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//===-- CodePlacementOpt.cpp - Code Placement pass. -----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the pass that optimizes code placement and aligns loop
// headers to target-specific alignment boundaries.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "code-placement"
#include "llvm/CodeGen/Passes.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
STATISTIC(NumLoopsAligned, "Number of loops aligned");
STATISTIC(NumIntraElim, "Number of intra loop branches eliminated");
STATISTIC(NumIntraMoved, "Number of intra loop branches moved");
namespace {
class CodePlacementOpt : public MachineFunctionPass {
const MachineLoopInfo *MLI;
const TargetInstrInfo *TII;
const TargetLowering *TLI;
public:
static char ID;
CodePlacementOpt() : MachineFunctionPass(ID) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineLoopInfo>();
AU.addPreservedID(MachineDominatorsID);
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
bool HasFallthrough(MachineBasicBlock *MBB);
bool HasAnalyzableTerminator(MachineBasicBlock *MBB);
void Splice(MachineFunction &MF,
MachineFunction::iterator InsertPt,
MachineFunction::iterator Begin,
MachineFunction::iterator End);
bool EliminateUnconditionalJumpsToTop(MachineFunction &MF,
MachineLoop *L);
bool MoveDiscontiguousLoopBlocks(MachineFunction &MF,
MachineLoop *L);
bool OptimizeIntraLoopEdgesInLoopNest(MachineFunction &MF, MachineLoop *L);
bool OptimizeIntraLoopEdges(MachineFunction &MF);
bool AlignLoops(MachineFunction &MF);
bool AlignLoop(MachineFunction &MF, MachineLoop *L, unsigned Align);
};
char CodePlacementOpt::ID = 0;
} // end anonymous namespace
char &llvm::CodePlacementOptID = CodePlacementOpt::ID;
INITIALIZE_PASS(CodePlacementOpt, "code-placement",
"Code Placement Optimizer", false, false)
/// HasFallthrough - Test whether the given branch has a fallthrough, either as
/// a plain fallthrough or as a fallthrough case of a conditional branch.
///
bool CodePlacementOpt::HasFallthrough(MachineBasicBlock *MBB) {
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
return false;
// This conditional branch has no fallthrough.
if (FBB)
return false;
// An unconditional branch has no fallthrough.
if (Cond.empty() && TBB)
return false;
// It has a fallthrough.
return true;
}
/// HasAnalyzableTerminator - Test whether AnalyzeBranch will succeed on MBB.
/// This is called before major changes are begun to test whether it will be
/// possible to complete the changes.
///
/// Target-specific code is hereby encouraged to make AnalyzeBranch succeed
/// whenever possible.
///
bool CodePlacementOpt::HasAnalyzableTerminator(MachineBasicBlock *MBB) {
// Conservatively ignore EH landing pads.
if (MBB->isLandingPad()) return false;
// Aggressively handle return blocks and similar constructs.
if (MBB->succ_empty()) return true;
// Ask the target's AnalyzeBranch if it can handle this block.
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
// Make sure the terminator is understood.
if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
return false;
// Ignore blocks which look like they might have EH-related control flow.
// AnalyzeBranch thinks it knows how to analyze such things, but it doesn't
// recognize the possibility of a control transfer through an unwind.
// Such blocks contain EH_LABEL instructions, however they may be in the
// middle of the block. Instead of searching for them, just check to see
// if the CFG disagrees with AnalyzeBranch.
if (1u + !Cond.empty() != MBB->succ_size())
return false;
// Make sure we have the option of reversing the condition.
if (!Cond.empty() && TII->ReverseBranchCondition(Cond))
return false;
return true;
}
/// Splice - Move the sequence of instructions [Begin,End) to just before
/// InsertPt. Update branch instructions as needed to account for broken
/// fallthrough edges and to take advantage of newly exposed fallthrough
/// opportunities.
///
void CodePlacementOpt::Splice(MachineFunction &MF,
MachineFunction::iterator InsertPt,
MachineFunction::iterator Begin,
MachineFunction::iterator End) {
assert(Begin != MF.begin() && End != MF.begin() && InsertPt != MF.begin() &&
"Splice can't change the entry block!");
MachineFunction::iterator OldBeginPrior = prior(Begin);
MachineFunction::iterator OldEndPrior = prior(End);
MF.splice(InsertPt, Begin, End);
prior(Begin)->updateTerminator();
OldBeginPrior->updateTerminator();
OldEndPrior->updateTerminator();
}
/// EliminateUnconditionalJumpsToTop - Move blocks which unconditionally jump
/// to the loop top to the top of the loop so that they have a fall through.
