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[MBP] Clean up of the comments, and a first attempt to better describe a part 

of the algorithm.

Differential Revision: https://reviews.llvm.org/D22364

llvm-svn: 275595
This commit is contained in:
Sjoerd Meijer 2016-07-15 18:41:56 +00:00
parent 27fefb2fcf
commit fd0ad4e193
1 changed files with 49 additions and 28 deletions
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77
llvm/lib/CodeGen/MachineBlockPlacement.cpp
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@ -109,7 +109,7 @@ static cl::opt<bool>
static cl::opt<unsigned> MisfetchCost(
"misfetch-cost",
cl::desc("Cost that models the probablistic risk of an instruction "
cl::desc("Cost that models the probabilistic risk of an instruction "
"misfetch due to a jump comparing to falling through, whose cost "
"is zero."),
cl::init(1), cl::Hidden);
@ -310,7 +310,7 @@ class MachineBlockPlacement : public MachineFunctionPass {
MachineFunction::iterator &PrevUnplacedBlockIt,
const BlockFilterSet *BlockFilter);
/// \brief Add a basic block to the work list if it is apropriate.
/// \brief Add a basic block to the work list if it is appropriate.
///
/// If the optional parameter BlockFilter is provided, only MBB
/// present in the set will be added to the worklist. If nullptr
@ -438,7 +438,7 @@ BranchProbability MachineBlockPlacement::collectViableSuccessors(
// Assume A->C is very hot (>90%), and C->D has a 50% probability, then after
// A->C is chosen as a fall-through, D won't be selected as a successor of C
// due to CFG constraint (the probability of C->D is not greater than
// HotProb to break top-oorder). If we exclude E that is not in BlockFilter
// HotProb to break top-order). If we exclude E that is not in BlockFilter
// when calculating the probability of C->D, D will be selected and we
// will get A C D B as the layout of this loop.
auto AdjustedSumProb = BranchProbability::getOne();
@ -466,7 +466,6 @@ BranchProbability MachineBlockPlacement::collectViableSuccessors(
/// The helper function returns the branch probability that is adjusted
/// or normalized over the new total \p AdjustedSumProb.
static BranchProbability
getAdjustedProbability(BranchProbability OrigProb,
BranchProbability AdjustedSumProb) {
@ -484,7 +483,7 @@ getAdjustedProbability(BranchProbability OrigProb,
/// When the option OutlineOptionalBranches is on, this method
/// checks if the fallthrough candidate block \p Succ (of block
/// \p BB) also has other unscheduled predecessor blocks which
/// are also successors of \p BB (forming triagular shape CFG).
/// are also successors of \p BB (forming triangular shape CFG).
/// If none of such predecessors are small, it returns true.
/// The caller can choose to select \p Succ as the layout successors
/// so that \p Succ's predecessors (optional branches) can be
@ -550,13 +549,13 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
BranchProbability SuccProb, BranchProbability RealSuccProb,
BlockChain &Chain, const BlockFilterSet *BlockFilter) {
// This is no global conflict, just return false.
// There isn't a better layout when there are no unscheduled predecessors.
if (SuccChain.UnscheduledPredecessors == 0)
return false;
// There are two basic scenarios here:
// -------------------------------------
// Case 1: triagular shape CFG:
// Case 1: triangular shape CFG (if-then):
// BB
// | \
// | \
@ -565,11 +564,13 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
// Succ
// In this case, we are evaluating whether to select edge -> Succ, e.g.
// set Succ as the layout successor of BB. Picking Succ as BB's
// successor breaks the CFG constraints. With this layout, Pred BB
// successor breaks the CFG constraints (FIXME: define these constraints).
// With this layout, Pred BB
// is forced to be outlined, so the overall cost will be cost of the
// branch taken from BB to Pred, plus the cost of back taken branch
// from Pred to Succ, as well as the additional cost asssociated
// from Pred to Succ, as well as the additional cost associated
// with the needed unconditional jump instruction from Pred To Succ.
// The cost of the topological order layout is the taken branch cost
// from BB to Succ, so to make BB->Succ a viable candidate, the following
// must hold:
@ -579,12 +580,12 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
// freq(BB->Succ) > 2 * freq(BB->Pred), i.e.,
// prob(BB->Succ) > 2 * prob(BB->Pred)
//
// When real profile data is available, we can precisely compute the the
// probabililty threshold that is needed for edge BB->Succ to be considered.
// With out profile data, the heuristic requires the branch bias to be
// When real profile data is available, we can precisely compute the
// probability threshold that is needed for edge BB->Succ to be considered.
// Without profile data, the heuristic requires the branch bias to be
