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[MBP] Renaming a confusing variable and add clarifying comments 

Was discussed as part of http://reviews.llvm.org/D17830

llvm-svn: 262571
This commit is contained in:
Philip Reames 2016-03-03 00:58:43 +00:00
parent 34ede34acd
commit ae27b2380f
1 changed files with 24 additions and 19 deletions
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43
llvm/lib/CodeGen/MachineBlockPlacement.cpp
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@ -151,7 +151,7 @@ public:
/// function. It also registers itself as the chain that block participates
/// in with the BlockToChain mapping.
BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
: Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
: Blocks(1, BB), BlockToChain(BlockToChain), UnscheduledPredecessors(0) {
assert(BB && "Cannot create a chain with a null basic block");
BlockToChain[BB] = this;
}
@ -203,11 +203,16 @@ public:
}
#endif // NDEBUG
/// \brief Count of predecessors within the loop currently being processed.
/// \brief Count of predecessors of any block within the chain which have not
/// yet been scheduled. In general, we will delay scheduling this chain
/// until those predecessors are scheduled (or we find a sufficiently good
/// reason to override this heuristic.) Note that when forming loop chains,
/// blocks outside the loop are ignored and treated as if they were already
/// scheduled.
///
/// This count is updated at each loop we process to represent the number of
/// in-loop predecessors of this chain.
unsigned LoopPredecessors;
/// Note: This field is reinitialized multiple times - once for each loop,
/// and then once for the function as a whole.
unsigned UnscheduledPredecessors;
};
}
@ -354,7 +359,7 @@ void MachineBlockPlacement::markChainSuccessors(
// This is a cross-chain edge that is within the loop, so decrement the
// loop predecessor count of the destination chain.
if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
if (SuccChain.UnscheduledPredecessors > 0 && --SuccChain.UnscheduledPredecessors == 0)
BlockWorkList.push_back(*SuccChain.begin());
}
}
@ -452,7 +457,7 @@ MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.
BlockChain &SuccChain = *BlockToChain[Succ];
if (SuccChain.LoopPredecessors != 0) {
if (SuccChain.UnscheduledPredecessors != 0) {
if (SuccProb < HotProb) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob) (CFG conflict)\n");
@ -486,7 +491,7 @@ MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob)"
<< (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
<< (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
if (BestSucc && BestProb >= SuccProb)
continue;
@ -526,7 +531,7 @@ MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
if (&SuccChain == &Chain)
continue;
assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
assert(SuccChain.UnscheduledPredecessors == 0 && "Found CFG-violating block");
BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
@ -604,9 +609,9 @@ void MachineBlockPlacement::buildChain(
// Place this block, updating the datastructures to reflect its placement.
BlockChain &SuccChain = *BlockToChain[BestSucc];
// Zero out LoopPredecessors for the successor we're about to merge in case
// Zero out UnscheduledPredecessors for the successor we're about to merge in case
// we selected a successor that didn't fit naturally into the CFG.
SuccChain.LoopPredecessors = 0;
SuccChain.UnscheduledPredecessors = 0;
DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to "
<< getBlockName(BestSucc) << "\n");
markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
@ -1059,7 +1064,7 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
// walk the blocks, and use a set to prevent visiting a particular chain
// twice.
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
assert(LoopChain.LoopPredecessors == 0);
assert(LoopChain.UnscheduledPredecessors == 0);
UpdatedPreds.insert(&LoopChain);
for (MachineBasicBlock *LoopBB : LoopBlockSet) {
@ -1067,17 +1072,17 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
if (!UpdatedPreds.insert(&Chain).second)
continue;
assert(Chain.LoopPredecessors == 0);
assert(Chain.UnscheduledPredecessors == 0);
for (MachineBasicBlock *ChainBB : Chain) {
assert(BlockToChain[ChainBB] == &Chain);
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
if (BlockToChain[Pred] == &Chain || !LoopBlockSet.count(Pred))
continue;
++Chain.LoopPredecessors;
++Chain.UnscheduledPredecessors;
}
}
if (Chain.LoopPredecessors == 0)
if (Chain.UnscheduledPredecessors == 0)
BlockWorkList.push_back(*Chain.begin());
}
@ -1091,7 +1096,7 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
DEBUG({
// Crash at the end so we get all of the debugging output first.
bool BadLoop = false;
if (LoopChain.LoopPredecessors) {
if (LoopChain.UnscheduledPredecessors) {
BadLoop = true;
dbgs() << "Loop chain contains a block without its preds placed!\n"
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
@ -1185,17 +1190,17 @@ void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
if (!UpdatedPreds.insert(&Chain).second)
continue;
assert(Chain.LoopPredecessors == 0);
assert(Chain.UnscheduledPredecessors == 0);
for (MachineBasicBlock *ChainBB : Chain) {
assert(BlockToChain[ChainBB] == &Chain);
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
if (BlockToChain[Pred] == &Chain)
continue;
++Chain.LoopPredecessors;
++Chain.UnscheduledPredecessors;
}
}
if (Chain.LoopPredecessors == 0)
if (Chain.UnscheduledPredecessors == 0)
BlockWorkList.push_back(*Chain.begin());
}