[JumpThreading] Split select that has constant conditions coming from the PHI node

Look for PHI/Select in the same BB of the form

bb:
  %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
  %s = select p, trueval, falseval

And expand the select into a branch structure. This later enables
jump-threading over bb in this pass.

Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
select if the associated PHI has at least one constant.  If the unfolded
select is not jump-threaded, it will be folded again in the later
optimizations.

llvm-svn: 257198

1 files changed, 63 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 087ce8ac50d..704d286ecac 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -163,6 +163,7 @@ namespace {
 
     bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
     bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
+    bool TryToUnfoldSelectInCurrBB(BasicBlock *BB);
 
   private:
     BasicBlock *SplitBlockPreds(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
@@ -736,6 +737,9 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
     }
   }
 
+  if (TryToUnfoldSelectInCurrBB(BB))
+    return true;
+
   // What kind of constant we're looking for.
   ConstantPreference Preference = WantInteger;
 
@@ -1903,3 +1907,62 @@ bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
   }
   return false;
 }
+
+/// TryToUnfoldSelectInCurrBB - Look for PHI/Select in the same BB of the form
+/// bb:
+///   %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
+///   %s = select p, trueval, falseval
+///
+/// And expand the select into a branch structure. This later enables
+/// jump-threading over bb in this pass.
+///
+/// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
+/// select if the associated PHI has at least one constant.  If the unfolded
+/// select is not jump-threaded, it will be folded again in the later
+/// optimizations.
+bool JumpThreading::TryToUnfoldSelectInCurrBB(BasicBlock *BB) {
+  // If threading this would thread across a loop header, don't thread the edge.
+  // See the comments above FindLoopHeaders for justifications and caveats.
+  if (LoopHeaders.count(BB))
+    return false;
+
+  // Look for a Phi/Select pair in the same basic block.  The Phi feeds the
+  // condition of the Select and at least one of the incoming values is a
+  // constant.
+  for (BasicBlock::iterator BI = BB->begin();
+       PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
+    unsigned NumPHIValues = PN->getNumIncomingValues();
+    if (NumPHIValues == 0 || !PN->hasOneUse())
+      continue;
+
+    SelectInst *SI = dyn_cast<SelectInst>(PN->user_back());
+    if (!SI || SI->getParent() != BB)
+      continue;
+
+    Value *Cond = SI->getCondition();
+    if (!Cond || Cond != PN || !Cond->getType()->isIntegerTy(1))
+      continue;
+
+    bool HasConst = false;
+    for (unsigned i = 0; i != NumPHIValues; ++i) {
+      if (PN->getIncomingBlock(i) == BB)
+        return false;
+      if (isa<ConstantInt>(PN->getIncomingValue(i)))
+        HasConst = true;
+    }
+
+    if (HasConst) {
+      // Expand the select.
+      TerminatorInst *Term =
+          SplitBlockAndInsertIfThen(SI->getCondition(), SI, false);
+      PHINode *NewPN = PHINode::Create(SI->getType(), 2, "", SI);
+      NewPN->addIncoming(SI->getTrueValue(), Term->getParent());
+      NewPN->addIncoming(SI->getFalseValue(), BB);
+      SI->replaceAllUsesWith(NewPN);
+      SI->eraseFromParent();
+      return true;
+    }
+  }
+  
+  return false;
+}