[llvm-commits] [llvm] r145195 - /llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
chandlerc at gmail.com
Sun Nov 27 12:18:01 PST 2011
Date: Sun Nov 27 14:18:00 2011
New Revision: 145195
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/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Sun Nov 27 14:18:00 2011
@@ -571,6 +571,11 @@
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 @@
BestExitEdgeFreq = ExitEdgeFreq;
ExitingBB = *I;
+ if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI))
+ if (ExitLoop->contains(&L))
// Restore the old exiting state, no viable looping successor was found.
@@ -659,6 +668,13 @@
if (!ExitingBB || L.getNumBlocks() == 1)
+ // 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() &&
+ 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");
More information about the llvm-commits