/// This can introduce a branch on entry to the loop, but it can eliminate a
/// branch within the loop. See the @simple case in
/// test/CodeGen/X86/loop_blocks.ll for an example of this.
bool CodePlacementOpt::EliminateUnconditionalJumpsToTop(MachineFunction &MF,
MachineLoop *L) {
bool Changed = false;
MachineBasicBlock *TopMBB = L->getTopBlock();
bool BotHasFallthrough = HasFallthrough(L->getBottomBlock());
if (TopMBB == MF.begin() ||
HasAnalyzableTerminator(prior(MachineFunction::iterator(TopMBB)))) {
new_top:
for (MachineBasicBlock::pred_iterator PI = TopMBB->pred_begin(),
PE = TopMBB->pred_end(); PI != PE; ++PI) {
MachineBasicBlock *Pred = *PI;
if (Pred == TopMBB) continue;
if (HasFallthrough(Pred)) continue;
if (!L->contains(Pred)) continue;
// Verify that we can analyze all the loop entry edges before beginning
// any changes which will require us to be able to analyze them.
if (Pred == MF.begin())
continue;
if (!HasAnalyzableTerminator(Pred))
continue;
if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(Pred))))
continue;
// Move the block.
DEBUG(dbgs() << "CGP: Moving blocks starting at BB#" << Pred->getNumber()
<< " to top of loop.\n");
Changed = true;
// Move it and all the blocks that can reach it via fallthrough edges
// exclusively, to keep existing fallthrough edges intact.
MachineFunction::iterator Begin = Pred;
MachineFunction::iterator End = llvm::next(Begin);
while (Begin != MF.begin()) {
MachineFunction::iterator Prior = prior(Begin);
if (Prior == MF.begin())
break;
// Stop when a non-fallthrough edge is found.
if (!HasFallthrough(Prior))
break;
// Stop if a block which could fall-through out of the loop is found.
if (Prior->isSuccessor(End))
break;
// If we've reached the top, stop scanning.
if (Prior == MachineFunction::iterator(TopMBB)) {
// We know top currently has a fall through (because we just checked
// it) which would be lost if we do the transformation, so it isn't
// worthwhile to do the transformation unless it would expose a new
// fallthrough edge.
if (!Prior->isSuccessor(End))
goto next_pred;
// Otherwise we can stop scanning and proceed to move the blocks.
break;
}
// If we hit a switch or something complicated, don't move anything
// for this predecessor.
if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(Prior))))
break;
// Ok, the block prior to Begin will be moved along with the rest.
// Extend the range to include it.
Begin = Prior;
++NumIntraMoved;
}
// Move the blocks.
Splice(MF, TopMBB, Begin, End);
// Update TopMBB.
TopMBB = L->getTopBlock();
// We have a new loop top. Iterate on it. We shouldn't have to do this
// too many times if BranchFolding has done a reasonable job.
goto new_top;
next_pred:;
}
}
// If the loop previously didn't exit with a fall-through and it now does,
// we eliminated a branch.
if (Changed &&
!BotHasFallthrough &&
HasFallthrough(L->getBottomBlock())) {
++NumIntraElim;
}
return Changed;
}
/// MoveDiscontiguousLoopBlocks - Move any loop blocks that are not in the
/// portion of the loop contiguous with the header. This usually makes the loop
/// contiguous, provided that AnalyzeBranch can handle all the relevant
/// branching. See the @cfg_islands case in test/CodeGen/X86/loop_blocks.ll
/// for an example of this.
bool CodePlacementOpt::MoveDiscontiguousLoopBlocks(MachineFunction &MF,
MachineLoop *L) {
bool Changed = false;
MachineBasicBlock *TopMBB = L->getTopBlock();
MachineBasicBlock *BotMBB = L->getBottomBlock();
// Determine a position to move orphaned loop blocks to. If TopMBB is not
// entered via fallthrough and BotMBB is exited via fallthrough, prepend them
// to the top of the loop to avoid losing that fallthrough. Otherwise append
// them to the bottom, even if it previously had a fallthrough, on the theory
// that it's worth an extra branch to keep the loop contiguous.
MachineFunction::iterator InsertPt =
llvm::next(MachineFunction::iterator(BotMBB));
bool InsertAtTop = false;
if (TopMBB != MF.begin() &&
!HasFallthrough(prior(MachineFunction::iterator(TopMBB))) &&
HasFallthrough(BotMBB)) {
InsertPt = TopMBB;
InsertAtTop = true;
}
// Keep a record of which blocks are in the portion of the loop contiguous
// with the loop header.
