[PPC] Correctly adjust branch probability in PPCReduceCRLogicals

In PPCReduceCRLogicals after splitting the original MBB into 2, the 2 impacted branches still use original branch probability. This is unreasonable. Suppose we have following code, and the probability of each successor is 50%.

    condc = conda || condb
    br condc, label %target, label %fallthrough

It can be transformed to following,

    br conda, label %target, label %newbb
  newbb:
    br condb, label %target, label %fallthrough

Since each branch has a probability of 50% to each successor, the total probability to %fallthrough is 25% now, and the total probability to %target is 75%. This actually changed the original profiling data. A more reasonable probability can be set to 70% to the false side for each branch instruction, so the total probability to %fallthrough is close to 50%.

This patch assumes the branch target with two incoming edges have same edge frequency and computes new probability fore each target, and keep the total probability to original targets unchanged.

Differential Revision: https://reviews.llvm.org/D62430

llvm-svn: 362237

1 files changed, 35 insertions, 6 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp b/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp
index 45f8907a08e..8eaa6dfe2bf 100644
--- a/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp
+++ b/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp
@@ -166,9 +166,33 @@ static bool splitMBB(BlockSplitInfo &BSI) {
                                            : *ThisMBB->succ_begin();
   MachineBasicBlock *NewBRTarget =
       BSI.BranchToFallThrough ? OrigFallThrough : OrigTarget;
-  BranchProbability ProbToNewTarget =
-      !BSI.MBPI ? BranchProbability::getUnknown()
-                : BSI.MBPI->getEdgeProbability(ThisMBB, NewBRTarget);
+
+  // It's impossible to know the precise branch probability after the split.
+  // But it still needs to be reasonable, the whole probability to original
+  // targets should not be changed.
+  // After split NewBRTarget will get two incoming edges. Assume P0 is the
+  // original branch probability to NewBRTarget, P1 and P2 are new branch
+  // probabilies to NewBRTarget after split. If the two edge frequencies are
+  // same, then
+  //      F * P1 = F * P0 / 2            ==>  P1 = P0 / 2
+  //      F * (1 - P1) * P2 = F * P1     ==>  P2 = P1 / (1 - P1)
+  BranchProbability ProbToNewTarget, ProbFallThrough;     // Prob for new Br.
+  BranchProbability ProbOrigTarget, ProbOrigFallThrough;  // Prob for orig Br.
+  ProbToNewTarget = ProbFallThrough = BranchProbability::getUnknown();
+  ProbOrigTarget = ProbOrigFallThrough = BranchProbability::getUnknown();
+  if (BSI.MBPI) {
+    if (BSI.BranchToFallThrough) {
+      ProbToNewTarget = BSI.MBPI->getEdgeProbability(ThisMBB, OrigFallThrough) / 2;
+      ProbFallThrough = ProbToNewTarget.getCompl();
+      ProbOrigFallThrough = ProbToNewTarget / ProbToNewTarget.getCompl();
+      ProbOrigTarget = ProbOrigFallThrough.getCompl();
+    } else {
+      ProbToNewTarget = BSI.MBPI->getEdgeProbability(ThisMBB, OrigTarget) / 2;
+      ProbFallThrough = ProbToNewTarget.getCompl();
+      ProbOrigTarget = ProbToNewTarget / ProbToNewTarget.getCompl();
+      ProbOrigFallThrough = ProbOrigTarget.getCompl();
+    }
+  }
 
   // Create a new basic block.
   MachineBasicBlock::iterator InsertPoint = BSI.SplitBefore;
@@ -180,11 +204,16 @@ static bool splitMBB(BlockSplitInfo &BSI) {
   // Move everything after SplitBefore into the new block.
   NewMBB->splice(NewMBB->end(), ThisMBB, InsertPoint, ThisMBB->end());
   NewMBB->transferSuccessors(ThisMBB);
+  if (!ProbOrigTarget.isUnknown()) {
+    auto MBBI = std::find(NewMBB->succ_begin(), NewMBB->succ_end(), OrigTarget);
+    NewMBB->setSuccProbability(MBBI, ProbOrigTarget);
+    MBBI = std::find(NewMBB->succ_begin(), NewMBB->succ_end(), OrigFallThrough);
+    NewMBB->setSuccProbability(MBBI, ProbOrigFallThrough);
+  }
 
-  // Add the two successors to ThisMBB. The probabilities come from the
-  // existing blocks if available.
+  // Add the two successors to ThisMBB.
   ThisMBB->addSuccessor(NewBRTarget, ProbToNewTarget);
-  ThisMBB->addSuccessor(NewMBB, ProbToNewTarget.getCompl());
+  ThisMBB->addSuccessor(NewMBB, ProbFallThrough);
 
   // Add the branches to ThisMBB.
   BuildMI(*ThisMBB, ThisMBB->end(), BSI.SplitBefore->getDebugLoc(),