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ARM: Fix and re-enable load/store optimizer for Thumb1.

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In a previous iteration of the pass, we would try to compensate for
writeback by updating later instructions and/or inserting a SUBS to
reset the base register if necessary.
Since such a SUBS sets the condition flags it's not generally safe to do
this. For now, only merge LDR/STRs if there is no writeback to the base
register (LDM that loads into the base register) or the base register is
killed by one of the merged instructions. These cases are clear wins
both in terms of instruction count and performance.

Also add three new test cases, and update the existing ones accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215729 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Moritz Roth
2014-08-15 17:00:30 +00:00
parent 713d9cbc3e
commit d84561bf69
8 changed files with 105 additions and 136 deletions
Download Patch File Download Diff File Expand all files Collapse all files

119
lib/Target/ARM/ARMLoadStoreOptimizer.cpp
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@@ -97,10 +97,6 @@ namespace {
void findUsesOfImpDef(SmallVectorImpl<MachineOperand *> &UsesOfImpDefs,
const MemOpQueue &MemOps, unsigned DefReg,
unsigned RangeBegin, unsigned RangeEnd);
void UpdateBaseRegUses(MachineBasicBlock &MBB,
MachineBasicBlock::iterator MBBI,
DebugLoc dl, unsigned Base, unsigned WordOffset,
ARMCC::CondCodes Pred, unsigned PredReg);
bool MergeOps(MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
int Offset, unsigned Base, bool BaseKill, int Opcode,
ARMCC::CondCodes Pred, unsigned PredReg, unsigned Scratch,
@@ -311,101 +307,6 @@ static bool isi32Store(unsigned Opc) {
return Opc == ARM::STRi12 || isT1i32Store(Opc) || isT2i32Store(Opc);
}
static unsigned getImmScale(unsigned Opc) {
switch (Opc) {
default: llvm_unreachable("Unhandled opcode!");
case ARM::tLDRi:
case ARM::tSTRi:
return 1;
case ARM::tLDRHi:
case ARM::tSTRHi:
return 2;
case ARM::tLDRBi:
case ARM::tSTRBi:
return 4;
}
}
/// Update future uses of the base register with the offset introduced
/// due to writeback. This function only works on Thumb1.
void
ARMLoadStoreOpt::UpdateBaseRegUses(MachineBasicBlock &MBB,
MachineBasicBlock::iterator MBBI,
DebugLoc dl, unsigned Base,
unsigned WordOffset,
ARMCC::CondCodes Pred, unsigned PredReg) {
assert(isThumb1 && "Can only update base register uses for Thumb1!");
// Start updating any instructions with immediate offsets. Insert a sub before
// the first non-updateable instruction (if any).
for (; MBBI != MBB.end(); ++MBBI) {
if (MBBI->readsRegister(Base)) {
unsigned Opc = MBBI->getOpcode();
int Offset;
bool InsertSub = false;
if (Opc == ARM::tLDRi || Opc == ARM::tSTRi ||
Opc == ARM::tLDRHi || Opc == ARM::tSTRHi ||
Opc == ARM::tLDRBi || Opc == ARM::tSTRBi) {
// Loads and stores with immediate offsets can be updated, but only if
// the new offset isn't negative.
// The MachineOperand containing the offset immediate is the last one
// before predicates.
MachineOperand &MO =
MBBI->getOperand(MBBI->getDesc().getNumOperands() - 3);
// The offsets are scaled by 1, 2 or 4 depending on the Opcode
Offset = MO.getImm() - WordOffset * getImmScale(Opc);
if (Offset >= 0)
MO.setImm(Offset);
else
InsertSub = true;
} else if (Opc == ARM::tSUBi8 || Opc == ARM::tADDi8) {
// SUB/ADD using this register. Merge it with the update.
// If the merged offset is too large, insert a new sub instead.
MachineOperand &MO =
MBBI->getOperand(MBBI->getDesc().getNumOperands() - 3);
Offset = (Opc == ARM::tSUBi8) ?
MO.getImm() + WordOffset * 4 :
MO.getImm() - WordOffset * 4 ;
if (TL->isLegalAddImmediate(Offset)) {
MO.setImm(Offset);
// The base register has now been reset, so exit early.
return;
} else {
InsertSub = true;
}
} else {
// Can't update the instruction.
InsertSub = true;
}
if (InsertSub) {
// An instruction above couldn't be updated, so insert a sub.
AddDefaultT1CC(BuildMI(MBB, MBBI, dl, TII->get(ARM::tSUBi8), Base))
.addReg(Base, getKillRegState(true)).addImm(WordOffset * 4)
.addImm(Pred).addReg(PredReg);
return;
}
}
if (MBBI->killsRegister(Base))
// Register got killed. Stop updating.
return;
}
// The end of the block was reached. This means register liveness escapes the
// block, and it's necessary to insert a sub before the last instruction.
if (MBB.succ_size() > 0)
// But only insert the SUB if there is actually a successor block.
// FIXME: Check more carefully if register is live at this point, e.g. by
// also examining the successor block's register liveness information.
AddDefaultT1CC(BuildMI(MBB, --MBBI, dl, TII->get(ARM::tSUBi8), Base))
.addReg(Base, getKillRegState(true)).addImm(WordOffset * 4)
.addImm(Pred).addReg(PredReg);
}
/// MergeOps - Create and insert a LDM or STM with Base as base register and
/// registers in Regs as the register operands that would be loaded / stored.
/// It returns true if the transformation is done.
@@ -512,6 +413,14 @@ ARMLoadStoreOpt::MergeOps(MachineBasicBlock &MBB,
break;
}
// If the merged instruction has writeback and the base register is not killed
// it's not safe to do the merge on Thumb1. This is because resetting the base
// register writeback by inserting a SUBS sets the condition flags.
// FIXME: Try something clever here to see if resetting the base register can
// be avoided, e.g. by updating a later ADD/SUB of the base register with the
// writeback.
if (isThumb1 && Writeback && !BaseKill) return false;
MachineInstrBuilder MIB;
if (Writeback) {
@@ -524,12 +433,6 @@ ARMLoadStoreOpt::MergeOps(MachineBasicBlock &MBB,
// Thumb1: we might need to set base writeback when building the MI.
MIB.addReg(Base, getDefRegState(true))
.addReg(Base, getKillRegState(BaseKill));
// The base isn't dead after a merged instruction with writeback. Update
// future uses of the base with the added offset (if possible), or reset
// the base register as necessary.
if (!BaseKill)
UpdateBaseRegUses(MBB, MBBI, dl, Base, NumRegs, Pred, PredReg);
} else {
// No writeback, simply build the MachineInstr.
MIB = BuildMI(MBB, MBBI, dl, TII->get(Opcode));
@@ -1740,12 +1643,6 @@ bool ARMLoadStoreOpt::runOnMachineFunction(MachineFunction &Fn) {
isThumb2 = AFI->isThumb2Function();
isThumb1 = AFI->isThumbFunction() && !isThumb2;
// FIXME: Temporarily disabling for Thumb-1 due to miscompiles
if (isThumb1) {
delete RS;
return false;
}
bool Modified = false;
for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
++MFI) {