//
//===----------------------------------------------------------------------===//
+#define DEBUG_TYPE "branchfolding"
#include "llvm/CodeGen/Passes.h"
-#include "llvm/CodeGen/MachineDebugInfo.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
+#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/MRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
+#include <algorithm>
using namespace llvm;
-static Statistic<> NumDeadBlocks("branchfold", "Number of dead blocks removed");
-static Statistic<> NumBranchOpts("branchfold", "Number of branches optimized");
-static Statistic<> NumTailMerge ("branchfold", "Number of block tails merged");
+STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
+STATISTIC(NumBranchOpts, "Number of branches optimized");
+STATISTIC(NumTailMerge , "Number of block tails merged");
+static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::Hidden);
namespace {
struct BranchFolder : public MachineFunctionPass {
+ static char ID;
+ BranchFolder() : MachineFunctionPass((intptr_t)&ID) {}
+
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual const char *getPassName() const { return "Control Flow Optimizer"; }
const TargetInstrInfo *TII;
- MachineDebugInfo *MDI;
+ MachineModuleInfo *MMI;
bool MadeChange;
private:
// Tail Merging.
bool TailMergeBlocks(MachineFunction &MF);
+ bool TryMergeBlocks(MachineBasicBlock* SuccBB,
+ MachineBasicBlock* PredBB);
void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
MachineBasicBlock *NewDest);
+ MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
+ MachineBasicBlock::iterator BBI1);
+ std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
+ const MRegisterInfo *RegInfo;
+ RegScavenger *RS;
// Branch optzn.
bool OptimizeBranches(MachineFunction &MF);
void OptimizeBlock(MachineBasicBlock *MBB);
void RemoveDeadBlock(MachineBasicBlock *MBB);
+
+ bool CanFallThrough(MachineBasicBlock *CurBB);
+ bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
+ MachineBasicBlock *TBB, MachineBasicBlock *FBB,
+ const std::vector<MachineOperand> &Cond);
};
+ char BranchFolder::ID = 0;
}
+static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB,
+ MachineBasicBlock *DestA,
+ MachineBasicBlock *DestB,
+ bool isCond,
+ MachineFunction::iterator FallThru);
+
FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); }
/// RemoveDeadBlock - Remove the specified dead machine basic block from the
/// function, updating the CFG.
void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
assert(MBB->pred_empty() && "MBB must be dead!");
+ DOUT << "\nRemoving MBB: " << *MBB;
MachineFunction *MF = MBB->getParent();
// drop all successors.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end()-1);
- // If there is DWARF info to active, check to see if there are any DWARF_LABEL
- // records in the basic block. If so, unregister them from MachineDebugInfo.
- if (MDI && !MBB->empty()) {
- unsigned DWARF_LABELOpc = TII->getDWARF_LABELOpcode();
- assert(DWARF_LABELOpc &&
- "Target supports dwarf but didn't implement getDWARF_LABELOpcode!");
-
+ // If there is DWARF info to active, check to see if there are any LABEL
+ // records in the basic block. If so, unregister them from MachineModuleInfo.
+ if (MMI && !MBB->empty()) {
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
I != E; ++I) {
- if ((unsigned)I->getOpcode() == DWARF_LABELOpc) {
+ if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) {
// The label ID # is always operand #0, an immediate.
- MDI->InvalidateLabel(I->getOperand(0).getImm());
+ MMI->InvalidateLabel(I->getOperand(0).getImm());
}
}
}
TII = MF.getTarget().getInstrInfo();
if (!TII) return false;
- MDI = getAnalysisToUpdate<MachineDebugInfo>();
-
+ // Fix CFG. The later algorithms expect it to be right.
bool EverMadeChange = false;
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
+ MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
+ std::vector<MachineOperand> Cond;
+ if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
+ EverMadeChange |= CorrectExtraCFGEdges(*MBB, TBB, FBB,
+ !Cond.empty(), next(I));
+ }
+
+ RegInfo = MF.getTarget().getRegisterInfo();
+ RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
+
+ MMI = getAnalysisToUpdate<MachineModuleInfo>();
+
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
EverMadeChange |= MadeChangeThisIteration;
}
+ // See if any jump tables have become mergable or dead as the code generator
+ // did its thing.
+ MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
+ const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
+ if (!JTs.empty()) {
+ // Figure out how these jump tables should be merged.
+ std::vector<unsigned> JTMapping;
+ JTMapping.reserve(JTs.size());
+
+ // We always keep the 0th jump table.
+ JTMapping.push_back(0);
+
+ // Scan the jump tables, seeing if there are any duplicates. Note that this
+ // is N^2, which should be fixed someday.
+ for (unsigned i = 1, e = JTs.size(); i != e; ++i)
+ JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
+
+ // If a jump table was merge with another one, walk the function rewriting
+ // references to jump tables to reference the new JT ID's. Keep track of
+ // whether we see a jump table idx, if not, we can delete the JT.
+ std::vector<bool> JTIsLive;
+ JTIsLive.resize(JTs.size());
+ for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
+ BB != E; ++BB) {
+ for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
+ I != E; ++I)
+ for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
+ MachineOperand &Op = I->getOperand(op);
+ if (!Op.isJumpTableIndex()) continue;
+ unsigned NewIdx = JTMapping[Op.getJumpTableIndex()];
+ Op.setJumpTableIndex(NewIdx);
+
+ // Remember that this JT is live.
+ JTIsLive[NewIdx] = true;
+ }
+ }
+
+ // Finally, remove dead jump tables. This happens either because the
+ // indirect jump was unreachable (and thus deleted) or because the jump
+ // table was merged with some other one.
+ for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
+ if (!JTIsLive[i]) {
+ JTI->RemoveJumpTable(i);
+ EverMadeChange = true;
+ }
+ }
+
+ delete RS;
return EverMadeChange;
}
++NumTailMerge;
}
-bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
- MadeChange = false;
+/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
+/// MBB so that the part before the iterator falls into the part starting at the
+/// iterator. This returns the new MBB.
+MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
+ MachineBasicBlock::iterator BBI1) {
+ // Create the fall-through block.
+ MachineFunction::iterator MBBI = &CurMBB;
+ MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock());
+ CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB);
+
+ // Move all the successors of this block to the specified block.
+ while (!CurMBB.succ_empty()) {
+ MachineBasicBlock *S = *(CurMBB.succ_end()-1);
+ NewMBB->addSuccessor(S);
+ CurMBB.removeSuccessor(S);
+ }
+
+ // Add an edge from CurMBB to NewMBB for the fall-through.
+ CurMBB.addSuccessor(NewMBB);
- return false;
+ // Splice the code over.
+ NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
+
+ // For targets that use the register scavenger, we must maintain LiveIns.
+ if (RS) {
+ RS->enterBasicBlock(&CurMBB);
+ if (!CurMBB.empty())
+ RS->forward(prior(CurMBB.end()));
+ BitVector RegsLiveAtExit(RegInfo->getNumRegs());
+ RS->getRegsUsed(RegsLiveAtExit, false);
+ for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++)
+ if (RegsLiveAtExit[i])
+ NewMBB->addLiveIn(i);
+ }
+
+ return NewMBB;
+}
+
+/// EstimateRuntime - Make a rough estimate for how long it will take to run
+/// the specified code.
+static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
+ MachineBasicBlock::iterator E,
+ const TargetInstrInfo *TII) {
+ unsigned Time = 0;
+ for (; I != E; ++I) {
+ const TargetInstrDescriptor &TID = TII->get(I->getOpcode());
+ if (TID.Flags & M_CALL_FLAG)
+ Time += 10;
+ else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG))
+ Time += 2;
+ else
+ ++Time;
+ }
+ return Time;
+}
+
+/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at
+/// MBB2I and then insert an unconditional branch in the other block. Determine
+/// which is the best to split
+static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
+ MachineBasicBlock::iterator MBB1I,
+ MachineBasicBlock *MBB2,
+ MachineBasicBlock::iterator MBB2I,
+ const TargetInstrInfo *TII,
+ MachineBasicBlock *PredBB) {
+ // If one block is the entry block, split the other one; we can't generate
+ // a branch to the entry block, as its label is not emitted.
