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blockfreq: Only one mass distribution per node

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Remove the concepts of "forward" and "general" mass distributions, which
was wrong.  The split might have made sense in an early version of the
algorithm, but it's definitely wrong now.

<rdar://problem/14292693>

llvm-svn: 207195
This commit is contained in:
Duncan P. N. Exon Smith 2014-04-25 04:38:43 +00:00
parent eebda28f4c
commit 6117699d7e
3 changed files with 48 additions and 94 deletions
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43
include/llvm/Analysis/BlockFrequencyInfoImpl.h
View File

@ -1024,17 +1024,14 @@ public:
/// This class collates the successor edge weights for later processing.
///
/// \a DidOverflow indicates whether \a Total did overflow while adding to
/// the distribution. It should never overflow twice. There's no flag for
/// whether \a ForwardTotal overflows, since when \a Total exceeds 32-bits
/// they both get re-computed during \a normalize().
/// the distribution. It should never overflow twice.
struct Distribution {
typedef SmallVector<Weight, 4> WeightList;
WeightList Weights; ///< Individual successor weights.
uint64_t Total; ///< Sum of all weights.
bool DidOverflow; ///< Whether \a Total did overflow.
uint32_t ForwardTotal; ///< Total excluding backedges.
Distribution() : Total(0), DidOverflow(false), ForwardTotal(0) {}
Distribution() : Total(0), DidOverflow(false) {}
void addLocal(const BlockNode &Node, uint64_t Amount) {
add(Node, Amount, Weight::Local);
}
@ -1079,9 +1076,8 @@ public:
/// \brief Add an edge to the distribution.
///
/// Adds an edge to Succ to Dist. If \c LoopHead.isValid(), then whether the
/// edge is forward/exit/backedge is in the context of LoopHead. Otherwise,
/// every edge should be a forward edge (since all the loops are packaged
/// up).
/// edge is local/exit/backedge is in the context of LoopHead. Otherwise,
/// every edge should be a local edge (since all the loops are packaged up).
void addToDist(Distribution &Dist, const LoopData *OuterLoop,
const BlockNode &Pred, const BlockNode &Succ, uint64_t Weight);
@ -1119,19 +1115,6 @@ public:
/// backedges and exits are stored in its entry in Loops.
///
/// Mass is distributed in parallel from two copies of the source mass.
///
/// The first mass (forward) represents the distribution of mass through the
/// local DAG. This distribution should lose mass at loop exits and ignore
/// backedges.
///
/// The second mass (general) represents the behavior of the loop in the
/// global context. In a given distribution from the head, how much mass
/// exits, and to where? How much mass returns to the loop head?
///
/// The forward mass should be split up between local successors and exits,
/// but only actually distributed to the local successors. The general mass
/// should be split up between all three types of successors, but distributed
/// only to exits and backedges.
void distributeMass(const BlockNode &Source, LoopData *OuterLoop,
Distribution &Dist);
@ -1270,23 +1253,17 @@ template <> inline std::string getBlockName(const BasicBlock *BB) {
/// in \a LoopData::Exits. Otherwise, fetch it from
/// BranchProbabilityInfo.
///
/// - Each successor is categorized as \a Weight::Local, a normal
/// forward edge within the current loop, \a Weight::Backedge, a
/// backedge to the loop header, or \a Weight::Exit, any successor
/// outside the loop. The weight, the successor, and its category
/// are stored in \a Distribution. There can be multiple edges to
/// each successor.
/// - Each successor is categorized as \a Weight::Local, a local edge
/// within the current loop, \a Weight::Backedge, a backedge to the
/// loop header, or \a Weight::Exit, any successor outside the loop.
/// The weight, the successor, and its category are stored in \a
/// Distribution. There can be multiple edges to each successor.
///
/// - Normalize the distribution: scale weights down so that their sum
/// is 32-bits, and coalesce multiple edges to the same node.
///
/// - Distribute the mass accordingly, dithering to minimize mass loss,
/// as described in \a distributeMass(). Mass is distributed in
/// parallel in two ways: forward, and general. Local successors
/// take their mass from the forward mass, while exit and backedge
/// successors take their mass from the general mass. Additionally,
/// exit edges use up (ignored) mass from the forward mass, and local
/// edges use up (ignored) mass from the general distribution.
/// as described in \a distributeMass().
///
/// Finally, calculate the loop scale from the accumulated backedge mass.
///

