[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

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;
+}

test/Transforms/JumpThreading/select.ll

@@ -250,3 +250,40 @@ if.end:                                           ; preds = %if.then, %cond.end4
 ; CHECK: br i1 %cmp6, label %if.then, label %if.end
 ; CHECK: br label %if.end
 }
+
+
+define i32 @unfold3(i32 %u, i32 %v, i32 %w, i32 %x, i32 %y, i32 %z, i32 %j) nounwind {
+entry:
+  %add3 = add nsw i32 %j, 2
+  %cmp.i = icmp slt i32 %u, %v
+  br i1 %cmp.i, label %.exit, label %cond.false.i
+
+cond.false.i:                                     ; preds = %entry
+  %cmp4.i = icmp sgt i32 %u, %v
+  br i1 %cmp4.i, label %.exit, label %cond.false.6.i
+
+cond.false.6.i:                                   ; preds = %cond.false.i
+  %cmp8.i = icmp slt i32 %w, %x
+  br i1 %cmp8.i, label %.exit, label %cond.false.10.i
+
+cond.false.10.i:                                  ; preds = %cond.false.6.i
+  %cmp13.i = icmp sgt i32 %w, %x
+  br i1 %cmp13.i, label %.exit, label %cond.false.15.i
+
+cond.false.15.i:                                  ; preds = %cond.false.10.i
+  %phitmp = icmp sge i32 %y, %z
+  br label %.exit
+
+.exit:                                  ; preds = %entry, %cond.false.i, %cond.false.6.i, %cond.false.10.i, %cond.false.15.i
+  %cond23.i = phi i1 [ false, %entry ], [ true, %cond.false.i ], [ false, %cond.false.6.i ], [ %phitmp, %cond.false.15.i ], [ true, %cond.false.10.i ]
+  %j.add3 = select i1 %cond23.i, i32 %j, i32 %add3
+  ret i32 %j.add3
+
+; CHECK-LABEL: @unfold3
+; CHECK: br i1 %cmp.i, label %.exit.thread2, label %cond.false.i 
+; CHECK: br i1 %cmp4.i, label %.exit.thread, label %cond.false.6.i
+; CHECK: br i1 %cmp8.i, label %.exit.thread2, label %cond.false.10.i
+; CHECK: br i1 %cmp13.i, label %.exit.thread, label %.exit
+; CHECK: br i1 %phitmp, label %.exit.thread, label %.exit.thread2
+; CHECK: br label %.exit.thread2
+}