blockfreq: Only one mass distribution per node

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

1 files changed, 11 insertions, 61 deletions

diff --git a/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp b/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
index cb92e5bbb11..744bbe2fe95 100644
--- a/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
+++ b/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
@@ -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"));
   }
 }