::getCondFromBranch(*JmpIt) !=
X86::COND_INVALID);
break;
}
// Otherwise we can just rewrite in-place.
if (X86::getCondFromCMov(MI) != X86::COND_INVALID) {
rewriteCMov(*TestMBB, TestPos, TestLoc, MI, *FlagUse, CondRegs);
} else if (getCondFromFCMOV(MI.getOpcode()) != X86::COND_INVALID) {
rewriteFCMov(*TestMBB, TestPos, TestLoc, MI, *FlagUse, CondRegs);
} else if (X86::getCondFromSETCC(MI) != X86::COND_INVALID) {
rewriteSetCC(*TestMBB, TestPos, TestLoc, MI, *FlagUse, CondRegs);
} else if (MI.getOpcode() == TargetOpcode::COPY) {
rewriteCopy(MI, *FlagUse, CopyDefI);
} else {
// We assume all other instructions that use flags also def them.
assert(MI.findRegisterDefOperand(X86::EFLAGS) &&
"Expected a def of EFLAGS for this instruction!");
// NB!!! Several arithmetic instructions only *partially* update
// flags. Theoretically, we could generate MI code sequences that
// would rely on this fact and observe different flags independently.
// But currently LLVM models all of these instructions as clobbering
// all the flags in an undef way. We rely on that to simplify the
// logic.
FlagsKilled = true;
// Generically handle remaining uses as arithmetic instructions.
rewriteArithmetic(*TestMBB, TestPos, TestLoc, MI, *FlagUse,
CondRegs);
}
// If this was the last use of the flags, we're done.
if (FlagsKilled)
break;
}
// If the flags were killed, we're done with this block.
if (FlagsKilled)
continue;
// Otherwise we need to scan successors for ones where the flags live-in
// and queue those up for processing.
for (MachineBasicBlock *SuccMBB : UseMBB.successors())
if (SuccMBB->isLiveIn(X86::EFLAGS) &&
VisitedBlocks.insert(SuccMBB).second) {
// We currently don't do any PHI insertion and so we require that the
// test basic block dominates all of the use basic blocks. Further, we
// can't have a cycle from the test block back to itself as that would
// create a cycle requiring a PHI to break it.
//
// We could in theory do PHI insertion here if it becomes useful by
// just taking undef values in along every edge that we don't trace
// this EFLAGS copy along. This isn't as bad as fully general PHI
// insertion, but still seems like a great deal of complexity.
//
// Because it is theoretically possible that some earlier MI pass or
// other lowering transformation could induce this to happen, we do
// a hard check even in non-debug builds here.
if (SuccMBB == TestMBB || !MDT->dominates(TestMBB, SuccMBB)) {
LLVM_DEBUG({
dbgs()
<< "ERROR: Encountered use that is not dominated by our test "
"basic block! Rewriting this would require inserting PHI "
"nodes to track the flag state across the CFG.\n\nTest "
"block:\n";
TestMBB->dump();
dbgs() << "Use block:\n";
SuccMBB->dump();
});
report_fatal_error(
"Cannot lower EFLAGS copy when original copy def "
"does not dominate all uses.");
}
Blocks.push_back(SuccMBB);
// After this, EFLAGS will be recreated before each use.
SuccMBB->removeLiveIn(X86::EFLAGS);
}
} while (!Blocks.empty());
// Now rewrite the jumps that use the flags. These we handle specially
// because if there are multiple jumps in a single basic block we'll have
// to do surgery on the CFG.
MachineBasicBlock *LastJmpMBB = nullptr;
for (MachineInstr *JmpI : JmpIs) {
// Past the first jump within a basic block we need to split the blocks
// apart.
if (JmpI->getParent() == LastJmpMBB)
splitBlock(*JmpI->getParent(), *JmpI, *TII);
else
LastJmpMBB = JmpI->getParent();
rewriteCondJmp(*TestMBB, TestPos, TestLoc, *JmpI, CondRegs);
}
// FIXME: Mark the last use of EFLAGS before the copy's def as a kill if
// the copy's def operand is itself a kill.
