//===--------------- PPCVSXFMAMutate.cpp - VSX FMA Mutation ---------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This pass mutates the form of VSX FMA instructions to avoid unnecessary
// copies.
//
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/PPCPredicates.h"
#include "PPC.h"
#include "PPCInstrBuilder.h"
#include "PPCInstrInfo.h"
#include "PPCMachineFunctionInfo.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/PseudoSourceValue.h"
#include "llvm/CodeGen/ScheduleDAG.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
// Temporarily disable FMA mutation by default, since it doesn't handle
// cross-basic-block intervals well.
// See: http://lists.llvm.org/pipermail/llvm-dev/2016-February/095669.html
// http://reviews.llvm.org/D17087
static cl::opt<bool> DisableVSXFMAMutate(
"disable-ppc-vsx-fma-mutation",
cl::desc("Disable VSX FMA instruction mutation"), cl::init(true),
cl::Hidden);
#define DEBUG_TYPE "ppc-vsx-fma-mutate"
namespace llvm { namespace PPC {
int getAltVSXFMAOpcode(uint16_t Opcode);
} }
namespace {
// PPCVSXFMAMutate pass - For copies between VSX registers and non-VSX registers
// (Altivec and scalar floating-point registers), we need to transform the
// copies into subregister copies with other restrictions.
struct PPCVSXFMAMutate : public MachineFunctionPass {
static char ID;
PPCVSXFMAMutate() : MachineFunctionPass(ID) {
initializePPCVSXFMAMutatePass(*PassRegistry::getPassRegistry());
}
LiveIntervals *LIS;
const PPCInstrInfo *TII;
protected:
bool processBlock(MachineBasicBlock &MBB) {
bool Changed = false;
MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
const TargetRegisterInfo *TRI = &TII->getRegisterInfo();
for (MachineBasicBlock::iterator I = MBB.begin(), IE = MBB.end();
I != IE; ++I) {
MachineInstr &MI = *I;
// The default (A-type) VSX FMA form kills the addend (it is taken from
// the target register, which is then updated to reflect the result of
// the FMA). If the instruction, however, kills one of the registers
// used for the product, then we can use the M-form instruction (which
// will take that value from the to-be-defined register).
int AltOpc = PPC::getAltVSXFMAOpcode(MI.getOpcode());
if (AltOpc == -1)
continue;
// This pass is run after register coalescing, and so we're looking for
// a situation like this:
// ...
// %5 = COPY %9; VSLRC:%5,%9
// %5<def,tied1> = XSMADDADP %5<tied0>, %17, %16,
// implicit %rm; VSLRC:%5,%17,%16
// ...
// %9<def,tied1> = XSMADDADP %9<tied0>, %17, %19,
// implicit %rm; VSLRC:%9,%17,%19
// ...
// Where we can eliminate the copy by changing from the A-type to the
// M-type instruction. Specifically, for this example, this means:
// %5<def,tied1> = XSMADDADP %5<tied0>, %17, %16,
// implicit %rm; VSLRC:%5,%17,%16
// is replaced by:
// %16<def,tied1> = XSMADDMDP %16<tied0>, %18, %9,
// implicit %rm; VSLRC:%16,%18,%9
// and we remove: %5 = COPY %9; VSLRC:%5,%9
SlotIndex FMAIdx = LIS->getInstructionIndex(MI);
VNInfo *AddendValNo =
LIS->getInterval(MI.getOperand(1).getReg()).Query(FMAIdx).valueIn();
// This can be null if the register is undef.
if (!AddendValNo)
continue;
MachineInstr *AddendMI = LIS->getInstructionFromIndex(AddendValNo->def);
// The addend and this instruction must be in the same block.
if (!AddendMI || AddendMI->getParent() != MI.getParent())
continue;
// The addend must be a full copy within the same register class.
if (!AddendMI->isFullCopy())
continue;
Register AddendSrcReg = AddendMI->getOperand(1).getReg();
if (Register::isVirtualRegister(AddendSrcReg)) {
if (MRI.getRegClass(AddendMI->getOperand(0).getReg()) !=
MRI.getRegClass(AddendSrcReg))
continue;
} else {
// If AddendSrcReg is a physical register, make sure the destination
// register class contains it.
if (!MRI.getRegClass(AddendMI->getOperand(0).getReg())
->contains(AddendSrcReg))
continue;
}
// In theory, there could be other uses of the addend copy before this
// fma. We could deal with this, but that would require additional
// logic below and I suspect it will not occur in any relevant
// situations. Additionally, check whether the copy source is killed
// prior to the fma. In order to replace the addend here with the
// source of the copy, it must still be live here. We can't use
// interval testing for a physical register, so as long as we're
// walking the MIs we may as well test liveness here.
