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Rewrite tail merging algorithm to handle the
case where there are multiple blocks with a large number of common tail instructions more efficiently (compile time optimization). llvm-svn: 50916
This commit is contained in:
Dale Johannesen
parent
9b85a8dc92
commit
09a45f2ca3
@ -72,7 +72,10 @@ namespace {
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MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
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MachineBasicBlock::iterator BBI1);
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std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
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typedef std::pair<unsigned,MachineBasicBlock*> MergePotentialsElt;
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std::vector<MergePotentialsElt> MergePotentials;
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typedef std::vector<MergePotentialsElt>::iterator MPIterator;
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const TargetRegisterInfo *RegInfo;
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RegScavenger *RS;
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// Branch optzn.
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@ -506,129 +509,153 @@ static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p,
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bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
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MachineBasicBlock* PredBB) {
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// We cannot jump to the entry block, which affects various choices below.
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MachineBasicBlock *Entry = MergePotentials.begin()->second->
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getParent()->begin();
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// It doesn't make sense to save a single instruction since tail merging
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// will add a jump.
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// FIXME: Ask the target to provide the threshold?
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unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1;
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MadeChange = false;
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DOUT << "\nTryMergeBlocks " << MergePotentials.size();
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// Sort by hash value so that blocks with identical end sequences sort
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// together.
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std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare);
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// Walk through equivalence sets looking for actual exact matches.
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while (MergePotentials.size() > 1) {
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unsigned CurHash = (MergePotentials.end()-1)->first;
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unsigned PrevHash = (MergePotentials.end()-2)->first;
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MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second;
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unsigned CurHash = prior(MergePotentials.end())->first;
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// If there is nothing that matches the hash of the current basic block,
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// give up.
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if (CurHash != PrevHash) {
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if (SuccBB && CurMBB != PredBB)
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FixTail(CurMBB, SuccBB, TII);
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MergePotentials.pop_back();
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continue;
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}
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// Look through all the pairs of blocks that have the same hash as this
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// one, and find the pair that has the largest number of instructions in
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// common.
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// Since instructions may get combined later (e.g. single stores into
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// store multiple) this measure is not particularly accurate.
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MachineBasicBlock::iterator BBI1, BBI2;
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unsigned FoundI = ~0U, FoundJ = ~0U;
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// Look through all the other blocks that have the same hash as this
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// one, and build a vector of all those that have the (same) largest number
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// of instructions in common.
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// Order of elements in SameTails is the reverse of the order in which
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// those blocks appear in MergePotentials (where they are not necessarily
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// consecutive).
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typedef std::pair<MPIterator, MachineBasicBlock::iterator> SameTailElt;
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std::vector<SameTailElt> SameTails;
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unsigned maxCommonTailLength = 0U;
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for (int i = MergePotentials.size()-1;
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i != -1 && MergePotentials[i].first == CurHash; --i) {
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for (int j = i-1;
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j != -1 && MergePotentials[j].first == CurHash; --j) {
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MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
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SameTails.clear();
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MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
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MPIterator HighestMPIter = prior(MergePotentials.end());
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for (MPIterator CurMPIter = prior(MergePotentials.end()),
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B = MergePotentials.begin();
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CurMPIter!=B && CurMPIter->first==CurHash;
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--CurMPIter) {
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for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) {
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unsigned CommonTailLen = ComputeCommonTailLength(
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MergePotentials[i].second,
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MergePotentials[j].second,
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TrialBBI1, TrialBBI2);
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if (CommonTailLen >= minCommonTailLength &&
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CommonTailLen > maxCommonTailLength) {
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FoundI = i;
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FoundJ = j;
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maxCommonTailLength = CommonTailLen;
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BBI1 = TrialBBI1;
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BBI2 = TrialBBI2;
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CurMPIter->second,
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I->second,
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TrialBBI1, TrialBBI2);
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if (CommonTailLen >= minCommonTailLength) {
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if (CommonTailLen > maxCommonTailLength) {
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SameTails.clear();
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maxCommonTailLength = CommonTailLen;
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HighestMPIter = CurMPIter;
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SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1));
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}
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if (HighestMPIter == CurMPIter &&
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CommonTailLen == maxCommonTailLength)
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SameTails.push_back(std::make_pair(I, TrialBBI2));
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}
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if (I==B)
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break;
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}
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}
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// If we didn't find any pair that has at least minCommonTailLength
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// instructions in common, bail out. All entries with this
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// hash code can go away now.
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if (FoundI == ~0U) {
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for (int i = MergePotentials.size()-1;
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i != -1 && MergePotentials[i].first == CurHash; --i) {
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// instructions in common, remove all blocks with this hash code and retry.
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if (SameTails.empty()) {
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for (MPIterator CurMPIter = prior(MergePotentials.end()),
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B = MergePotentials.begin();
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CurMPIter->first==CurHash;
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--CurMPIter) {
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// Put the unconditional branch back, if we need one.
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CurMBB = MergePotentials[i].second;
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MachineBasicBlock *CurMBB = CurMPIter->second;
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if (SuccBB && CurMBB != PredBB)
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FixTail(CurMBB, SuccBB, TII);
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MergePotentials.pop_back();
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MergePotentials.erase(CurMPIter);
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if (CurMPIter==B)
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break;
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}
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continue;
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}
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// Otherwise, move the block(s) to the right position(s). So that
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// BBI1/2 will be valid, the last must be I and the next-to-last J.
