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Branch Folding: Accept explicit threshold for tail merge size.

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This is prep work for allowing the threshold to be different during layout,
and to enforce a single threshold between merging and duplicating during
layout. No observable change intended.

llvm-svn: 279117
This commit is contained in:
Kyle Butt 2016-08-18 18:57:29 +00:00
parent 61bc562db4
commit 7a6eab8609
3 changed files with 44 additions and 22 deletions
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50
lib/CodeGen/BranchFolding.cpp
View File

@ -110,9 +110,12 @@ bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
MBFIWrapper &FreqInfo,
const MachineBranchProbabilityInfo &ProbInfo)
: EnableHoistCommonCode(CommonHoist), MBBFreqInfo(FreqInfo),
MBPI(ProbInfo) {
const MachineBranchProbabilityInfo &ProbInfo,
unsigned MinTailLength)
: EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
MBBFreqInfo(FreqInfo), MBPI(ProbInfo) {
if (MinCommonTailLength == 0)
MinCommonTailLength = TailMergeSize;
switch (FlagEnableTailMerge) {
case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
case cl::BOU_TRUE: EnableTailMerge = true; break;
@ -591,9 +594,21 @@ static unsigned CountTerminators(MachineBasicBlock *MBB,
/// and decide if it would be profitable to merge those tails. Return the
/// length of the common tail and iterators to the first common instruction
/// in each block.
/// MBB1, MBB2 The blocks to check
/// MinCommonTailLength Minimum size of tail block to be merged.
/// CommonTailLen Out parameter to record the size of the shared tail between
/// MBB1 and MBB2
/// I1, I2 Iterator references that will be changed to point to the first
/// instruction in the common tail shared by MBB1,MBB2
/// SuccBB A common successor of MBB1, MBB2 which are in a canonical form
/// relative to SuccBB
/// PredBB The layout predecessor of SuccBB, if any.
/// FuncletMembership map from block to funclet #.
/// AfterPlacement True if we are merging blocks after layout. Stricter
/// thresholds apply to prevent undoing tail-duplication.
static bool
ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
unsigned minCommonTailLength, unsigned &CommonTailLen,
unsigned MinCommonTailLength, unsigned &CommonTailLen,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB,
@ -651,7 +666,7 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
++EffectiveTailLen;
// Check if the common tail is long enough to be worthwhile.
if (EffectiveTailLen >= minCommonTailLength)
if (EffectiveTailLen >= MinCommonTailLength)
return true;
// If we are optimizing for code size, 2 instructions in common is enough if
@ -674,7 +689,7 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
/// those blocks appear in MergePotentials (where they are not necessarily
/// consecutive).
unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
unsigned minCommonTailLength,
unsigned MinCommonTailLength,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
unsigned maxCommonTailLength = 0U;
@ -687,7 +702,7 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
unsigned CommonTailLen;
if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
minCommonTailLength,
MinCommonTailLength,
CommonTailLen, TrialBBI1, TrialBBI2,
SuccBB, PredBB,
FuncletMembership,
@ -832,14 +847,13 @@ mergeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
// branch to Succ added (but the predecessor/successor lists need no
// adjustment). The lone predecessor of Succ that falls through into Succ,
// if any, is given in PredBB.
// MinCommonTailLength - Except for the special cases below, tail-merge if
// there are at least this many instructions in common.
bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
MachineBasicBlock *PredBB,
unsigned MinCommonTailLength) {
bool MadeChange = false;
// Except for the special cases below, tail-merge if there are at least
// this many instructions in common.
unsigned minCommonTailLength = TailMergeSize;
DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
@ -852,8 +866,8 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
<< PredBB->getNumber() << "\n";
}
dbgs() << "Looking for common tails of at least "
<< minCommonTailLength << " instruction"
<< (minCommonTailLength == 1 ? "" : "s") << '\n';
<< MinCommonTailLength << " instruction"
<< (MinCommonTailLength == 1 ? "" : "s") << '\n';
);
// Sort by hash value so that blocks with identical end sequences sort
@ -867,10 +881,10 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
// Build SameTails, identifying the set of blocks with this hash code
// and with the maximum number of instructions in common.
unsigned maxCommonTailLength = ComputeSameTails(CurHash,
minCommonTailLength,
MinCommonTailLength,
SuccBB, PredBB);
// If we didn't find any pair that has at least minCommonTailLength
// If we didn't find any pair that has at least MinCommonTailLength
// instructions in common, remove all blocks with this hash code and retry.
if (SameTails.empty()) {
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
@ -976,7 +990,7 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
// See if we can do any tail merging on those.
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(nullptr, nullptr);
MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
}
// Look at blocks (IBB) with multiple predecessors (PBB).
@ -1110,7 +1124,7 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
TriedMerging.insert(MergePotentials[i].getBlock());
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(IBB, PredBB);
MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength);
// Reinsert an unconditional branch if needed. The 1 below can occur as a
// result of removing blocks in TryTailMergeBlocks.

12
lib/CodeGen/BranchFolding.h
View File

@ -29,9 +29,13 @@ namespace llvm {
public:
class MBFIWrapper;
explicit BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
explicit BranchFolder(bool defaultEnableTailMerge,
bool CommonHoist,
MBFIWrapper &MBFI,
const MachineBranchProbabilityInfo &MBPI);
const MachineBranchProbabilityInfo &MBPI,
// Min tail length to merge. Defaults to commandline
// flag. Ignored for optsize.
unsigned MinCommonTailLength = 0);
bool OptimizeFunction(MachineFunction &MF, const TargetInstrInfo *tii,
const TargetRegisterInfo *tri, MachineModuleInfo *mmi,
@ -99,6 +103,7 @@ namespace llvm {
bool EnableTailMerge;
bool EnableHoistCommonCode;
bool UpdateLiveIns;
unsigned MinCommonTailLength;
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineModuleInfo *MMI;
@ -129,7 +134,8 @@ namespace llvm {
bool TailMergeBlocks(MachineFunction &MF);
bool TryTailMergeBlocks(MachineBasicBlock* SuccBB,
MachineBasicBlock* PredBB);
MachineBasicBlock* PredBB,
unsigned MinCommonTailLength);
void setCommonTailEdgeWeights(MachineBasicBlock &TailMBB);
void computeLiveIns(MachineBasicBlock &MBB);
void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,

4
lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -1747,8 +1747,10 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
BranchFoldPlacement;
// No tail merging opportunities if the block number is less than four.
if (MF.size() > 3 && EnableTailMerge) {
// Default to the standard tail-merge-size option.
unsigned TailMergeSize = 0;
BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI,
*MBPI);
*MBPI, TailMergeSize);
if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
getAnalysisIfAvailable<MachineModuleInfo>(), MLI,