[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
diff --git a/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp b/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp
index 45f8907..8eaa6df 100644
--- a/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp
+++ b/llvm/lib/Target/PowerPC/PPCReduceCRLogicals.cpp
@@ -166,9 +166,33 @@
: *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 @@
// 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(),