SmallPtrSet<MachineBasicBlock *, 8> ContiguousBlocks;
for (MachineFunction::iterator I = TopMBB,
E = llvm::next(MachineFunction::iterator(BotMBB)); I != E; ++I)
ContiguousBlocks.insert(I);
// Find non-contigous blocks and fix them.
if (InsertPt != MF.begin() && HasAnalyzableTerminator(prior(InsertPt)))
for (MachineLoop::block_iterator BI = L->block_begin(), BE = L->block_end();
BI != BE; ++BI) {
MachineBasicBlock *BB = *BI;
// Verify that we can analyze all the loop entry edges before beginning
// any changes which will require us to be able to analyze them.
if (!HasAnalyzableTerminator(BB))
continue;
if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(BB))))
continue;
// If the layout predecessor is part of the loop, this block will be
// processed along with it. This keeps them in their relative order.
if (BB != MF.begin() &&
L->contains(prior(MachineFunction::iterator(BB))))
continue;
// Check to see if this block is already contiguous with the main
// portion of the loop.
if (!ContiguousBlocks.insert(BB))
continue;
// Move the block.
DEBUG(dbgs() << "CGP: Moving blocks starting at BB#" << BB->getNumber()
<< " to be contiguous with loop.\n");
Changed = true;
// Process this block and all loop blocks contiguous with it, to keep
// them in their relative order.
MachineFunction::iterator Begin = BB;
MachineFunction::iterator End = llvm::next(MachineFunction::iterator(BB));
for (; End != MF.end(); ++End) {
if (!L->contains(End)) break;
if (!HasAnalyzableTerminator(End)) break;
ContiguousBlocks.insert(End);
++NumIntraMoved;
}
// If we're inserting at the bottom of the loop, and the code we're
// moving originally had fall-through successors, bring the sucessors
// up with the loop blocks to preserve the fall-through edges.
if (!InsertAtTop)
for (; End != MF.end(); ++End) {
if (L->contains(End)) break;
if (!HasAnalyzableTerminator(End)) break;
if (!HasFallthrough(prior(End))) break;
}
// Move the blocks. This may invalidate TopMBB and/or BotMBB, but
// we don't need them anymore at this point.
Splice(MF, InsertPt, Begin, End);
}
return Changed;
}
/// OptimizeIntraLoopEdgesInLoopNest - Reposition loop blocks to minimize
/// intra-loop branching and to form contiguous loops.
///
/// This code takes the approach of making minor changes to the existing
/// layout to fix specific loop-oriented problems. Also, it depends on
/// AnalyzeBranch, which can't understand complex control instructions.
///
bool CodePlacementOpt::OptimizeIntraLoopEdgesInLoopNest(MachineFunction &MF,
MachineLoop *L) {
bool Changed = false;
// Do optimization for nested loops.
for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
Changed |= OptimizeIntraLoopEdgesInLoopNest(MF, *I);
// Do optimization for this loop.
Changed |= EliminateUnconditionalJumpsToTop(MF, L);
Changed |= MoveDiscontiguousLoopBlocks(MF, L);
return Changed;
}
/// OptimizeIntraLoopEdges - Reposition loop blocks to minimize
/// intra-loop branching and to form contiguous loops.
///
bool CodePlacementOpt::OptimizeIntraLoopEdges(MachineFunction &MF) {
bool Changed = false;
if (!TLI->shouldOptimizeCodePlacement())
return Changed;
// Do optimization for each loop in the function.
for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end();
I != E; ++I)
if (!(*I)->getParentLoop())
Changed |= OptimizeIntraLoopEdgesInLoopNest(MF, *I);
return Changed;
}
/// AlignLoops - Align loop headers to target preferred alignments.
///
bool CodePlacementOpt::AlignLoops(MachineFunction &MF) {
const Function *F = MF.getFunction();
if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
Attribute::OptimizeForSize))
return false;
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return false; // Don't care about loop alignment.
bool Changed = false;
for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end();
I != E; ++I)
Changed |= AlignLoop(MF, *I, Align);
return Changed;
}
/// AlignLoop - Align loop headers to target preferred alignments.
///
bool CodePlacementOpt::AlignLoop(MachineFunction &MF, MachineLoop *L,
unsigned Align) {
bool Changed = false;
// Do alignment for nested loops.
for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
Changed |= AlignLoop(MF, *I, Align);
L->getTopBlock()->setAlignment(Align);
Changed = true;
++NumLoopsAligned;
return Changed;
}
bool CodePlacementOpt::runOnMachineFunction(MachineFunction &MF) {
MLI = &getAnalysis<MachineLoopInfo>();
if (MLI->empty())
return false; // No loops.
TLI = MF.getTarget().getTargetLowering();
TII = MF.getTarget().getInstrInfo();
bool Changed = OptimizeIntraLoopEdges(MF);
Changed |= AlignLoops(MF);
return Changed;
}