[SimplifyCFG] Handle tail-sinking of more than 2 incoming branches

This was a real restriction in the original version of SinkIfThenCodeToEnd. Now it's been rewritten, the restriction can be lifted.

As part of this, we handle a very common and useful case where one of the incoming branches is actually conditional. Consider:

   if (a)
     x(1);
   else if (b)
     x(2);

This produces the following CFG:

         [if]
        /    \
      [x(1)] [if]
        |     | \
        |     |  \
        |  [x(2)] |
         \    |  /
          [ end ]

[end] has two unconditional predecessor arcs and one conditional. The conditional refers to the implicit empty 'else' arc. This same pattern can also be caused by an empty default block in a switch.

We can't sink the call to x() down to end because no call to x() happens on the third incoming arc (assume that x() has sideeffects for the sake of argument; if something is safe to speculate we could indeed sink nevertheless but this cannot happen in the general case and causes many extra selects).

We are now able to detect this case and split off the unconditional arcs to a common successor:

         [if]
        /    \
      [x(1)] [if]
        |     | \
        |     |  \
        |  [x(2)] |
         \   /    |
     [sink.split] |
           \     /
           [ end ]

Now we can sink the call to x() into %sink.split. This can cause significant code simplification in many testcases.

llvm-svn: 280217

1 files changed, 89 insertions, 27 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a0d4541..a9fa7e7 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1547,20 +1547,62 @@ static bool SinkThenElseCodeToEnd(BranchInst *BI1) {
   assert(BI1->isUnconditional());
   BasicBlock *BBEnd = BI1->getSuccessor(0);
 
-  // We currently only support branch targets with two predecessors.
-  // FIXME: this is an arbitrary restriction and should be lifted.
-  SmallVector<BasicBlock*,4> Blocks;
-  for (auto *BB : predecessors(BBEnd))
-    Blocks.push_back(BB);
-  if (Blocks.size() != 2 ||
-      !all_of(Blocks, [](const BasicBlock *BB) {
-        auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
-        return BI && BI->isUnconditional();
-      }))
+  // We support two situations:
+  //   (1) all incoming arcs are unconditional
+  //   (2) one incoming arc is conditional
+  //
+  // (2) is very common in switch defaults and
+  // else-if patterns;
+  //
+  //   if (a) f(1);
+  //   else if (b) f(2);
+  //
+  // produces:
+  //
+  //       [if]
+  //      /    \
+  //    [f(1)] [if]
+  //      |     | \
+  //      |     |  \
+  //      |  [f(2)]|
+  //       \    | /
+  //        [ end ]
+  //
+  // [end] has two unconditional predecessor arcs and one conditional. The
+  // conditional refers to the implicit empty 'else' arc. This conditional
+  // arc can also be caused by an empty default block in a switch.
+  //
+  // In this case, we attempt to sink code from all *unconditional* arcs.
+  // If we can sink instructions from these arcs (determined during the scan
+  // phase below) we insert a common successor for all unconditional arcs and
+  // connect that to [end], to enable sinking:
+  //
+  //       [if]
+  //      /    \
+  //    [x(1)] [if]
+  //      |     | \
+  //      |     |  \
+  //      |  [x(2)] |
+  //       \   /    |
+  //   [sink.split] |
+  //         \     /
+  //         [ end ]
+  //
+  SmallVector<BasicBlock*,4> UnconditionalPreds;
+  Instruction *Cond = nullptr;
+  for (auto *B : predecessors(BBEnd)) {
+    auto *T = B->getTerminator();
+    if (isa<BranchInst>(T) && cast<BranchInst>(T)->isUnconditional())
+      UnconditionalPreds.push_back(B);
+    else if ((isa<BranchInst>(T) || isa<SwitchInst>(T)) && !Cond)
+      Cond = T;
+    else
+      return false;
+  }
+  if (UnconditionalPreds.size() < 2)
     return false;
-
+  
   bool Changed = false;
-
   // We take a two-step approach to tail sinking. First we scan from the end of
   // each block upwards in lockstep. If the n'th instruction from the end of each
   // block can be sunk, those instructions are added to ValuesToSink and we
@@ -1570,7 +1612,7 @@ static bool SinkThenElseCodeToEnd(BranchInst *BI1) {
   unsigned ScanIdx = 0;
   SmallPtrSet<Value*,4> InstructionsToSink;
   DenseMap<Instruction*, SmallVector<Value*,4>> PHIOperands;
-  LockstepReverseIterator LRI(Blocks);
+  LockstepReverseIterator LRI(UnconditionalPreds);
   while (LRI.isValid() &&
          canSinkInstructions(*LRI, PHIOperands)) {
     DEBUG(dbgs() << "SINK: instruction can be sunk: " << (*LRI)[0] << "\n");
@@ -1579,6 +1621,35 @@ static bool SinkThenElseCodeToEnd(BranchInst *BI1) {
     --LRI;
   }
 