+ MachineBasicBlock *Entry = MBB1->getParent()->begin();
+ if (MBB1 == Entry)
+ return false;
+ if (MBB2 == Entry)
+ return true;
+
+ // If one block falls through into the common successor, choose that
+ // one to split; it is one instruction less to do that.
+ if (PredBB) {
+ if (MBB1 == PredBB)
+ return true;
+ else if (MBB2 == PredBB)
+ return false;
+ }
+ // TODO: if we had some notion of which block was hotter, we could split
+ // the hot block, so it is the fall-through. Since we don't have profile info
+ // make a decision based on which will hurt most to split.
+ unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII);
+ unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII);
- // Find blocks with no successors.
- std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
- for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
- if (I->succ_empty())
- MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I));
+ // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the
+ // MBB1 block so it falls through. This will penalize the MBB2 path, but will
+ // have a lower overall impact on the program execution.
+ return MBB1Time < MBB2Time;
+}
+
+// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
+// branches temporarily for tail merging). In the case where CurMBB ends
+// with a conditional branch to the next block, optimize by reversing the
+// test and conditionally branching to SuccMBB instead.
+
+static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB,
+ const TargetInstrInfo *TII) {
+ MachineFunction *MF = CurMBB->getParent();
+ MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
+ MachineBasicBlock *TBB = 0, *FBB = 0;
+ std::vector<MachineOperand> Cond;
+ if (I != MF->end() &&
+ !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) {
+ MachineBasicBlock *NextBB = I;
+ if (TBB == NextBB && Cond.size() && !FBB) {
+ if (!TII->ReverseBranchCondition(Cond)) {
+ TII->RemoveBranch(*CurMBB);
+ TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
+ return;
+ }
+ }
}
+ TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>());
+}
+
+// See if any of the blocks in MergePotentials (which all have a common single
+// successor, or all have no successor) can be tail-merged. If there is a
+// successor, any blocks in MergePotentials that are not tail-merged and
+// are not immediately before Succ must have an unconditional 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.
+
+bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
+ MachineBasicBlock* PredBB) {
+ MadeChange = false;
// Sort by hash value so that blocks with identical end sequences sort
// together.
// If there is nothing that matches the hash of the current basic block,
// give up.
if (CurHash != PrevHash) {
+ if (SuccBB && CurMBB != PredBB)
+ FixTail(CurMBB, SuccBB, TII);
MergePotentials.pop_back();
continue;
}
// If the tails don't have at least two instructions in common, see if there
// is anything else in the equivalence class that does match.
+ // Since instructions may get combined later (e.g. single stores into
+ // store multiple) this measure is not particularly accurate.
if (CommonTailLen < 2) {
unsigned FoundMatch = ~0U;
for (int i = MergePotentials.size()-2;
// If we didn't find anything that has at least two instructions matching
// this one, bail out.
if (FoundMatch == ~0U) {
+ // Put the unconditional branch back, if we need one.
+ if (SuccBB && CurMBB != PredBB)
+ FixTail(CurMBB, SuccBB, TII);
MergePotentials.pop_back();
continue;
}
// Otherwise, move the matching block to the right position.
std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2));
}
-
- // If either block is the entire common tail, make the longer one branch to
- // the shorter one.
+
MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
- if (CurMBB->begin() == BBI1) {
- // Hack the end off MBB2, making it jump to CurMBB instead.
- ReplaceTailWithBranchTo(BBI2, CurMBB);
- // This modifies MBB2, so remove it from the worklist.
- MergePotentials.erase(MergePotentials.end()-2);
- MadeChange = true;
- continue;
- } else if (MBB2->begin() == BBI2) {
+
+ // If neither block is the entire common tail, split the tail of one block
+ // to make it redundant with the other tail. Also, we cannot jump to the
+ // entry block, so if one block is the entry block, split the other one.