72
lib/Analysis/BlockFrequencyInfoImpl.cpp
View File

@ -389,31 +389,12 @@ typedef BlockFrequencyInfoImplBase::FrequencyData FrequencyData;
/// 2. Calculate the portion's mass as \a RemMass times P.
/// 3. Update \a RemWeight and \a RemMass at each portion by subtracting
/// the current portion's weight and mass.
///
/// Mass is distributed in two ways: full distribution and forward
/// distribution. The latter ignores backedges, and uses the parallel fields
/// \a RemForwardWeight and \a RemForwardMass.
struct DitheringDistributer {
uint32_t RemWeight;
uint32_t RemForwardWeight;
BlockMass RemMass;
BlockMass RemForwardMass;
DitheringDistributer(Distribution &Dist, const BlockMass &Mass);
BlockMass takeLocalMass(uint32_t Weight) {
(void)takeMass(Weight);
return takeForwardMass(Weight);
}
BlockMass takeExitMass(uint32_t Weight) {
(void)takeForwardMass(Weight);
return takeMass(Weight);
}
BlockMass takeBackedgeMass(uint32_t Weight) { return takeMass(Weight); }
private:
BlockMass takeForwardMass(uint32_t Weight);
BlockMass takeMass(uint32_t Weight);
};
}
@ -422,22 +403,9 @@ DitheringDistributer::DitheringDistributer(Distribution &Dist,
const BlockMass &Mass) {
Dist.normalize();
RemWeight = Dist.Total;
RemForwardWeight = Dist.ForwardTotal;
RemMass = Mass;
RemForwardMass = Dist.ForwardTotal ? Mass : BlockMass();
}
BlockMass DitheringDistributer::takeForwardMass(uint32_t Weight) {
// Compute the amount of mass to take.
assert(Weight && "invalid weight");
assert(Weight <= RemForwardWeight);
BlockMass Mass = RemForwardMass * BranchProbability(Weight, RemForwardWeight);
// Decrement totals (dither).
RemForwardWeight -= Weight;
RemForwardMass -= Mass;
return Mass;
}
BlockMass DitheringDistributer::takeMass(uint32_t Weight) {
assert(Weight && "invalid weight");
assert(Weight <= RemWeight);
@ -468,13 +436,6 @@ void Distribution::add(const BlockNode &Node, uint64_t Amount,
W.Amount = Amount;
W.Type = Type;
Weights.push_back(W);
if (Type == Weight::Backedge)
return;
// Update forward total. Don't worry about overflow here, since then Total
// will exceed 32-bits and they'll both be recomputed in normalize().
ForwardTotal += Amount;
}
static void combineWeight(Weight &W, const Weight &OtherW) {
@ -554,7 +515,6 @@ void Distribution::normalize() {
// Early exit when combined into a single successor.
if (Weights.size() == 1) {
Total = 1;
ForwardTotal = Weights.front().Type != Weight::Backedge;
Weights.front().Amount = 1;
return;
}
@ -574,9 +534,8 @@ void Distribution::normalize() {
return;
// Recompute the total through accumulation (rather than shifting it) so that
// it's accurate after shifting. ForwardTotal is dirty here anyway.
// it's accurate after shifting.
Total = 0;
ForwardTotal = 0;
// Sum the weights to each node and shift right if necessary.
for (Weight &W : Weights) {
@ -588,11 +547,6 @@ void Distribution::normalize() {
// Update the total.
Total += W.Amount;
if (W.Type == Weight::Backedge)
continue;
// Update the forward total.