+  auto ProfitableToSinkLastInstruction = [&]() {
+    LRI.reset();
+    unsigned NumPHIdValues = 0;
+    for (auto *I : *LRI)
+      for (auto *V : PHIOperands[I])
+        if (InstructionsToSink.count(V) == 0)
+          ++NumPHIdValues;
+    DEBUG(dbgs() << "SINK: #phid values: " << NumPHIdValues << "\n");
+    assert((NumPHIdValues % UnconditionalPreds.size() == 0) &&
+           "Every operand must either be PHId or not PHId!");
+    
+    return NumPHIdValues / UnconditionalPreds.size() <= 1;
+  };
+
+  if (ScanIdx > 0 && Cond) {
+    // Check if we would actually sink anything first!
+    if (!ProfitableToSinkLastInstruction())
+      return false;
+    
+    DEBUG(dbgs() << "SINK: Splitting edge\n");
+    // We have a conditional edge and we're going to sink some instructions.
+    // Insert a new block postdominating all blocks we're going to sink from.
+    if (!SplitBlockPredecessors(BI1->getSuccessor(0), UnconditionalPreds,
+                                ".sink.split"))
+      // Edges couldn't be split.
+      return false;
+    Changed = true;
+  }
+  
   // Now that we've analyzed all potential sinking candidates, perform the
   // actual sink. We iteratively sink the last non-terminator of the source
   // blocks into their common successor unless doing so would require too
@@ -1593,29 +1664,20 @@ static bool SinkThenElseCodeToEnd(BranchInst *BI1) {
   // This is unlikely in practice though.
   for (unsigned SinkIdx = 0; SinkIdx != ScanIdx; ++SinkIdx) {
     DEBUG(dbgs() << "SINK: Sink: "
-                 << *Blocks[0]->getTerminator()->getPrevNode()
+                 << *UnconditionalPreds[0]->getTerminator()->getPrevNode()
                  << "\n");
     // Because we've sunk every instruction in turn, the current instruction to
     // sink is always at index 0.
-    LRI.reset();
-    unsigned NumPHIdValues = 0;
-    for (auto *I : *LRI)
-      for (auto *V : PHIOperands[I])
-        if (InstructionsToSink.count(V) == 0)
-          ++NumPHIdValues;
-    DEBUG(dbgs() << "SINK: #phid values: " << NumPHIdValues << "\n");
-    assert((NumPHIdValues % Blocks.size() == 0) &&
-           "Every operand must either be PHId or not PHId!");
-    
-    if (NumPHIdValues / Blocks.size() > 1)
+    if (!ProfitableToSinkLastInstruction()) {
       // Too many PHIs would be created.
+      DEBUG(dbgs() << "SINK: stopping here, too many PHIs would be created!\n");
       break;
+    }
     
-    sinkLastInstruction(Blocks);
+    sinkLastInstruction(UnconditionalPreds);
     NumSinkCommons++;
     Changed = true;
   }
-  
   return Changed;
 }