}
#ifndef NDEBUG
for (MachineBasicBlock &MBB : MF)
for (MachineInstr &MI : MBB)
if (MI.getOpcode() == TargetOpcode::COPY &&
(MI.getOperand(0).getReg() == X86::EFLAGS ||
MI.getOperand(1).getReg() == X86::EFLAGS)) {
LLVM_DEBUG(dbgs() << "ERROR: Found a COPY involving EFLAGS: ";
MI.dump());
llvm_unreachable("Unlowered EFLAGS copy!");
}
#endif
return true;
}
/// Collect any conditions that have already been set in registers so that we
/// can re-use them rather than adding duplicates.
CondRegArray X86FlagsCopyLoweringPass::collectCondsInRegs(
MachineBasicBlock &MBB, MachineBasicBlock::iterator TestPos) {
CondRegArray CondRegs = {};
// Scan backwards across the range of instructions with live EFLAGS.
for (MachineInstr &MI :
llvm::reverse(llvm::make_range(MBB.begin(), TestPos))) {
X86::CondCode Cond = X86::getCondFromSETCC(MI);
if (Cond != X86::COND_INVALID && !MI.mayStore() &&
MI.getOperand(0).isReg() &&
Register::isVirtualRegister(MI.getOperand(0).getReg())) {
assert(MI.getOperand(0).isDef() &&
"A non-storing SETcc should always define a register!");
CondRegs[Cond] = MI.getOperand(0).getReg();
}
// Stop scanning when we see the first definition of the EFLAGS as prior to
// this we would potentially capture the wrong flag state.
if (MI.findRegisterDefOperand(X86::EFLAGS))
break;
}
return CondRegs;
}
unsigned X86FlagsCopyLoweringPass::promoteCondToReg(
MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc, X86::CondCode Cond) {
Register Reg = MRI->createVirtualRegister(PromoteRC);
auto SetI = BuildMI(TestMBB, TestPos, TestLoc,
TII->get(X86::SETCCr), Reg).addImm(Cond);
(void)SetI;
LLVM_DEBUG(dbgs() << " save cond: "; SetI->dump());
++NumSetCCsInserted;
return Reg;
}
std::pair<unsigned, bool> X86FlagsCopyLoweringPass::getCondOrInverseInReg(
MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc, X86::CondCode Cond, CondRegArray &CondRegs) {
unsigned &CondReg = CondRegs[Cond];
unsigned &InvCondReg = CondRegs[X86::GetOppositeBranchCondition(Cond)];
if (!CondReg && !InvCondReg)
CondReg = promoteCondToReg(TestMBB, TestPos, TestLoc, Cond);
if (CondReg)
return {CondReg, false};
else
return {InvCondReg, true};
}
void X86FlagsCopyLoweringPass::insertTest(MachineBasicBlock &MBB,
MachineBasicBlock::iterator Pos,
DebugLoc Loc, unsigned Reg) {
auto TestI =
BuildMI(MBB, Pos, Loc, TII->get(X86::TEST8rr)).addReg(Reg).addReg(Reg);
(void)TestI;
LLVM_DEBUG(dbgs() << " test cond: "; TestI->dump());
++NumTestsInserted;
}
void X86FlagsCopyLoweringPass::rewriteArithmetic(
MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc, MachineInstr &MI, MachineOperand &FlagUse,
CondRegArray &CondRegs) {
// Arithmetic is either reading CF or OF. Figure out which condition we need
// to preserve in a register.
X86::CondCode Cond = X86::COND_INVALID;
// The addend to use to reset CF or OF when added to the flag value.
int Addend = 0;
switch (getMnemonicFromOpcode(MI.getOpcode())) {
case FlagArithMnemonic::ADC:
case FlagArithMnemonic::ADCX:
case FlagArithMnemonic::RCL:
case FlagArithMnemonic::RCR:
case FlagArithMnemonic::SBB:
case FlagArithMnemonic::SETB:
Cond = X86::COND_B; // CF == 1
// Set up an addend that when one is added will need a carry due to not
// having a higher bit available.
Addend = 255;
break;
case FlagArithMnemonic::ADOX:
Cond = X86::COND_O; // OF == 1
// Set up an addend that when one is added will turn from positive to
// negative and thus overflow in the signed domain.
Addend = 127;
break;
}
// Now get a register that contains the value of the flag input to the
// arithmetic. We require exactly this flag to simplify the arithmetic
// required to materialize it back into the flag.
unsigned &CondReg = CondRegs[Cond];
if (!CondReg)
CondReg = promoteCondToReg(TestMBB, TestPos, TestLoc, Cond);
MachineBasicBlock &MBB = *MI.getParent();
// Insert an instruction that will set the flag back to the desired value.
Register TmpReg = MRI->createVirtualRegister(PromoteRC);
auto AddI =
BuildMI(MBB, MI.getIterator(), MI.getDebugLoc(), TII->get(X86::ADD8ri))
.addDef(TmpReg, RegState::Dead)
.addReg(CondReg)
.addImm(Addend);
(void)AddI;
LLVM_DEBUG(dbgs() << " add cond: "; AddI->dump());
++NumAddsInserted;
FlagUse.setIsKill(true);
}
void X86FlagsCopyLoweringPass::rewriteCMov(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc,
MachineInstr &CMovI,
MachineOperand &FlagUse,
CondRegArray &CondRegs) {
// First get the register containing this specific condition.
X86::CondCode Cond = X86::getCondFromCMov(CMovI);
unsigned CondReg;
bool Inverted;
std::tie(CondReg, Inverted) =
getCondOrInverseInReg(TestMBB, TestPos, TestLoc, Cond, CondRegs);
MachineBasicBlock &MBB = *CMovI.getParent();
// Insert a direct test of the saved register.
insertTest(MBB, CMovI.getIterator(), CMovI.getDebugLoc(), CondReg);
// Rewrite the CMov to use the !ZF flag from the test, and then kill its use
// of the flags afterward.
CMovI.getOperand(CMovI.getDesc().getNumOperands() - 1)
.setImm(Inverted ? X86::COND_E : X86::COND_NE);
FlagUse.setIsKill(true);
LLVM_DEBUG(dbgs() << " fixed cmov: "; CMovI.dump());
}
void X86FlagsCopyLoweringPass::rewriteFCMov(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc,
MachineInstr &CMovI,
MachineOperand &FlagUse,
CondRegArray &CondRegs) {
// First get the register containing this specific condition.
X86::CondCode Cond = getCondFromFCMOV(CMovI.getOpcode());
unsigned CondReg;
bool Inverted;
std::tie(CondReg, Inverted) =
getCondOrInverseInReg(TestMBB, TestPos, TestLoc, Cond, CondRegs);
MachineBasicBlock &MBB = *CMovI.getParent();
// Insert a direct test of the saved register.
insertTest(MBB, CMovI.getIterator(), CMovI.getDebugLoc(), CondReg);
auto getFCMOVOpcode = [](unsigned Opcode, bool Inverted) {
switch (Opcode) {
default: llvm_unreachable("Unexpected opcode!");
case X86::CMOVBE_Fp32: case X86::CMOVNBE_Fp32:
case X86::CMOVB_Fp32: case X86::CMOVNB_Fp32:
case X86::CMOVE_Fp32: case X86::CMOVNE_Fp32:
case X86::CMOVP_Fp32: case X86::CMOVNP_Fp32:
return Inverted ? X86::CMOVE_Fp32 : X86::CMOVNE_Fp32;
case X86::CMOVBE_Fp64: case X86::CMOVNBE_Fp64:
case X86::CMOVB_Fp64: case X86::CMOVNB_Fp64:
case X86::CMOVE_Fp64: case X86::CMOVNE_Fp64:
case X86::CMOVP_Fp64: case X86::CMOVNP_Fp64:
return Inverted ? X86::CMOVE_Fp64 : X86::CMOVNE_Fp64;
case X86::CMOVBE_Fp80: case X86::CMOVNBE_Fp80:
case X86::CMOVB_Fp80: case X86::CMOVNB_Fp80:
case X86::CMOVE_Fp80: case X86::CMOVNE_Fp80:
case X86::CMOVP_Fp80: case X86::CMOVNP_Fp80:
return Inverted ? X86::CMOVE_Fp80 : X86::CMOVNE_Fp80;
}
};
// Rewrite the CMov to use the !ZF flag from the test.