//
// FIXME: There is a case that occurs in practice, like this:
// %9 = COPY %f1; VSSRC:%9
// ...
// %6 = COPY %9; VSSRC:%6,%9
// %7 = COPY %9; VSSRC:%7,%9
// %9<def,tied1> = XSMADDASP %9<tied0>, %1, %4; VSSRC:
// %6<def,tied1> = XSMADDASP %6<tied0>, %1, %2; VSSRC:
// %7<def,tied1> = XSMADDASP %7<tied0>, %1, %3; VSSRC:
// which prevents an otherwise-profitable transformation.
bool OtherUsers = false, KillsAddendSrc = false;
for (auto J = std::prev(I), JE = MachineBasicBlock::iterator(AddendMI);
J != JE; --J) {
if (J->readsVirtualRegister(AddendMI->getOperand(0).getReg())) {
OtherUsers = true;
break;
}
if (J->modifiesRegister(AddendSrcReg, TRI) ||
J->killsRegister(AddendSrcReg, TRI)) {
KillsAddendSrc = true;
break;
}
}
if (OtherUsers || KillsAddendSrc)
continue;
// The transformation doesn't work well with things like:
// %5 = A-form-op %5, %11, %5;
// unless %11 is also a kill, so skip when it is not,
// and check operand 3 to see it is also a kill to handle the case:
// %5 = A-form-op %5, %5, %11;
// where %5 and %11 are both kills. This case would be skipped
// otherwise.
Register OldFMAReg = MI.getOperand(0).getReg();
// Find one of the product operands that is killed by this instruction.
unsigned KilledProdOp = 0, OtherProdOp = 0;
Register Reg2 = MI.getOperand(2).getReg();
Register Reg3 = MI.getOperand(3).getReg();
if (LIS->getInterval(Reg2).Query(FMAIdx).isKill()
&& Reg2 != OldFMAReg) {
KilledProdOp = 2;
OtherProdOp = 3;
} else if (LIS->getInterval(Reg3).Query(FMAIdx).isKill()
&& Reg3 != OldFMAReg) {
KilledProdOp = 3;
OtherProdOp = 2;
}
// If there are no usable killed product operands, then this
// transformation is likely not profitable.
if (!KilledProdOp)
continue;
// If the addend copy is used only by this MI, then the addend source
// register is likely not live here. This could be fixed (based on the
// legality checks above, the live range for the addend source register
// could be extended), but it seems likely that such a trivial copy can
// be coalesced away later, and thus is not worth the effort.
if (Register::isVirtualRegister(AddendSrcReg) &&
!LIS->getInterval(AddendSrcReg).liveAt(FMAIdx))
continue;
// Transform: (O2 * O3) + O1 -> (O2 * O1) + O3.
Register KilledProdReg = MI.getOperand(KilledProdOp).getReg();
Register OtherProdReg = MI.getOperand(OtherProdOp).getReg();
unsigned AddSubReg = AddendMI->getOperand(1).getSubReg();
unsigned KilledProdSubReg = MI.getOperand(KilledProdOp).getSubReg();
unsigned OtherProdSubReg = MI.getOperand(OtherProdOp).getSubReg();
bool AddRegKill = AddendMI->getOperand(1).isKill();
bool KilledProdRegKill = MI.getOperand(KilledProdOp).isKill();
bool OtherProdRegKill = MI.getOperand(OtherProdOp).isKill();
bool AddRegUndef = AddendMI->getOperand(1).isUndef();
bool KilledProdRegUndef = MI.getOperand(KilledProdOp).isUndef();
bool OtherProdRegUndef = MI.getOperand(OtherProdOp).isUndef();
// If there isn't a class that fits, we can't perform the transform.
// This is needed for correctness with a mixture of VSX and Altivec
// instructions to make sure that a low VSX register is not assigned to
// the Altivec instruction.
if (!MRI.constrainRegClass(KilledProdReg,
MRI.getRegClass(OldFMAReg)))
continue;
assert(OldFMAReg == AddendMI->getOperand(0).getReg() &&
"Addend copy not tied to old FMA output!");
LLVM_DEBUG(dbgs() << "VSX FMA Mutation:\n " << MI);
MI.getOperand(0).setReg(KilledProdReg);
MI.getOperand(1).setReg(KilledProdReg);
MI.getOperand(3).setReg(AddendSrcReg);
MI.getOperand(0).setSubReg(KilledProdSubReg);
MI.getOperand(1).setSubReg(KilledProdSubReg);
MI.getOperand(3).setSubReg(AddSubReg);
MI.getOperand(1).setIsKill(KilledProdRegKill);
MI.getOperand(3).setIsKill(AddRegKill);
MI.getOperand(1).setIsUndef(KilledProdRegUndef);
MI.getOperand(3).setIsUndef(AddRegUndef);
MI.setDesc(TII->get(AltOpc));
// If the addend is also a multiplicand, replace it with the addend
// source in both places.