+ MachineBasicBlock *Entry = CurMBB->getParent()->begin();
+ if (CurMBB->begin() == BBI1 && CurMBB != Entry)
+ ; // CurMBB is common tail
+ else if (MBB2->begin() == BBI2 && MBB2 != Entry)
+ ; // MBB2 is common tail
+ else {
+ if (0) { // Enable this to disable partial tail merges.
+ MergePotentials.pop_back();
+ continue;
+ }
+
+ // Decide whether we want to split CurMBB or MBB2.
+ if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII, PredBB)) {
+ CurMBB = SplitMBBAt(*CurMBB, BBI1);
+ BBI1 = CurMBB->begin();
+ MergePotentials.back().second = CurMBB;
+ } else {
+ MBB2 = SplitMBBAt(*MBB2, BBI2);
+ BBI2 = MBB2->begin();
+ (MergePotentials.end()-2)->second = MBB2;
+ }
+ }
+
+ if (MBB2->begin() == BBI2 && MBB2 != Entry) {
// Hack the end off CurMBB, making it jump to MBBI@ instead.
ReplaceTailWithBranchTo(BBI1, MBB2);
// This modifies CurMBB, so remove it from the worklist.
MergePotentials.pop_back();
- MadeChange = true;
- continue;
+ } else {
+ assert(CurMBB->begin() == BBI1 && CurMBB != Entry &&
+ "Didn't split block correctly?");
+ // Hack the end off MBB2, making it jump to CurMBB instead.
+ ReplaceTailWithBranchTo(BBI2, CurMBB);
+ // This modifies MBB2, so remove it from the worklist.
+ MergePotentials.erase(MergePotentials.end()-2);
}
-
- MergePotentials.pop_back();
+ MadeChange = true;
}
-
return MadeChange;
}
+bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
+
+ if (!EnableTailMerge) return false;
+
+ MadeChange = false;
+
+ // First find blocks with no successors.
+ MergePotentials.clear();
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+ if (I->succ_empty())
+ MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I));
+ }
+ // See if we can do any tail merging on those.
+ MadeChange |= TryMergeBlocks(NULL, NULL);
+
+ // Look at blocks (IBB) with multiple predecessors (PBB).
+ // We change each predecessor to a canonical form, by
+ // (1) temporarily removing any unconditional branch from the predecessor
+ // to IBB, and
+ // (2) alter conditional branches so they branch to the other block
+ // not IBB; this may require adding back an unconditional branch to IBB
+ // later, where there wasn't one coming in. E.g.
+ // Bcc IBB
+ // fallthrough to QBB
+ // here becomes
+ // Bncc QBB
+ // with a conceptual B to IBB after that, which never actually exists.
+ // With those changes, we see whether the predecessors' tails match,
+ // and merge them if so. We change things out of canonical form and
+ // back to the way they were later in the process. (OptimizeBranches
+ // would undo some of this, but we can't use it, because we'd get into
+ // a compile-time infinite loop repeatedly doing and undoing the same
+ // transformations.)
+
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+ if (!I->succ_empty() && I->pred_size() >= 2) {
+ MachineBasicBlock *IBB = I;
+ MachineBasicBlock *PredBB = prior(I);
+ MergePotentials.clear();
+ for (MachineBasicBlock::pred_iterator P = I->pred_begin(), E2 = I->pred_end();
+ P != E2; ++P) {
+ MachineBasicBlock* PBB = *P;
+ // Skip blocks that loop to themselves, can't tail merge these.
+ if (PBB==IBB)
+ continue;
+ MachineBasicBlock *TBB = 0, *FBB = 0;
+ std::vector<MachineOperand> Cond;
+ if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) {
+ // Failing case: IBB is the target of a cbr, and
+ // we cannot reverse the branch.
+ std::vector<MachineOperand> NewCond(Cond);
+ if (Cond.size() && TBB==IBB) {
+ if (TII->ReverseBranchCondition(NewCond))
+ continue;
+ // This is the QBB case described above
+ if (!FBB)
+ FBB = next(MachineFunction::iterator(PBB));
+ }
+ // Remove the unconditional branch at the end, if any.
+ if (TBB && (Cond.size()==0 || FBB)) {
+ TII->RemoveBranch(*PBB);
+ if (Cond.size())
+ // reinsert conditional branch only, for now
+ TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond);
+ }
+ MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB), *P));
+ }
+ }
+ if (MergePotentials.size() >= 2)
+ MadeChange |= TryMergeBlocks(I, PredBB);
+ // Reinsert an unconditional branch if needed.
+ // The 1 below can be either an original single predecessor, or a result
+ // of removing blocks in TryMergeBlocks.
+ if (MergePotentials.size()==1 &&
+ (MergePotentials.begin())->second != PredBB)
+ FixTail((MergePotentials.begin())->second, I, TII);
+ }
+ }
+ return MadeChange;
+}
//===----------------------------------------------------------------------===//
// Branch Optimization
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
MadeChange = false;
+ // Make sure blocks are numbered in order
+ MF.RenumberBlocks();
+
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
MachineBasicBlock *MBB = I++;
OptimizeBlock(MBB);
} else if (*SI == DestB) {
DestB = 0;
++SI;
+ } else if ((*SI)->isLandingPad()) {
+ ++SI;
} else {
// Otherwise, this is a superfluous edge, remove it.
MBB.removeSuccessor(SI);
}
}
-/// CanFallThrough - Return true of the specified branch condition can transfer
-/// control to FallthroughBlock, the block immediately after the branch.
-static bool CanFallThrough(MachineBasicBlock *TBB,
- MachineBasicBlock *FBB,
- const std::vector<MachineOperand> &Cond,
- MachineFunction::iterator FallthroughBlock) {
+/// CanFallThrough - Return true if the specified block (with the specified
+/// branch condition) can implicitly transfer control to the block after it by
+/// falling off the end of it. This should return false if it can reach the
+/// block after it, but it uses an explicit branch to do so (e.g. a table jump).
+///
+/// True is a conservative answer.
+///
+bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
+ bool BranchUnAnalyzable,
+ MachineBasicBlock *TBB, MachineBasicBlock *FBB,
+ const std::vector<MachineOperand> &Cond) {
+ MachineFunction::iterator Fallthrough = CurBB;
+ ++Fallthrough;
+ // If FallthroughBlock is off the end of the function, it can't fall through.
+ if (Fallthrough == CurBB->getParent()->end())
+ return false;
+
+ // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
+ if (!CurBB->isSuccessor(Fallthrough))
+ return false;
+
+ // If we couldn't analyze the branch, assume it could fall through.
+ if (BranchUnAnalyzable) return true;
+
// If there is no branch, control always falls through.
if (TBB == 0) return true;
// If there is some explicit branch to the fallthrough block, it can obviously
// reach, even though the branch should get folded to fall through implicitly.
- if (MachineFunction::iterator(TBB) == FallthroughBlock ||
- MachineFunction::iterator(FBB) == FallthroughBlock)
+ if (MachineFunction::iterator(TBB) == Fallthrough ||
+ MachineFunction::iterator(FBB) == Fallthrough)
return true;
// If it's an unconditional branch to some block not the fall through, it
return FBB == 0;
}
+/// CanFallThrough - Return true if the specified can implicitly transfer
+/// control to the block after it by falling off the end of it. This should
+/// return false if it can reach the block after it, but it uses an explicit
+/// branch to do so (e.g. a table jump).
+///
+/// True is a conservative answer.
+///
+bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
+ MachineBasicBlock *TBB = 0, *FBB = 0;
+ std::vector<MachineOperand> Cond;
+ bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond);
+ return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
+}
+
+/// IsBetterFallthrough - Return true if it would be clearly better to
+/// fall-through to MBB1 than to fall through into MBB2. This has to return
+/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
+/// result in infinite loops.
+static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
+ MachineBasicBlock *MBB2,
+ const TargetInstrInfo &TII) {
+ // Right now, we use a simple heuristic. If MBB2 ends with a call, and
+ // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
+ // optimize branches that branch to either a return block or an assert block
+ // into a fallthrough to the return.
