Codegen: MachineBlockPlacement Improve probability layout.

The following pattern was being layed out poorly:

              A
             / \
            B   C
           / \ / \
          D   E   ? (Doesn't matter)

Where A->B is far more likely than A->C, and prob(B->D) = prob(B->E)

The current algorithm gives:
A,B,C,E (D goes on worklist)

It does this even if C has a frequency count of 0. This patch
adjusts the layout calculation so that if freq(B->E) >> freq(C->E)
then we go ahead and layout E rather than C. Fallthrough half the time
is better than fallthrough never, or fallthrough very rarely. The
resulting layout is:

A,B,E, (C and D are in a worklist)

llvm-svn: 277187

1 files changed, 45 insertions, 15 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 409c3cc26ab..27da8498897 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -631,18 +631,46 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
   // 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).
-
+  // --------------------------------------------------------------------------
+  // Case 3: forked diamond
+  //       S
+  //      / \
+  //     /   \
+  //   BB    Pred
+  //   | \   / |
+  //   |  \ /  |
+  //   |   X   |
+  //   |  / \  |
+  //   | /   \ |
+  //   S1     S2
+  //
+  // The current block is BB and edge BB->S1 is now being evaluated.
+  // As above S->BB was already selected because
+  // prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2).
+  //
+  // topo-order:
+  //
+  //     S-------|                     ---S
+  //     |       |                     |  |
+  //  ---BB      |                     |  BB
+  //  |          |                     |  |
+  //  |  Pred----|                     |  S1----
+  //  |  |                             |       |
+  //  --(S1 or S2)                     ---Pred--
+  //
+  // topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2)
+  //    + min(freq(Pred->S1), freq(Pred->S2))
+  // Non-topo-order cost:
+  // In the worst case, S2 will not get laid out after Pred.
+  // non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2).
+  // To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2))
+  // is 0. Then the non topo layout is better when
+  // freq(S->Pred) < freq(BB->S1).
+  // This is exactly what is checked below.
+  // Note there are other shapes that apply (Pred may not be a single block,
+  // but they all fit this general pattern.)
   BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
 
-  // Forward checking. For case 2, SuccProb will be 1.
-  if (SuccProb < HotProb) {
-    DEBUG(dbgs() << "    Not a candidate: " << getBlockName(Succ) << " "
-                 << "Respecting topological ordering because "
-                 << "probability is less than prob treshold: "
-                 << SuccProb << "\n");
-    return true;
-  }
-
   // Make sure that a hot successor doesn't have a globally more
   // important predecessor.
   BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb;
@@ -653,11 +681,11 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
         (BlockFilter && !BlockFilter->count(Pred)) ||
         BlockToChain[Pred] == &Chain)
       continue;
-    // Do backward checking. For case 1, it is actually redundant check. For
-    // case 2 above, we need a backward checking to filter out edges that are
-    // not 'strongly' biased. With profile data available, the check is mostly
-    // redundant too (when threshold prob is set at 50%) unless S has more than
-    // two successors.
+    // Do backward checking.
+    // For all cases above, we need a backward checking to filter out edges that
+    // are not 'strongly' biased. With profile data available, the check is
+    // mostly redundant for case 2 (when threshold prob is set at 50%) unless S
+    // has more than two successors.
     // BB  Pred
     //  \ /
     //  Succ
@@ -666,6 +694,8 @@ bool MachineBlockPlacement::hasBetterLayoutPredecessor(
     //      i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
     //      HotProb
     //      i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb
+    // Case 1 is covered too, because the first equation reduces to:
+    // prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle)
     BlockFrequency PredEdgeFreq =
         MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
     if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {