[llvm-commits] CVS: llvm/lib/CodeGen/BranchFolding.cpp
Chris Lattner
sabre at nondot.org
Fri Oct 20 17:48:03 PDT 2006
Changes in directory llvm/lib/CodeGen:
BranchFolding.cpp updated: 1.14 -> 1.15
---
Log message:
Add an experimental cross-jumping implementation.
This is currently disabled by default and limited in several ways, but does
have a positive effect.
---
Diffs of the changes: (+232 -14)
BranchFolding.cpp | 246 ++++++++++++++++++++++++++++++++++++++++++++++++++----
1 files changed, 232 insertions(+), 14 deletions(-)
Index: llvm/lib/CodeGen/BranchFolding.cpp
diff -u llvm/lib/CodeGen/BranchFolding.cpp:1.14 llvm/lib/CodeGen/BranchFolding.cpp:1.15
--- llvm/lib/CodeGen/BranchFolding.cpp:1.14 Tue Oct 17 18:17:27 2006
+++ llvm/lib/CodeGen/BranchFolding.cpp Fri Oct 20 19:47:49 2006
@@ -22,9 +22,16 @@
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
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");
+static cl::opt<bool> EnableTailMerge("enable-tail-merge");
+
namespace {
struct BranchFolder : public MachineFunctionPass {
virtual bool runOnMachineFunction(MachineFunction &MF);
@@ -33,6 +40,13 @@
MachineDebugInfo *MDI;
bool MadeChange;
private:
+ // Tail Merging.
+ bool TailMergeBlocks(MachineFunction &MF);
+ void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
+ MachineBasicBlock *NewDest);
+
+ // Branch optzn.
+ bool OptimizeBranches(MachineFunction &MF);
void OptimizeBlock(MachineFunction::iterator MBB);
void RemoveDeadBlock(MachineBasicBlock *MBB);
};
@@ -77,26 +91,228 @@
MDI = getAnalysisToUpdate<MachineDebugInfo>();
bool EverMadeChange = false;
- MadeChange = true;
- while (MadeChange) {
- MadeChange = false;
-
- for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
- MachineBasicBlock *MBB = I++;
- OptimizeBlock(MBB);
+ bool MadeChangeThisIteration = true;
+ while (MadeChangeThisIteration) {
+ MadeChangeThisIteration = false;
+ MadeChangeThisIteration |= TailMergeBlocks(MF);
+ MadeChangeThisIteration |= OptimizeBranches(MF);
+ EverMadeChange |= MadeChangeThisIteration;
+ }
+
+ return EverMadeChange;
+}
+
+//===----------------------------------------------------------------------===//
+// Tail Merging of Blocks
+//===----------------------------------------------------------------------===//
+
+/// HashMachineInstr - Compute a hash value for MI and its operands.
+static unsigned HashMachineInstr(const MachineInstr *MI) {
+ unsigned Hash = MI->getOpcode();
+ for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+ const MachineOperand &Op = MI->getOperand(i);
+
+ // Merge in bits from the operand if easy.
+ unsigned OperandHash = 0;
+ switch (Op.getType()) {
+ case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
+ case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
+ case MachineOperand::MO_MachineBasicBlock:
+ OperandHash = Op.getMachineBasicBlock()->getNumber();
+ break;
+ case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break;
+ case MachineOperand::MO_ConstantPoolIndex:
+ OperandHash = Op.getConstantPoolIndex();
+ break;
+ case MachineOperand::MO_JumpTableIndex:
+ OperandHash = Op.getJumpTableIndex();
+ break;
+ case MachineOperand::MO_GlobalAddress:
+ case MachineOperand::MO_ExternalSymbol:
+ // Global address / external symbol are too hard, don't bother, but do
+ // pull in the offset.
+ OperandHash = Op.getOffset();
+ break;
+ default: break;
+ }
+
+ Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
+ }
+ return Hash;
+}
+
+/// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two
+/// instructions, because cross-jumping only saves code when at least two
+/// instructions are removed (since a branch must be inserted).
+static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
+ MachineBasicBlock::const_iterator I = MBB->end();
+ if (I == MBB->begin())
+ return 0; // Empty MBB.
+
+ --I;
+ unsigned Hash = HashMachineInstr(I);
+
+ if (I == MBB->begin())
+ return Hash; // Single instr MBB.
+
+ --I;
+ // Hash in the second-to-last instruction.
