Codegen: Tail Merge: Be less aggressive with special cases.

This change makes it possible for tail-duplication and tail-merging to
be disjoint. By being less aggressive when merging during layout, there are no
overlapping cases between tail-duplication and tail-merging, provided the
thresholds are disjoint.

There is a remaining TODO to benchmark the succ_size() test for non-layout tail
merging.

llvm-svn: 278265

1 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index d2ffd214b28..1a783b61276 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -597,7 +597,8 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
                   MachineBasicBlock::iterator &I1,
                   MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
                   MachineBasicBlock *PredBB,
-                  DenseMap<const MachineBasicBlock *, int> &FuncletMembership) {
+                  DenseMap<const MachineBasicBlock *, int> &FuncletMembership,
+                  bool AfterPlacement) {
   // It is never profitable to tail-merge blocks from two different funclets.
   if (!FuncletMembership.empty()) {
     auto Funclet1 = FuncletMembership.find(MBB1);
@@ -617,7 +618,11 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
 
   // It's almost always profitable to merge any number of non-terminator
   // instructions with the block that falls through into the common successor.
-  if (MBB1 == PredBB || MBB2 == PredBB) {
+  // This is true only for a single successor. For multiple successors, we are
+  // trading a conditional branch for an unconditional one.
+  // TODO: Re-visit successor size for non-layout tail merging.
+  if ((MBB1 == PredBB || MBB2 == PredBB) &&
+      (!AfterPlacement || MBB1->succ_size() == 1)) {
     MachineBasicBlock::iterator I;
     unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
     if (CommonTailLen > NumTerms)
@@ -635,9 +640,12 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
 
   // If both blocks have an unconditional branch temporarily stripped out,
   // count that as an additional common instruction for the following
-  // heuristics.
+  // heuristics. This heuristic is only accurate for single-succ blocks, so to
+  // make sure that during layout merging and duplicating don't crash, we check
+  // for that when merging during layout.
   unsigned EffectiveTailLen = CommonTailLen;
   if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
+      (MBB1->succ_size() == 1 || !AfterPlacement) &&
       !MBB1->back().isBarrier() &&
       !MBB2->back().isBarrier())
     ++EffectiveTailLen;
@@ -682,7 +690,8 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
                             minCommonTailLength,
                             CommonTailLen, TrialBBI1, TrialBBI2,
                             SuccBB, PredBB,
-                            FuncletMembership)) {
+                            FuncletMembership,
+                            AfterBlockPlacement)) {
         if (CommonTailLen > maxCommonTailLength) {
           SameTails.clear();
           maxCommonTailLength = CommonTailLen;