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Register Allocator: refactoring and add comments. 

No functionality change. Thanks Andy for reviewing.

rdar://16162005

llvm-svn: 204962
This commit is contained in:
Manman Ren 2014-03-27 21:21:57 +00:00
parent c03f44ca8a
commit 9dee449ee3
1 changed files with 58 additions and 35 deletions
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93
llvm/lib/CodeGen/RegAllocGreedy.cpp
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@ -321,6 +321,11 @@ private:
unsigned doRegionSplit(LiveInterval &VirtReg, unsigned BestCand,
bool HasCompact,
SmallVectorImpl<unsigned> &NewVRegs);
/// Check other options before using a callee-saved register for the first
/// time.
unsigned tryAssignCSRFirstTime(LiveInterval &VirtReg, AllocationOrder &Order,
unsigned PhysReg, unsigned &CostPerUseLimit,
SmallVectorImpl<unsigned> &NewVRegs);
unsigned tryBlockSplit(LiveInterval&, AllocationOrder&,
SmallVectorImpl<unsigned>&);
unsigned tryInstructionSplit(LiveInterval&, AllocationOrder&,
@ -2107,6 +2112,49 @@ unsigned RAGreedy::selectOrSplit(LiveInterval &VirtReg,
return selectOrSplitImpl(VirtReg, NewVRegs, FixedRegisters);
}
/// Using a CSR for the first time has a cost because it causes push|pop
/// to be added to prologue|epilogue. Splitting a cold section of the live
/// range can have lower cost than using the CSR for the first time;
/// Spilling a live range in the cold path can have lower cost than using
/// the CSR for the first time. Returns the physical register if we decide
/// to use the CSR; otherwise return 0.
unsigned RAGreedy::tryAssignCSRFirstTime(LiveInterval &VirtReg,
AllocationOrder &Order,
unsigned PhysReg,
unsigned &CostPerUseLimit,
SmallVectorImpl<unsigned> &NewVRegs) {
BlockFrequency CSRCost(CSRFirstTimeCost);
if (getStage(VirtReg) == RS_Spill && VirtReg.isSpillable()) {
// We choose spill over using the CSR for the first time if the spill cost
// is lower than CSRCost.
SA->analyze(&VirtReg);
if (calcSpillCost() >= CSRCost)
return PhysReg;
// We are going to spill, set CostPerUseLimit to 1 to make sure that
// we will not use a callee-saved register in tryEvict.
CostPerUseLimit = 1;
return 0;
}
if (getStage(VirtReg) < RS_Split) {
// We choose pre-splitting over using the CSR for the first time if
// the cost of splitting is lower than CSRCost.
SA->analyze(&VirtReg);
unsigned NumCands = 0;
unsigned BestCand =
calculateRegionSplitCost(VirtReg, Order, CSRCost, NumCands,
true/*IgnoreCSR*/);
if (BestCand == NoCand)
// Use the CSR if we can't find a region split below CSRCost.
return PhysReg;
// Perform the actual pre-splitting.
doRegionSplit(VirtReg, BestCand, false/*HasCompact*/, NewVRegs);
return 0;
}
return PhysReg;
}
unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
SmallVectorImpl<unsigned> &NewVRegs,
SmallVirtRegSet &FixedRegisters,
@ -2121,41 +2169,16 @@ unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
if (!MRI->isPhysRegUsed(CSR))
CSRFirstUse = true;
BlockFrequency CSRCost(CSRFirstTimeCost);
// Using a CSR for the first time has a cost because it causes push|pop
// to be added to prologue|epilogue. Splitting a cold section of the live
// range can have lower cost than using the CSR for the first time;
// Spilling a live range in the cold path can have lower cost than using
// the CSR for the first time.
if (getStage(VirtReg) == RS_Spill && CSRFirstUse && NewVRegs.empty() &&
CSRFirstTimeCost > 0 && VirtReg.isSpillable()) {
// We choose spill over using the CSR for the first time if the spill cost
// is lower than CSRCost.
SA->analyze(&VirtReg);
if (calcSpillCost() >= CSRCost)
return PhysReg;
// We are going to spill, set CostPerUseLimit to 1 to make sure that
// we will not use a callee-saved register in tryEvict.
CostPerUseLimit = 1;
}
else if (getStage(VirtReg) < RS_Split && CSRFirstUse &&
NewVRegs.empty() && CSRFirstTimeCost > 0) {
// We choose pre-splitting over using the CSR for the first time if
// the cost of splitting is lower than CSRCost.
SA->analyze(&VirtReg);
unsigned NumCands = 0;
unsigned BestCand =
calculateRegionSplitCost(VirtReg, Order, CSRCost, NumCands,
true/*IgnoreCSR*/);
if (BestCand == NoCand)
// Use the CSR if we can't find a region split below CSRCost.
return PhysReg;
// Perform the actual pre-splitting.
doRegionSplit(VirtReg, BestCand, false/*HasCompact*/, NewVRegs);
if (!NewVRegs.empty())
return 0;
// When NewVRegs is not empty, we may have made decisions such as evicting
// a virtual register, go with the earlier decisions and use the physical
// register.
if (CSRFirstTimeCost > 0 && CSRFirstUse && NewVRegs.empty()) {
unsigned CSRReg = tryAssignCSRFirstTime(VirtReg, Order, PhysReg,
CostPerUseLimit, NewVRegs);
if (CSRReg || !NewVRegs.empty())
// Return now if we decide to use a CSR or create new vregs due to
// pre-splitting.
return CSRReg;
} else
return PhysReg;
}