// a lot larger to make sure the signal is very strong (e.g. 80% default).
// -----------------------------------------------------------------
// Case 2: diamond like CFG:
// Case 2: diamond like CFG (if-then-else):
// S
// / \
// | \
@ -592,18 +593,38 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
// \ /
// Succ
// ..
// In this case, edge S->BB has already been selected, and we are evaluating
// candidate edge BB->Succ. Edge S->BB is selected because prob(S->BB)
// is no less than prob(S->Pred). When real profile data is *available*, if
// the condition is true, it will be always better to continue the trace with
// edge BB->Succ instead of laying out with topological order (i.e. laying
// Pred first). The cost of S->BB->Succ is 2 * freq (S->Pred), while with
// the topo order, the cost is freq(S-> Pred) + Pred(S->BB) which is larger.
//
// The current block is BB and edge BB->Succ is now being evaluated.
// Note that edge S->BB was previously already selected because
// prob(S->BB) > prob(S->Pred).
// At this point, 2 blocks can be placed after BB: Pred or Succ. If we
// choose Pred, we will have a topological ordering as shown on the left
// in the picture below. If we choose Succ, we have the solution as shown
// on the right:
//
// topo-order:
//
// S----- ---S
// | | | |
// ---BB | | BB
// | | | |
// | pred-- | Succ--
// | | | |
// ---succ ---pred--
//
// cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred)
// = freq(S->Pred) + freq(S->BB)
//
// If we have profile data (i.e, branch probabilities can be trusted), the
// cost (number of taken branches) with layout S->BB->Succ->Pred is 2 *
// freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB).
// We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which
// means the cost of topological order is greater.
// When profile data is not available, however, we need to be more
// conservative. If the branch prediction is wrong, breaking the topo-order
// will actually yield a layout with large cost. For this reason, we need
// strong biaaed branch at block S with Prob(S->BB) in order to select
// BB->Succ. This is equialant to looking the CFG backward with backward
// strong biased branch at block S with Prob(S->BB) in order to select
// BB->Succ. This is equivalent to looking the CFG backward with backward
// edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without
// profile data).
@ -634,7 +655,7 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
// BB Pred
// \ /
// Succ
// We select edgee BB->Succ if
// We select edge BB->Succ if
// freq(BB->Succ) > freq(Succ) * HotProb
// i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
// HotProb
@ -716,7 +737,7 @@ MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
/// profitable only really makes sense in the context of a loop. This returns
/// the most frequently visited block in the worklist, which in the case of
/// a loop, is the one most desirable to be physically close to the rest of the
/// loop body in order to improve icache behavior.
/// loop body in order to improve i-cache behavior.
///
/// \returns The best block found, or null if none are viable.
MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
@ -971,7 +992,7 @@ MachineBlockPlacement::findBestLoopExit(MachineLoop &L,
unsigned BestExitLoopDepth = 0;
MachineBasicBlock *ExitingBB = nullptr;
// If there are exits to outer loops, loop rotation can severely limit
// fallthrough opportunites unless it selects such an exit. Keep a set of
// fallthrough opportunities 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;
@ -1379,7 +1400,7 @@ void MachineBlockPlacement::buildLoopChains(MachineLoop &L) {
EHPadWorkList.clear();
}
/// When OutlineOpitonalBranches is on, this method colects BBs that
/// When OutlineOpitonalBranches is on, this method collects BBs that
/// dominates all terminator blocks of the function \p F.
void MachineBlockPlacement::collectMustExecuteBBs() {
if (OutlineOptionalBranches) {
@ -1502,7 +1523,7 @@ void MachineBlockPlacement::buildCFGChains() {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
// The "PrevBB" is not yet updated to reflect current code layout, so,
// o. it may fall-through to a block without explict "goto" instruction
// o. it may fall-through to a block without explicit "goto" instruction
// before layout, and no longer fall-through it after layout; or
// o. just opposite.
//
@ -1661,7 +1682,7 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
// Changing the layout can create new tail merging opportunities.
TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
// TailMerge can create jump into if branches that make CFG irreducible for
// HW that requires structurized CFG.
// HW that requires structured CFG.
bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
PassConfig->getEnableTailMerge() &&
BranchFoldPlacement;