+ if (MBB1->empty() || MBB2->empty()) return false;
+
+ MachineInstr *MBB1I = --MBB1->end();
+ MachineInstr *MBB2I = --MBB2->end();
+ return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode());
+}
+
/// OptimizeBlock - Analyze and optimize control flow related to the specified
/// block. This is never called on the entry block.
void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
// If MBB was the target of a jump table, update jump tables to go to the
// fallthrough instead.
- MBB->getParent()->getJumpTableInfo()->ReplaceMBBInJumpTables(MBB,
- FallThrough);
+ MBB->getParent()->getJumpTableInfo()->
+ ReplaceMBBInJumpTables(MBB, FallThrough);
MadeChange = true;
}
return;
MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
std::vector<MachineOperand> PriorCond;
- bool PriorUnAnalyzable = false;
- PriorUnAnalyzable = TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
+ bool PriorUnAnalyzable =
+ TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
if (!PriorUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB,
return OptimizeBlock(MBB);
}
}
+
+ // If this block doesn't fall through (e.g. it ends with an uncond branch or
+ // has no successors) and if the pred falls through into this block, and if
+ // it would otherwise fall through into the block after this, move this
+ // block to the end of the function.
+ //
+ // We consider it more likely that execution will stay in the function (e.g.
+ // due to loops) than it is to exit it. This asserts in loops etc, moving
+ // the assert condition out of the loop body.
+ if (!PriorCond.empty() && PriorFBB == 0 &&
+ MachineFunction::iterator(PriorTBB) == FallThrough &&
+ !CanFallThrough(MBB)) {
+ bool DoTransform = true;
+
+ // We have to be careful that the succs of PredBB aren't both no-successor
+ // blocks. If neither have successors and if PredBB is the second from
+ // last block in the function, we'd just keep swapping the two blocks for
+ // last. Only do the swap if one is clearly better to fall through than
+ // the other.
+ if (FallThrough == --MBB->getParent()->end() &&
+ !IsBetterFallthrough(PriorTBB, MBB, *TII))
+ DoTransform = false;
+
+ // We don't want to do this transformation if we have control flow like:
+ // br cond BB2
+ // BB1:
+ // ..
+ // jmp BBX
+ // BB2:
+ // ..
+ // ret
+ //
+ // In this case, we could actually be moving the return block *into* a
+ // loop!
+ if (DoTransform && !MBB->succ_empty() &&
+ (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
+ DoTransform = false;
+
+
+ if (DoTransform) {
+ // Reverse the branch so we will fall through on the previous true cond.
+ std::vector<MachineOperand> NewPriorCond(PriorCond);
+ if (!TII->ReverseBranchCondition(NewPriorCond)) {
+ DOUT << "\nMoving MBB: " << *MBB;
+ DOUT << "To make fallthrough to: " << *PriorTBB << "\n";
+
+ TII->RemoveBranch(PrevBB);
+ TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
+
+ // Move this block to the end of the function.
+ MBB->moveAfter(--MBB->getParent()->end());
+ MadeChange = true;
+ ++NumBranchOpts;
+ return;
+ }
+ }
+ }
}
// Analyze the branch in the current block.
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
std::vector<MachineOperand> CurCond;
- if (!TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond)) {
+ bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond);
+ if (!CurUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB,
!CurCond.empty(),
++MachineFunction::iterator(MBB));
+ // If this is a two-way branch, and the FBB branches to this block, reverse
+ // the condition so the single-basic-block loop is faster. Instead of:
+ // Loop: xxx; jcc Out; jmp Loop
+ // we want:
+ // Loop: xxx; jncc Loop; jmp Out
+ if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
+ std::vector<MachineOperand> NewCond(CurCond);
+ if (!TII->ReverseBranchCondition(NewCond)) {
+ TII->RemoveBranch(*MBB);
+ TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
+ MadeChange = true;
+ ++NumBranchOpts;
+ return OptimizeBlock(MBB);
+ }
+ }
+
+
// If this branch is the only thing in its block, see if we can forward
// other blocks across it.
if (CurTBB && CurCond.empty() && CurFBB == 0 &&
// completely eliminate the block is when the block before this one
// falls through into MBB and we can't understand the prior block's branch
// condition.
