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Add a smarter heuristic to determine when to coalesce a virtual register with a physical one. More specifically, it avoid tying a virtual register in the loop with a physical register defined / used outside the loop. When it determines it's not profitable, it will use the physical register as the allocation preference instead.

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This is *not* turned on by default. Testing indicates this is just as likely to pessimize code. The main issue seems to be allocation preference doesn't work effectively. That will change once I've taught register allocator "swapping".


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70503 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng 2009-04-30 18:39:57 +00:00
parent 440548dd1f
commit 0490dcb1b7
2 changed files with 172 additions and 19 deletions
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179
lib/CodeGen/SimpleRegisterCoalescing.cpp
View File

@ -60,6 +60,11 @@ CrossClassJoin("join-cross-class-copies",
cl::desc("Coalesce cross register class copies"),
cl::init(false), cl::Hidden);
static cl::opt<bool>
PhysJoinTweak("tweak-phys-join-heuristics",
cl::desc("Tweak heuristics for joining phys reg with vr"),
cl::init(false), cl::Hidden);
static RegisterPass<SimpleRegisterCoalescing>
X("simple-register-coalescing", "Simple Register Coalescing");
@ -1000,6 +1005,117 @@ void SimpleRegisterCoalescing::RemoveCopiesFromValNo(LiveInterval &li,
}
}
/// isWinToJoinVRWithSrcPhysReg - Return true if it's worth while to join a
/// a virtual destination register with physical source register.
bool
SimpleRegisterCoalescing::isWinToJoinVRWithSrcPhysReg(MachineInstr *CopyMI,
MachineBasicBlock *CopyMBB,
LiveInterval &DstInt,
LiveInterval &SrcInt) {
// If the virtual register live interval is long but it has low use desity,
// do not join them, instead mark the physical register as its allocation
// preference.
const TargetRegisterClass *RC = mri_->getRegClass(DstInt.reg);
unsigned Threshold = allocatableRCRegs_[RC].count() * 2;
unsigned Length = li_->getApproximateInstructionCount(DstInt);
if (Length > Threshold &&
(((float)std::distance(mri_->use_begin(DstInt.reg),
mri_->use_end()) / Length) < (1.0 / Threshold)))
return false;
// If the virtual register live interval extends into a loop, turn down
// aggressiveness.
unsigned CopyIdx = li_->getDefIndex(li_->getInstructionIndex(CopyMI));
const MachineLoop *L = loopInfo->getLoopFor(CopyMBB);
if (!L) {
// Let's see if the virtual register live interval extends into the loop.
LiveInterval::iterator DLR = DstInt.FindLiveRangeContaining(CopyIdx);
assert(DLR != DstInt.end() && "Live range not found!");
DLR = DstInt.FindLiveRangeContaining(DLR->end+1);
if (DLR != DstInt.end()) {
CopyMBB = li_->getMBBFromIndex(DLR->start);
L = loopInfo->getLoopFor(CopyMBB);
}
}
if (!L || Length <= Threshold)
return true;
unsigned UseIdx = li_->getUseIndex(CopyIdx);
LiveInterval::iterator SLR = SrcInt.FindLiveRangeContaining(UseIdx);
MachineBasicBlock *SMBB = li_->getMBBFromIndex(SLR->start);
if (loopInfo->getLoopFor(SMBB) != L) {
if (!loopInfo->isLoopHeader(CopyMBB))
return false;
// If vr's live interval extends pass the loop header, do not join.
for (MachineBasicBlock::succ_iterator SI = CopyMBB->succ_begin(),
SE = CopyMBB->succ_end(); SI != SE; ++SI) {
MachineBasicBlock *SuccMBB = *SI;
if (SuccMBB == CopyMBB)
continue;
if (DstInt.overlaps(li_->getMBBStartIdx(SuccMBB),
li_->getMBBEndIdx(SuccMBB)+1))
return false;
}
}
return true;
}
/// isWinToJoinVRWithDstPhysReg - Return true if it's worth while to join a
/// copy from a virtual source register to a physical destination register.
bool
SimpleRegisterCoalescing::isWinToJoinVRWithDstPhysReg(MachineInstr *CopyMI,
MachineBasicBlock *CopyMBB,
LiveInterval &DstInt,
LiveInterval &SrcInt) {
// If the virtual register live interval is long but it has low use desity,
// do not join them, instead mark the physical register as its allocation
// preference.
const TargetRegisterClass *RC = mri_->getRegClass(SrcInt.reg);
unsigned Threshold = allocatableRCRegs_[RC].count() * 2;
unsigned Length = li_->getApproximateInstructionCount(SrcInt);
if (Length > Threshold &&
(((float)std::distance(mri_->use_begin(SrcInt.reg),
mri_->use_end()) / Length) < (1.0 / Threshold)))
return false;
if (SrcInt.empty())
// Must be implicit_def.
return false;
// If the virtual register live interval is defined or cross a loop, turn
// down aggressiveness.
unsigned CopyIdx = li_->getDefIndex(li_->getInstructionIndex(CopyMI));
unsigned UseIdx = li_->getUseIndex(CopyIdx);
LiveInterval::iterator SLR = SrcInt.FindLiveRangeContaining(UseIdx);
assert(SLR != SrcInt.end() && "Live range not found!");
SLR = SrcInt.FindLiveRangeContaining(SLR->start-1);
if (SLR == SrcInt.end())
return true;
MachineBasicBlock *SMBB = li_->getMBBFromIndex(SLR->start);
const MachineLoop *L = loopInfo->getLoopFor(SMBB);
if (!L || Length <= Threshold)
return true;
if (loopInfo->getLoopFor(CopyMBB) != L) {
if (SMBB != L->getLoopLatch())
return false;
// If vr's live interval is extended from before the loop latch, do not
// join.
for (MachineBasicBlock::pred_iterator PI = SMBB->pred_begin(),
PE = SMBB->pred_end(); PI != PE; ++PI) {
MachineBasicBlock *PredMBB = *PI;
if (PredMBB == SMBB)
continue;
if (SrcInt.overlaps(li_->getMBBStartIdx(PredMBB),
li_->getMBBEndIdx(PredMBB)+1))
return false;
}
}
return true;
}
/// isWinToJoinCrossClass - Return true if it's profitable to coalesce
/// two virtual registers from different register classes.
bool
@ -1440,26 +1556,51 @@ bool SimpleRegisterCoalescing::JoinCopy(CopyRec &TheCopy, bool &Again) {
// these are not spillable! If the destination interval uses are far away,
// think twice about coalescing them!
if (!isDead && (SrcIsPhys || DstIsPhys)) {
LiveInterval &JoinVInt = SrcIsPhys ? DstInt : SrcInt;
unsigned JoinVReg = SrcIsPhys ? DstReg : SrcReg;
unsigned JoinPReg = SrcIsPhys ? SrcReg : DstReg;
const TargetRegisterClass *RC = mri_->getRegClass(JoinVReg);
unsigned Threshold = allocatableRCRegs_[RC].count() * 2;
if (TheCopy.isBackEdge)
Threshold *= 2; // Favors back edge copies.
// If the copy is in a loop, take care not to coalesce aggressively if the
// src is coming in from outside the loop (or the dst is out of the loop).
// If it's not in a loop, then determine whether to join them base purely
// by the length of the interval.
if (PhysJoinTweak) {
if (SrcIsPhys) {
if (!isWinToJoinVRWithSrcPhysReg(CopyMI, CopyMBB, DstInt, SrcInt)) {
DstInt.preference = SrcReg;
++numAborts;
DOUT << "\tMay tie down a physical register, abort!\n";
Again = true; // May be possible to coalesce later.
return false;
}
} else {
if (!isWinToJoinVRWithDstPhysReg(CopyMI, CopyMBB, DstInt, SrcInt)) {
SrcInt.preference = DstReg;
++numAborts;
DOUT << "\tMay tie down a physical register, abort!\n";
Again = true; // May be possible to coalesce later.
return false;
}
}
} else {
// If the virtual register live interval is long but it has low use desity,
// do not join them, instead mark the physical register as its allocation
// preference.
LiveInterval &JoinVInt = SrcIsPhys ? DstInt : SrcInt;
unsigned JoinVReg = SrcIsPhys ? DstReg : SrcReg;
unsigned JoinPReg = SrcIsPhys ? SrcReg : DstReg;
const TargetRegisterClass *RC = mri_->getRegClass(JoinVReg);
unsigned Threshold = allocatableRCRegs_[RC].count() * 2;
if (TheCopy.isBackEdge)
Threshold *= 2; // Favors back edge copies.
// If the virtual register live interval is long but it has low use desity,
// do not join them, instead mark the physical register as its allocation
// preference.
unsigned Length = li_->getApproximateInstructionCount(JoinVInt);
if (Length > Threshold &&
(((float)std::distance(mri_->use_begin(JoinVReg), mri_->use_end())
/ Length) < (1.0 / Threshold))) {
JoinVInt.preference = JoinPReg;
++numAborts;
DOUT << "\tMay tie down a physical register, abort!\n";
Again = true; // May be possible to coalesce later.
return false;
unsigned Length = li_->getApproximateInstructionCount(JoinVInt);
float Ratio = 1.0 / Threshold;
if (Length > Threshold &&
(((float)std::distance(mri_->use_begin(JoinVReg),
mri_->use_end()) / Length) < Ratio)) {
JoinVInt.preference = JoinPReg;
++numAborts;
DOUT << "\tMay tie down a physical register, abort!\n";
Again = true; // May be possible to coalesce later.
return false;
}
}
}
}

12
lib/CodeGen/SimpleRegisterCoalescing.h
View File

@ -224,6 +224,18 @@ namespace llvm {
/// identity copies so they will be removed.
void RemoveCopiesFromValNo(LiveInterval &li, VNInfo *VNI);
/// isWinToJoinVRWithSrcPhysReg - Return true if it's worth while to join a
/// a virtual destination register with physical source register.
bool isWinToJoinVRWithSrcPhysReg(MachineInstr *CopyMI,
MachineBasicBlock *CopyMBB,
LiveInterval &DstInt, LiveInterval &SrcInt);
/// isWinToJoinVRWithDstPhysReg - Return true if it's worth while to join a
/// copy from a virtual source register to a physical destination register.
bool isWinToJoinVRWithDstPhysReg(MachineInstr *CopyMI,
MachineBasicBlock *CopyMBB,
LiveInterval &DstInt, LiveInterval &SrcInt);
/// isWinToJoinCrossClass - Return true if it's profitable to coalesce
/// two virtual registers from different register classes.
bool isWinToJoinCrossClass(unsigned LargeReg, unsigned SmallReg,