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Prevent rotating the blocks of a loop (and thus getting a backedge to be 

fallthrough) in cases where we might fail to rotate an exit to an outer
loop onto the end of the loop chain.

Having *some* rotation, but not performing this rotation, is the primary
fix of thep performance regression with -enable-block-placement for
Olden/em3d (a whopping 30% regression). Still working on reducing the
test case that actually exercises this and the new rotation strategy out
of this code, but I want to check if this regresses other test cases
first as that may indicate it isn't the correct fix.

llvm-svn: 145195
This commit is contained in:
Chandler Carruth 2011-11-27 20:18:00 +00:00
parent a6416a7c73
commit 4f56720754
1 changed files with 16 additions and 0 deletions
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16
llvm/lib/CodeGen/MachineBlockPlacement.cpp
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@ -571,6 +571,11 @@ MachineBlockPlacement::findBestLoopTop(MachineFunction &F,
BlockFrequency BestExitEdgeFreq;
MachineBasicBlock *ExitingBB = 0;
MachineBasicBlock *LoopingBB = 0;
// If there are exits to outer loops, loop rotation can severely limit
// fallthrough opportunites unless it selects such an exit. Keep a set of
// blocks where rotating to exit with that block will reach an outer loop.
SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
DEBUG(dbgs() << "Finding best loop exit for: "
<< getBlockName(L.getHeader()) << "\n");
for (MachineLoop::block_iterator I = L.block_begin(),
@ -641,6 +646,10 @@ MachineBlockPlacement::findBestLoopTop(MachineFunction &F,
BestExitEdgeFreq = ExitEdgeFreq;
ExitingBB = *I;
}
if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI))
if (ExitLoop->contains(&L))
BlocksExitingToOuterLoop.insert(*I);
}
// Restore the old exiting state, no viable looping successor was found.
@ -659,6 +668,13 @@ MachineBlockPlacement::findBestLoopTop(MachineFunction &F,
if (!ExitingBB || L.getNumBlocks() == 1)
return L.getHeader();
// Also, if we have exit blocks which lead to outer loops but didn't select
// one of them as the exiting block we are rotating toward, disable loop
// rotation altogether.
if (!BlocksExitingToOuterLoop.empty() &&
!BlocksExitingToOuterLoop.count(ExitingBB))
return L.getHeader();
assert(LoopingBB && "All successors of a loop block are exit blocks!");
DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
DEBUG(dbgs() << " Best top block: " << getBlockName(LoopingBB) << "\n");