- if (MBB->empty() && (!PriorUnAnalyzable || !PrevBB.isSuccessor(MBB))) {
- // If the prior block falls through into us, turn it into an
- // explicit branch to us to make updates simpler.
- if (PrevBB.isSuccessor(MBB) && PriorTBB != MBB && PriorFBB != MBB) {
- if (PriorTBB == 0) {
- assert(PriorCond.empty() && PriorFBB == 0 && "Bad branch analysis");
- PriorTBB = MBB;
- } else {
- assert(PriorFBB == 0 && "Machine CFG out of date!");
- PriorFBB = MBB;
+ if (MBB->empty()) {
+ bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
+ if (PredHasNoFallThrough || !PriorUnAnalyzable ||
+ !PrevBB.isSuccessor(MBB)) {
+ // If the prior block falls through into us, turn it into an
+ // explicit branch to us to make updates simpler.
+ if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
+ PriorTBB != MBB && PriorFBB != MBB) {
+ if (PriorTBB == 0) {
+ assert(PriorCond.empty() && PriorFBB == 0 &&
+ "Bad branch analysis");
+ PriorTBB = MBB;
+ } else {
+ assert(PriorFBB == 0 && "Machine CFG out of date!");
+ PriorFBB = MBB;
+ }
+ TII->RemoveBranch(PrevBB);
+ TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
}
- TII->RemoveBranch(PrevBB);
- TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
- }
- // Iterate through all the predecessors, revectoring each in-turn.
- MachineBasicBlock::pred_iterator PI = MBB->pred_begin();
- bool DidChange = false;
- bool HasBranchToSelf = false;
- while (PI != MBB->pred_end()) {
- if (*PI == MBB) {
- // If this block has an uncond branch to itself, leave it.
- ++PI;
- HasBranchToSelf = true;
- } else {
- DidChange = true;
- ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII);
+ // Iterate through all the predecessors, revectoring each in-turn.
+ MachineBasicBlock::pred_iterator PI = MBB->pred_begin();
+ bool DidChange = false;
+ bool HasBranchToSelf = false;
+ while (PI != MBB->pred_end()) {
+ if (*PI == MBB) {
+ // If this block has an uncond branch to itself, leave it.
+ ++PI;
+ HasBranchToSelf = true;
+ } else {
+ DidChange = true;
+ ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII);
+ }
}
- }
- // Change any jumptables to go to the new MBB.
- MBB->getParent()->getJumpTableInfo()->ReplaceMBBInJumpTables(MBB,
- CurTBB);
- if (DidChange) {
- ++NumBranchOpts;
- MadeChange = true;
- if (!HasBranchToSelf) return;
+ // Change any jumptables to go to the new MBB.
+ MBB->getParent()->getJumpTableInfo()->
+ ReplaceMBBInJumpTables(MBB, CurTBB);
+ if (DidChange) {
+ ++NumBranchOpts;
+ MadeChange = true;
+ if (!HasBranchToSelf) return;
+ }
}
}
// Add the branch back if the block is more than just an uncond branch.
TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
}
-
- // If the prior block doesn't fall through into this block, and if this
- // block doesn't fall through into some other block, see if we can find a
- // place to move this block where a fall-through will happen.
- if (!PriorUnAnalyzable && !CanFallThrough(PriorTBB, PriorFBB,
- PriorCond, MBB)) {
- // Now we know that there was no fall-through into this block, check to
- // see if it has fall-throughs.
- if (!CanFallThrough(CurTBB, CurFBB, CurCond, FallThrough)) {
-
- // Check all the predecessors of this block. If one of them has no fall
- // throughs, move this block right after it.
- for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
- E = MBB->pred_end(); PI != E; ++PI) {
- // Analyze the branch at the end of the pred.
- MachineBasicBlock *PredBB = *PI;
- MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
- MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
- std::vector<MachineOperand> PredCond;
- if (PredBB != MBB &&
- !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond) &&
- !CanFallThrough(PredTBB, PredFBB, PredCond, PredFallthrough)) {
- MBB->moveAfter(PredBB);
- MadeChange = true;
- return OptimizeBlock(MBB);
+ }
+
+ // If the prior block doesn't fall through into this block, and if this
+ // block doesn't fall through into some other block, see if we can find a
+ // place to move this block where a fall-through will happen.
