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Add a somewhat hacky heuristic to do something different from whole-loop

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rotation. When there is a loop backedge which is an unconditional
branch, we will end up with a branch somewhere no matter what. Try
placing this backedge in a fallthrough position above the loop header as
that will definitely remove at least one branch from the loop iteration,
where whole loop rotation may not.

I haven't seen any benchmarks where this is important but loop-blocks.ll
tests for it, and so this will be covered when I flip the default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154812 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2012-04-16 13:33:36 +00:00
parent 656dc62606
commit e773e8c3e5
2 changed files with 83 additions and 8 deletions
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81
lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -211,6 +211,8 @@ class MachineBlockPlacement : public MachineFunctionPass {
void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter = 0);
MachineBasicBlock *findBestLoopTop(MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
MachineBasicBlock *findBestLoopExit(MachineFunction &F,
MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
@ -541,6 +543,67 @@ void MachineBlockPlacement::buildChain(
/// \brief Find the best loop top block for layout.
///
/// Look for a block which is strictly better than the loop header for laying
/// out at the top of the loop. This looks for one and only one pattern:
/// a latch block with no conditional exit. This block will cause a conditional
/// jump around it or will be the bottom of the loop if we lay it out in place,
/// but if it it doesn't end up at the bottom of the loop for any reason,
/// rotation alone won't fix it. Because such a block will always result in an
/// unconditional jump (for the backedge) rotating it in front of the loop
/// header is always profitable.
MachineBasicBlock *
MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
// Check that the header hasn't been fused with a preheader block due to
// crazy branches. If it has, we need to start with the header at the top to
// prevent pulling the preheader into the loop body.
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
if (!LoopBlockSet.count(*HeaderChain.begin()))
return L.getHeader();
DEBUG(dbgs() << "Finding best loop top for: "
<< getBlockName(L.getHeader()) << "\n");
BlockFrequency BestPredFreq;
MachineBasicBlock *BestPred = 0;
for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(),
PE = L.getHeader()->pred_end();
PI != PE; ++PI) {
MachineBasicBlock *Pred = *PI;
if (!LoopBlockSet.count(Pred))
continue;
DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
<< Pred->succ_size() << " successors, "
<< MBFI->getBlockFreq(Pred) << " freq\n");
if (Pred->succ_size() > 1)
continue;
BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
if (!BestPred || PredFreq > BestPredFreq ||
(!(PredFreq < BestPredFreq) &&
Pred->isLayoutSuccessor(L.getHeader()))) {
BestPred = Pred;
BestPredFreq = PredFreq;
}
}
// If no direct predecessor is fine, just use the loop header.
if (!BestPred)
return L.getHeader();
// Walk backwards through any straight line of predecessors.
while (BestPred->pred_size() == 1 &&
(*BestPred->pred_begin())->succ_size() == 1 &&
*BestPred->pred_begin() != L.getHeader())
BestPred = *BestPred->pred_begin();
DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
return BestPred;
}
/// \brief Find the best loop exiting block for layout.
///
/// This routine implements the logic to analyze the loop looking for the best
/// block to layout at the top of the loop. Typically this is done to maximize
/// fallthrough opportunities.
@ -725,8 +788,20 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
MachineBasicBlock *ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
BlockChain &LoopChain = *BlockToChain[L.getHeader()];
// First check to see if there is an obviously preferable top block for the
// loop. This will default to the header, but may end up as one of the
// predecessors to the header if there is one which will result in strictly
// fewer branches in the loop body.
MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
// If we selected just the header for the loop top, look for a potentially
// profitable exit block in the event that rotating the loop can eliminate
// branches by placing an exit edge at the bottom.
MachineBasicBlock *ExitingBB = 0;
if (LoopTop == L.getHeader())
ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
BlockChain &LoopChain = *BlockToChain[LoopTop];
// FIXME: This is a really lame way of walking the chains in the loop: we
// walk the blocks, and use a set to prevent visiting a particular chain
@ -758,7 +833,7 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
BlockWorkList.push_back(*Chain.begin());
}
buildChain(L.getHeader(), LoopChain, BlockWorkList, &LoopBlockSet);
buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
DEBUG({

10
test/CodeGen/X86/block-placement.ll
View File

@ -974,9 +974,14 @@ define void @benchmark_heapsort(i32 %n, double* nocapture %ra) {
; CHECK: %entry
; First rotated loop top.
; CHECK: .align
; CHECK: %while.end
; CHECK: %for.cond
; CHECK: %if.then
; CHECK: %if.else
; CHECK: %if.end10
; Second rotated loop top
; CHECK: .align
; CHECK: %if.then24
; CHECK: %while.cond.outer
; Third rotated loop top
; CHECK: .align
@ -985,11 +990,6 @@ define void @benchmark_heapsort(i32 %n, double* nocapture %ra) {
; CHECK: %land.lhs.true
; CHECK: %if.then19
; CHECK: %if.then19
; CHECK: %if.then24
; CHECK: %while.end
; CHECK: %for.cond
; CHECK: %if.then
; CHECK: %if.else
; CHECK: %if.then8
; CHECK: ret