ForwardTotal += W.Amount;
}
assert(Total <= UINT32_MAX);
}
@ -732,8 +686,7 @@ void BlockFrequencyInfoImplBase::distributeMass(const BlockNode &Source,
LoopData *OuterLoop,
Distribution &Dist) {
BlockMass Mass = getPackageMass(*this, Source);
DEBUG(dbgs() << " => mass: " << Mass
<< " ( general | forward )\n");
DEBUG(dbgs() << " => mass: " << Mass << "\n");
// Distribute mass to successors as laid out in Dist.
DitheringDistributer D(Dist, Mass);
@ -741,8 +694,7 @@ void BlockFrequencyInfoImplBase::distributeMass(const BlockNode &Source,
#ifndef NDEBUG
auto debugAssign = [&](const BlockNode &T, const BlockMass &M,
const char *Desc) {
dbgs() << " => assign " << M << " (" << D.RemMass << "|"
<< D.RemForwardMass << ")";
dbgs() << " => assign " << M << " (" << D.RemMass << ")";
if (Desc)
dbgs() << " [" << Desc << "]";
if (T.isValid())
@ -753,11 +705,11 @@ void BlockFrequencyInfoImplBase::distributeMass(const BlockNode &Source,
#endif
for (const Weight &W : Dist.Weights) {
// Check for a local edge (forward and non-exit).
// Check for a local edge (non-backedge and non-exit).
BlockMass Taken = D.takeMass(W.Amount);
if (W.Type == Weight::Local) {
BlockMass Local = D.takeLocalMass(W.Amount);
getPackageMass(*this, W.TargetNode) += Local;
DEBUG(debugAssign(W.TargetNode, Local, nullptr));
getPackageMass(*this, W.TargetNode) += Taken;
DEBUG(debugAssign(W.TargetNode, Taken, nullptr));
continue;
}
@ -766,17 +718,15 @@ void BlockFrequencyInfoImplBase::distributeMass(const BlockNode &Source,
// Check for a backedge.
if (W.Type == Weight::Backedge) {
BlockMass Back = D.takeBackedgeMass(W.Amount);
OuterLoop->BackedgeMass += Back;
DEBUG(debugAssign(BlockNode(), Back, "back"));
OuterLoop->BackedgeMass += Taken;
DEBUG(debugAssign(BlockNode(), Taken, "back"));
continue;
}
// This must be an exit.
assert(W.Type == Weight::Exit);
BlockMass Exit = D.takeExitMass(W.Amount);
OuterLoop->Exits.push_back(std::make_pair(W.TargetNode, Exit));
DEBUG(debugAssign(W.TargetNode, Exit, "exit"));
OuterLoop->Exits.push_back(std::make_pair(W.TargetNode, Taken));
DEBUG(debugAssign(W.TargetNode, Taken, "exit"));
}
}

27
test/Analysis/BlockFrequencyInfo/double_backedge.ll Normal file
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@ -0,0 +1,27 @@
; RUN: opt < %s -analyze -block-freq | FileCheck %s
define void @double_backedge(i1 %x) {
; CHECK-LABEL: Printing analysis {{.*}} for function 'double_backedge':
; CHECK-NEXT: block-frequency-info: double_backedge
entry:
; CHECK-NEXT: entry: float = 1.0, int = [[ENTRY:[0-9]+]]
br label %loop
loop:
; CHECK-NEXT: loop: float = 10.0,
br i1 %x, label %exit, label %loop.1, !prof !0
loop.1:
; CHECK-NEXT: loop.1: float = 9.0,
br i1 %x, label %loop, label %loop.2, !prof !1
loop.2:
; CHECK-NEXT: loop.2: float = 5.0,
br label %loop
exit:
; CHECK-NEXT: exit: float = 1.0, int = [[ENTRY]]
ret void
}
!0 = metadata !{metadata !"branch_weights", i32 1, i32 9}
!1 = metadata !{metadata !"branch_weights", i32 4, i32 5}