+ if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
+ PriorTBB, PriorFBB, PriorCond)) {
+ // Now we know that there was no fall-through into this block, check to
+ // see if it has a fall-through into its successor.
+ bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
+ CurCond);
+
+ if (!MBB->isLandingPad()) {
+ // Check all the predecessors of this block. If one of them has no fall
+ // throughs, move this block right after it.
+ for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
+ E = MBB->pred_end(); PI != E; ++PI) {
+ // Analyze the branch at the end of the pred.
+ MachineBasicBlock *PredBB = *PI;
+ MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
+ if (PredBB != MBB && !CanFallThrough(PredBB)
+ && (!CurFallsThru || !CurTBB || !CurFBB)
+ && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
+ // If the current block doesn't fall through, just move it.
+ // If the current block can fall through and does not end with a
+ // conditional branch, we need to append an unconditional jump to
+ // the (current) next block. To avoid a possible compile-time
+ // infinite loop, move blocks only backward in this case.
+ // Also, if there are already 2 branches here, we cannot add a third;
+ // this means we have the case
+ // Bcc next
+ // B elsewhere
+ // next:
+ if (CurFallsThru) {
+ MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
+ CurCond.clear();
+ TII->InsertBranch(*MBB, NextBB, 0, CurCond);
}
+ MBB->moveAfter(PredBB);
+ MadeChange = true;
+ return OptimizeBlock(MBB);
}
+ }
+ }
- // Check all successors to see if we can move this block before it.
- for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
- E = MBB->succ_end(); SI != E; ++SI) {
- // Analyze the branch at the end of the block before the succ.
- MachineBasicBlock *SuccBB = *SI;
- MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
- MachineBasicBlock *SuccPrevTBB = 0, *SuccPrevFBB = 0;
- std::vector<MachineOperand> SuccPrevCond;
- if (SuccBB != MBB &&
- !TII->AnalyzeBranch(*SuccPrev, SuccPrevTBB, SuccPrevFBB,
- SuccPrevCond) &&
- !CanFallThrough(SuccPrevTBB, SuccPrevFBB, SuccPrevCond, SuccBB)) {
- MBB->moveBefore(SuccBB);
- MadeChange = true;
- return OptimizeBlock(MBB);
- }
- }
+ if (!CurFallsThru) {
+ // Check all successors to see if we can move this block before it.
+ for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
+ E = MBB->succ_end(); SI != E; ++SI) {
+ // Analyze the branch at the end of the block before the succ.
+ MachineBasicBlock *SuccBB = *SI;
+ MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
+ std::vector<MachineOperand> SuccPrevCond;
- // Okay, there is no really great place to put this block. If, however,
- // the block before this one would be a fall-through if this block were
- // removed, move this block to the end of the function.
- if (FallThrough != MBB->getParent()->end() &&
- CanFallThrough(PriorTBB, PriorFBB, PriorCond, FallThrough)) {
- MBB->moveAfter(--MBB->getParent()->end());
+ // If this block doesn't already fall-through to that successor, and if
+ // the succ doesn't already have a block that can fall through into it,
+ // and if the successor isn't an EH destination, we can arrange for the
+ // fallthrough to happen.
+ if (SuccBB != MBB && !CanFallThrough(SuccPrev) &&
+ !SuccBB->isLandingPad()) {
+ MBB->moveBefore(SuccBB);
MadeChange = true;
- return;
+ return OptimizeBlock(MBB);
}
}
+
+ // Okay, there is no really great place to put this block. If, however,
+ // the block before this one would be a fall-through if this block were
+ // removed, move this block to the end of the function.
+ if (FallThrough != MBB->getParent()->end() &&
+ PrevBB.isSuccessor(FallThrough)) {
+ MBB->moveAfter(--MBB->getParent()->end());
+ MadeChange = true;
+ return;
+ }
}
}
}
projects / oota-llvm.git / blobdiff