// structure and branch probability estimates.
//
// The pass strives to preserve the structure of the CFG (that is, retain
-// a topological ordering of basic blocks) in the absense of a *strong* signal
+// a topological ordering of basic blocks) in the absence of a *strong* signal
// to the contrary from probabilities. However, within the CFG structure, it
// attempts to choose an ordering which favors placing more likely sequences of
// blocks adjacent to each other.
///
/// This is the datastructure representing a chain of consecutive blocks that
/// are profitable to layout together in order to maximize fallthrough
-/// probabilities. We also can use a block chain to represent a sequence of
-/// basic blocks which have some external (correctness) requirement for
-/// sequential layout.
+/// probabilities and code locality. We also can use a block chain to represent
+/// a sequence of basic blocks which have some external (correctness)
+/// requirement for sequential layout.
///
-/// Eventually, the block chains will form a directed graph over the function.
-/// We provide an SCC-supporting-iterator in order to quicky build and walk the
-/// SCCs of block chains within a function.
-///
-/// The block chains also have support for calculating and caching probability
-/// information related to the chain itself versus other chains. This is used
-/// for ranking during the final layout of block chains.
+/// Chains can be built around a single basic block and can be merged to grow
+/// them. They participate in a block-to-chain mapping, which is updated
+/// automatically as chains are merged together.
class BlockChain {
/// \brief The sequence of blocks belonging to this chain.
///
/// \brief Allocator and owner of BlockChain structures.
///
- /// We build BlockChains lazily by merging together high probability BB
- /// sequences acording to the "Algo2" in the paper mentioned at the top of
- /// the file. To reduce malloc traffic, we allocate them using this slab-like
- /// allocator, and destroy them after the pass completes.
+ /// We build BlockChains lazily while processing the loop structure of
+ /// a function. To reduce malloc traffic, we allocate them using this
+ /// slab-like allocator, and destroy them after the pass completes. An
+ /// important guarantee is that this allocator produces stable pointers to
+ /// the chains.
SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
/// \brief Function wide BasicBlock to BlockChain mapping.
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);
// the MBPI analysis, we manually compute probabilities using the edge
// weights. This is suboptimal as it means that the somewhat subtle
// definition of edge weight semantics is encoded here as well. We should
- // improve the MBPI interface to effeciently support query patterns such as
+ // improve the MBPI interface to efficiently support query patterns such as
// this.
uint32_t BestWeight = 0;
uint32_t WeightScale = 0;
assert(BB);
assert(BlockToChain[BB] == &Chain);
assert(*llvm::prior(Chain.end()) == BB);
- MachineBasicBlock *BestSucc = 0;
// Look for the best viable successor if there is one to place immediately
// after this block.
- BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
+ MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
// If an immediate successor isn't available, look for the best viable
// block among those we've identified as not violating the loop's CFG at
/// \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.
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
BlockWorkList.push_back(*Chain.begin());
}
- buildChain(L.getHeader(), LoopChain, BlockWorkList, &LoopBlockSet);
+ buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
DEBUG({
// boiler plate.
Cond.clear();
MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
- if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
+ if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
+ // If PrevBB has a two-way branch, try to re-order the branches
+ // such that we branch to the successor with higher weight first.
+ if (TBB && !Cond.empty() && FBB &&
+ MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
+ !TII->ReverseBranchCondition(Cond)) {
+ DEBUG(dbgs() << "Reverse order of the two branches: "
+ << getBlockName(PrevBB) << "\n");
+ DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
+ << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
+ DebugLoc dl; // FIXME: this is nowhere
+ TII->RemoveBranch(*PrevBB);
+ TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
+ }
PrevBB->updateTerminator();
+ }
}
// Fixup the last block.
// Walk through the backedges of the function now that we have fully laid out
// the basic blocks and align the destination of each backedge. We don't rely
- // on the loop info here so that we can align backedges in unnatural CFGs and
- // backedges that were introduced purely because of the loop rotations done
- // during this layout pass.
- // FIXME: This isn't quite right, we shouldn't align backedges that result
- // from blocks being sunken below the exit block for the function.
- if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
+ // exclusively on the loop info here so that we can align backedges in
+ // unnatural CFGs and backedges that were introduced purely because of the
+ // loop rotations done during this layout pass.
+ if (F.getFunction()->getFnAttributes().
+ hasAttribute(Attributes::OptimizeForSize))
return;
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return; // Don't care about loop alignment.
+ if (FunctionChain.begin() == FunctionChain.end())
+ return; // Empty chain.
- SmallPtrSet<MachineBasicBlock *, 16> PreviousBlocks;
- for (BlockChain::iterator BI = FunctionChain.begin(),
+ const BranchProbability ColdProb(1, 5); // 20%
+ BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
+ BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
+ for (BlockChain::iterator BI = llvm::next(FunctionChain.begin()),
BE = FunctionChain.end();
BI != BE; ++BI) {
- PreviousBlocks.insert(*BI);
- // Set alignment on the destination of all the back edges in the new
- // ordering.
- for (MachineBasicBlock::succ_iterator SI = (*BI)->succ_begin(),
- SE = (*BI)->succ_end();
- SI != SE; ++SI)
- if (PreviousBlocks.count(*SI))
- (*SI)->setAlignment(Align);
+ // Don't align non-looping basic blocks. These are unlikely to execute
+ // enough times to matter in practice. Note that we'll still handle
+ // unnatural CFGs inside of a natural outer loop (the common case) and
+ // rotated loops.
+ MachineLoop *L = MLI->getLoopFor(*BI);
+ if (!L)
+ continue;
+
+ // If the block is cold relative to the function entry don't waste space
+ // aligning it.
+ BlockFrequency Freq = MBFI->getBlockFreq(*BI);
+ if (Freq < WeightedEntryFreq)
+ continue;
+
+ // If the block is cold relative to its loop header, don't align it
+ // regardless of what edges into the block exist.
+ MachineBasicBlock *LoopHeader = L->getHeader();
+ BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
+ if (Freq < (LoopHeaderFreq * ColdProb))
+ continue;
+
+ // Check for the existence of a non-layout predecessor which would benefit
+ // from aligning this block.
+ MachineBasicBlock *LayoutPred = *llvm::prior(BI);
+
+ // Force alignment if all the predecessors are jumps. We already checked
+ // that the block isn't cold above.
+ if (!LayoutPred->isSuccessor(*BI)) {
+ (*BI)->setAlignment(Align);
+ continue;
+ }
+
+ // Align this block if the layout predecessor's edge into this block is
+ // cold relative to the block. When this is true, othe predecessors make up
+ // all of the hot entries into the block and thus alignment is likely to be
+ // important.
+ BranchProbability LayoutProb = MBPI->getEdgeProbability(LayoutPred, *BI);
+ BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
+ if (LayoutEdgeFreq <= (Freq * ColdProb))
+ (*BI)->setAlignment(Align);
}
}
///
/// A separate pass to compute interesting statistics for evaluating block
/// placement. This is separate from the actual placement pass so that they can
-/// be computed in the absense of any placement transformations or when using
+/// be computed in the absence of any placement transformations or when using
/// alternative placement strategies.
class MachineBlockPlacementStats : public MachineFunctionPass {
/// \brief A handle to the branch probability pass.
projects / oota-llvm.git / blobdiff