if (OtherProdReg == AddendMI->getOperand(0).getReg()) {
MI.getOperand(2).setReg(AddendSrcReg);
MI.getOperand(2).setSubReg(AddSubReg);
MI.getOperand(2).setIsKill(AddRegKill);
MI.getOperand(2).setIsUndef(AddRegUndef);
} else {
MI.getOperand(2).setReg(OtherProdReg);
MI.getOperand(2).setSubReg(OtherProdSubReg);
MI.getOperand(2).setIsKill(OtherProdRegKill);
MI.getOperand(2).setIsUndef(OtherProdRegUndef);
}
LLVM_DEBUG(dbgs() << " -> " << MI);
// The killed product operand was killed here, so we can reuse it now
// for the result of the fma.
LiveInterval &FMAInt = LIS->getInterval(OldFMAReg);
VNInfo *FMAValNo = FMAInt.getVNInfoAt(FMAIdx.getRegSlot());
for (auto UI = MRI.reg_nodbg_begin(OldFMAReg), UE = MRI.reg_nodbg_end();
UI != UE;) {
MachineOperand &UseMO = *UI;
MachineInstr *UseMI = UseMO.getParent();
++UI;
// Don't replace the result register of the copy we're about to erase.
if (UseMI == AddendMI)
continue;
UseMO.substVirtReg(KilledProdReg, KilledProdSubReg, *TRI);
}
// Extend the live intervals of the killed product operand to hold the
// fma result.
LiveInterval &NewFMAInt = LIS->getInterval(KilledProdReg);
for (auto &AI : FMAInt) {
// Don't add the segment that corresponds to the original copy.
if (AI.valno == AddendValNo)
continue;
VNInfo *NewFMAValNo =
NewFMAInt.getNextValue(AI.start, LIS->getVNInfoAllocator());
NewFMAInt.addSegment(
LiveInterval::Segment(AI.start, AI.end, NewFMAValNo));
}
LLVM_DEBUG(dbgs() << " extended: " << NewFMAInt << '\n');
// Extend the live interval of the addend source (it might end at the
// copy to be removed, or somewhere in between there and here). This
// is necessary only if it is a physical register.
if (!AddendSrcReg.isVirtual())
for (MCRegUnitIterator Units(AddendSrcReg.asMCReg(), TRI);
Units.isValid(); ++Units) {
unsigned Unit = *Units;
LiveRange &AddendSrcRange = LIS->getRegUnit(Unit);
AddendSrcRange.extendInBlock(LIS->getMBBStartIdx(&MBB),
FMAIdx.getRegSlot());
LLVM_DEBUG(dbgs() << " extended: " << AddendSrcRange << '\n');
}
FMAInt.removeValNo(FMAValNo);
LLVM_DEBUG(dbgs() << " trimmed: " << FMAInt << '\n');
// Remove the (now unused) copy.
LLVM_DEBUG(dbgs() << " removing: " << *AddendMI << '\n');
LIS->RemoveMachineInstrFromMaps(*AddendMI);
AddendMI->eraseFromParent();
Changed = true;
}
return Changed;
}
public:
bool runOnMachineFunction(MachineFunction &MF) override {
if (skipFunction(MF.getFunction()))
return false;
// If we don't have VSX then go ahead and return without doing
// anything.
const PPCSubtarget &STI = MF.getSubtarget<PPCSubtarget>();
if (!STI.hasVSX())
return false;
LIS = &getAnalysis<LiveIntervals>();
TII = STI.getInstrInfo();
bool Changed = false;
if (DisableVSXFMAMutate)
return Changed;
for (MachineBasicBlock &B : llvm::make_early_inc_range(MF))
if (processBlock(B))
Changed = true;
return Changed;
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<LiveIntervals>();
AU.addPreserved<LiveIntervals>();
AU.addRequired<SlotIndexes>();
AU.addPreserved<SlotIndexes>();
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
INITIALIZE_PASS_BEGIN(PPCVSXFMAMutate, DEBUG_TYPE,
"PowerPC VSX FMA Mutation", false, false)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_END(PPCVSXFMAMutate, DEBUG_TYPE,
"PowerPC VSX FMA Mutation", false, false)
char &llvm::PPCVSXFMAMutateID = PPCVSXFMAMutate::ID;
char PPCVSXFMAMutate::ID = 0;
FunctionPass *llvm::createPPCVSXFMAMutatePass() {
return new PPCVSXFMAMutate();
}