+ Hash ^= HashMachineInstr(I) << 2;
+ return Hash;
+}
+
+/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
+/// of instructions they actually have in common together at their end. Return
+/// iterators for the first shared instruction in each block.
+static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
+ MachineBasicBlock *MBB2,
+ MachineBasicBlock::iterator &I1,
+ MachineBasicBlock::iterator &I2) {
+ I1 = MBB1->end();
+ I2 = MBB2->end();
+
+ unsigned TailLen = 0;
+ while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
+ --I1; --I2;
+ if (!I1->isIdenticalTo(I2)) {
+ ++I1; ++I2;
+ break;
+ }
+ ++TailLen;
+ }
+ return TailLen;
+}
+
+/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
+/// after it, replacing it with an unconditional branch to NewDest.
+void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
+ MachineBasicBlock *NewDest) {
+ MachineBasicBlock *OldBB = OldInst->getParent();
+
+ // Remove all the old successors of OldBB from the CFG.
+ while (!OldBB->succ_empty())
+ OldBB->removeSuccessor(OldBB->succ_begin());
+
+ // Remove all the dead instructions from the end of OldBB.
+ OldBB->erase(OldInst, OldBB->end());
+
+ TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>());
+ OldBB->addSuccessor(NewDest);
+ ++NumTailMerge;
+}
+
+bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
+ MadeChange = false;
+
+ if (!EnableTailMerge)
+ return false;
+
+ // 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));
+ }
+
+ // Sort by hash value so that blocks with identical end sequences sort
+ // together.
+ std::stable_sort(MergePotentials.begin(), MergePotentials.end());
+
+ // Walk through equivalence sets looking for actual exact matches.
+ while (MergePotentials.size() > 1) {
+ unsigned CurHash = (MergePotentials.end()-1)->first;
+ unsigned PrevHash = (MergePotentials.end()-2)->first;
+ MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second;
+
+ // If there is nothing that matches the hash of the current basic block,
+ // give up.
+ if (CurHash != PrevHash) {
+ MergePotentials.pop_back();
+ continue;
+ }
+
+ // Determine the actual length of the shared tail between these two basic
+ // blocks. Because the hash can have collisions, it's possible that this is
+ // less than 2.
+ MachineBasicBlock::iterator BBI1, BBI2;
+ unsigned CommonTailLen =
+ ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second,
+ BBI1, BBI2);
+
+ // 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.
+ if (CommonTailLen < 2) {
+ unsigned FoundMatch = ~0U;
+ for (int i = MergePotentials.size()-2;
+ i != -1 && MergePotentials[i].first == CurHash; --i) {
+ CommonTailLen = ComputeCommonTailLength(CurMBB,
+ MergePotentials[i].second,
+ BBI1, BBI2);
+ if (CommonTailLen >= 2) {
+ FoundMatch = i;
+ break;
+ }
+ }
- // If it is dead, remove it.
- if (MBB->pred_empty()) {
- RemoveDeadBlock(MBB);
- MadeChange = true;
+ // If we didn't find anything that has at least two instructions matching
+ // this one, bail out.
+ if (FoundMatch == ~0U) {
+ MergePotentials.pop_back();
+ continue;
}
- }
- EverMadeChange |= MadeChange;
+
+ // 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) {
+ // 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;
+ }
+
+ MergePotentials.pop_back();
}
+
+ return MadeChange;
+}
- return EverMadeChange;
+
+//===----------------------------------------------------------------------===//
+// Branch Optimization
+//===----------------------------------------------------------------------===//
+
+bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
+ MadeChange = false;
+
+ for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
+ MachineBasicBlock *MBB = I++;
+ OptimizeBlock(MBB);
+
+ // If it is dead, remove it.
+ if (MBB->pred_empty()) {
+ RemoveDeadBlock(MBB);
+ MadeChange = true;
+ ++NumDeadBlocks;
+ }
+ }
+ return MadeChange;
}
+
/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
/// 'Old', change the code and CFG so that it branches to 'New' instead.
static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
@@ -171,6 +387,7 @@
if (PriorTBB != &*MBB)
TII->InsertBranch(*prior(MBB), PriorTBB, 0, PriorCond);
MadeChange = true;
+ ++NumBranchOpts;
return OptimizeBlock(MBB);
}
@@ -179,6 +396,7 @@
if (PriorTBB == &*MBB && PriorFBB == 0) {
TII->RemoveBranch(*prior(MBB));
MadeChange = true;
+ ++NumBranchOpts;
return OptimizeBlock(MBB);
}
}
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