CMovI.setDesc(TII->get(getFCMOVOpcode(CMovI.getOpcode(), Inverted)));
FlagUse.setIsKill(true);
LLVM_DEBUG(dbgs() << " fixed fcmov: "; CMovI.dump());
}
void X86FlagsCopyLoweringPass::rewriteCondJmp(
MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc, MachineInstr &JmpI, CondRegArray &CondRegs) {
// First get the register containing this specific condition.
X86::CondCode Cond = X86::getCondFromBranch(JmpI);
unsigned CondReg;
bool Inverted;
std::tie(CondReg, Inverted) =
getCondOrInverseInReg(TestMBB, TestPos, TestLoc, Cond, CondRegs);
MachineBasicBlock &JmpMBB = *JmpI.getParent();
// Insert a direct test of the saved register.
insertTest(JmpMBB, JmpI.getIterator(), JmpI.getDebugLoc(), CondReg);
// Rewrite the jump to use the !ZF flag from the test, and kill its use of
// flags afterward.
JmpI.getOperand(1).setImm(Inverted ? X86::COND_E : X86::COND_NE);
JmpI.findRegisterUseOperand(X86::EFLAGS)->setIsKill(true);
LLVM_DEBUG(dbgs() << " fixed jCC: "; JmpI.dump());
}
void X86FlagsCopyLoweringPass::rewriteCopy(MachineInstr &MI,
MachineOperand &FlagUse,
MachineInstr &CopyDefI) {
// Just replace this copy with the original copy def.
MRI->replaceRegWith(MI.getOperand(0).getReg(),
CopyDefI.getOperand(0).getReg());
MI.eraseFromParent();
}
void X86FlagsCopyLoweringPass::rewriteSetCC(MachineBasicBlock &TestMBB,
MachineBasicBlock::iterator TestPos,
DebugLoc TestLoc,
MachineInstr &SetCCI,
MachineOperand &FlagUse,
CondRegArray &CondRegs) {
X86::CondCode Cond = X86::getCondFromSETCC(SetCCI);
// Note that we can't usefully rewrite this to the inverse without complex
// analysis of the users of the setCC. Largely we rely on duplicates which
// could have been avoided already being avoided here.
unsigned &CondReg = CondRegs[Cond];
if (!CondReg)
CondReg = promoteCondToReg(TestMBB, TestPos, TestLoc, Cond);
// Rewriting a register def is trivial: we just replace the register and
// remove the setcc.
if (!SetCCI.mayStore()) {
assert(SetCCI.getOperand(0).isReg() &&
"Cannot have a non-register defined operand to SETcc!");
MRI->replaceRegWith(SetCCI.getOperand(0).getReg(), CondReg);
SetCCI.eraseFromParent();
return;
}
// Otherwise, we need to emit a store.
auto MIB = BuildMI(*SetCCI.getParent(), SetCCI.getIterator(),
SetCCI.getDebugLoc(), TII->get(X86::MOV8mr));
// Copy the address operands.
for (int i = 0; i < X86::AddrNumOperands; ++i)
MIB.add(SetCCI.getOperand(i));
MIB.addReg(CondReg);
MIB.setMemRefs(SetCCI.memoperands());
SetCCI.eraseFromParent();
return;
}
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