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if (FoundI != MergePotentials.size()-1)
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std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1));
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if (FoundJ != MergePotentials.size()-2)
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std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2));
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// If one of the blocks is the entire common tail (and not the entry
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// block, which we can't jump to), treat all blocks with this same
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// tail at once.
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unsigned int i;
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for (i=0; i<SameTails.size(); i++) {
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MachineBasicBlock *MBB = SameTails[i].first->second;
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if (MBB->begin() == SameTails[i].second && MBB != Entry)
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break;
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}
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if (i!=SameTails.size()) {
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MachineBasicBlock *MBB = SameTails[i].first->second;
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// MBB is common tail. Adjust all other BB's to jump to this one.
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// Traversal must be forwards so erases work.
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DOUT << "\nUsing common tail " << MBB->getNumber() << " for ";
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for (unsigned int j=0; j<SameTails.size(); ++j) {
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if (i==j)
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continue;
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DOUT << SameTails[j].first->second->getNumber() << ",";
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// Hack the end off BB j, making it jump to BB i instead.
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ReplaceTailWithBranchTo(SameTails[j].second, MBB);
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// This modifies BB j, so remove it from the worklist.
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MergePotentials.erase(SameTails[j].first);
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}
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DOUT << "\n";
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// We leave i in the worklist in case there are other blocks that
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// match it with a smaller number of instructions.
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MadeChange = true;
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continue;
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}
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CurMBB = (MergePotentials.end()-1)->second;
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MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
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// Otherwise, merge the 2 blocks in SameTails that are latest in
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// MergePotentials; these are at indices 0 and 1 in SameTails.
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MachineBasicBlock::iterator BBI1 = (SameTails[0]).second;
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MachineBasicBlock::iterator BBI2 = (SameTails[1]).second;
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MachineBasicBlock *MBB1 = (SameTails[0]).first->second;
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MachineBasicBlock *MBB2 = (SameTails[1]).first->second;
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// If neither block is the entire common tail, split the tail of one block
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// to make it redundant with the other tail. Also, we cannot jump to the
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DOUT << "\nMerging " << MBB1->getNumber() << "," <<
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MBB2->getNumber() << ", size " << maxCommonTailLength;
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// Neither block is the entire common tail; split the tail of one block
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// to make it redundant with the other tail. We cannot jump to the
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// entry block, so if one block is the entry block, split the other one.
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MachineBasicBlock *Entry = CurMBB->getParent()->begin();
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if (CurMBB->begin() == BBI1 && CurMBB != Entry)
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; // CurMBB is common tail
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else if (MBB2->begin() == BBI2 && MBB2 != Entry)
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; // MBB2 is common tail
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else {
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if (0) { // Enable this to disable partial tail merges.
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MergePotentials.pop_back();
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continue;
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}
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MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
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unsigned CommonTailLen = ComputeCommonTailLength(CurMBB, MBB2,
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TrialBBI1, TrialBBI2);
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if (CommonTailLen < minCommonTailLength)
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continue;
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// Decide whether we want to split CurMBB or MBB2.
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if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, PredBB)) {
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CurMBB = SplitMBBAt(*CurMBB, BBI1);
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BBI1 = CurMBB->begin();
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MergePotentials.back().second = CurMBB;
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} else {
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MBB2 = SplitMBBAt(*MBB2, BBI2);
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BBI2 = MBB2->begin();
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(MergePotentials.end()-2)->second = MBB2;
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}
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// The second half of the split block will remain in SameTails, and will
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// consist entirely of common code. Thus in the case where there are multiple
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// blocks that would all need to be split, the next iteration of the
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// outer loop will handle all the rest of them.
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// Decide whether we want to split MBB1 or MBB2.
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if (ShouldSplitFirstBlock(MBB1, BBI1, MBB2, BBI2, PredBB)) {
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MBB1 = SplitMBBAt(*MBB1, BBI1);
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BBI1 = MBB1->begin();
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SameTails[0].first->second = MBB1;
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} else {
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MBB2 = SplitMBBAt(*MBB2, BBI2);
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BBI2 = MBB2->begin();
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SameTails[1].first->second = MBB2;
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}
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if (MBB2->begin() == BBI2 && MBB2 != Entry) {
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// Hack the end off CurMBB, making it jump to MBBI@ instead.
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// Hack the end off MBB1, making it jump to MBB2 instead.
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ReplaceTailWithBranchTo(BBI1, MBB2);
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// This modifies CurMBB, so remove it from the worklist.
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MergePotentials.pop_back();
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// This modifies MBB1, so remove it from the worklist.
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MergePotentials.erase(SameTails[0].first);
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} else {
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assert(CurMBB->begin() == BBI1 && CurMBB != Entry &&
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assert(MBB1->begin() == BBI1 && MBB1 != Entry &&
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"Didn't split block correctly?");
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// Hack the end off MBB2, making it jump to CurMBB instead.
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ReplaceTailWithBranchTo(BBI2, CurMBB);
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// Hack the end off MBB2, making it jump to MBB1 instead.
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ReplaceTailWithBranchTo(BBI2, MBB1);
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// This modifies MBB2, so remove it from the worklist.
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MergePotentials.erase(MergePotentials.end()-2);
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MergePotentials.erase(SameTails[1].first);
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}
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MadeChange = true;
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}
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@ -648,7 +675,8 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
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MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I));
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}
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// See if we can do any tail merging on those.
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if (MergePotentials.size() < TailMergeThreshold)
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if (MergePotentials.size() < TailMergeThreshold &&
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MergePotentials.size() >= 2)
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MadeChange |= TryMergeBlocks(NULL, NULL);
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// Look at blocks (IBB) with multiple predecessors (PBB).
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