ART: Implement HSelect

This patch adds a new HIR instruction to Optimizing. HSelect returns
one of two inputs based on the outcome of a condition.

This is only initial implementation which:
 - defines the new instruction,
 - repurposes BooleanSimplifier to emit it,
 - extends InstructionSimplifier to statically resolve it,
 - updates existing code and tests accordingly.

Code generators currently emit fallback if/then/else code and will be
updated in follow-up CLs to use platform-specific conditional moves
when possible.

Change-Id: Ib61b17146487ebe6b55350c2b589f0b971dcaaee

1 files changed, 152 insertions, 0 deletions

diff --git a/compiler/optimizing/select_generator.cc b/compiler/optimizing/select_generator.cc
new file mode 100644
index 0000000000..105b30ae5d
--- /dev/null
+++ b/compiler/optimizing/select_generator.cc
@@ -0,0 +1,152 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "select_generator.h"
+
+namespace art {
+
+static constexpr size_t kMaxInstructionsInBranch = 1u;
+
+// Returns true if `block` has only one predecessor, ends with a Goto and
+// contains at most `kMaxInstructionsInBranch` other movable instruction with
+// no side-effects.
+static bool IsSimpleBlock(HBasicBlock* block) {
+  if (block->GetPredecessors().size() != 1u) {
+    return false;
+  }
+  DCHECK(block->GetPhis().IsEmpty());
+
+  size_t num_instructions = 0u;
+  for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
+    HInstruction* instruction = it.Current();
+    if (instruction->IsControlFlow()) {
+      return instruction->IsGoto() && num_instructions <= kMaxInstructionsInBranch;
+    } else if (instruction->CanBeMoved() && !instruction->HasSideEffects()) {
+      num_instructions++;
+    } else {
+      return false;
+    }
+  }
+
+  LOG(FATAL) << "Unreachable";
+  UNREACHABLE();
+}
+
+// Returns true if 'block1' and 'block2' are empty, merge into the same single
+// successor and the successor can only be reached from them.
+static bool BlocksMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
+  return block1->GetSingleSuccessor() == block2->GetSingleSuccessor();
+}
+
+// Returns nullptr if `block` has either no phis or there is more than one phi
+// with different inputs at `index1` and `index2`. Otherwise returns that phi.
+static HPhi* GetSingleChangedPhi(HBasicBlock* block, size_t index1, size_t index2) {
+  DCHECK_NE(index1, index2);
+
+  HPhi* select_phi = nullptr;
+  for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
+    HPhi* phi = it.Current()->AsPhi();
+    if (phi->InputAt(index1) != phi->InputAt(index2)) {
+      if (select_phi == nullptr) {
+        // First phi with different inputs for the two indices found.
+        select_phi = phi;
+      } else {
+        // More than one phis has different inputs for the two indices.
+        return nullptr;
+      }
+    }
+  }
+  return select_phi;
+}
+
+void HSelectGenerator::Run() {
+  // Iterate in post order in the unlikely case that removing one occurrence of
+  // the selection pattern empties a branch block of another occurrence.
+  // Otherwise the order does not matter.
+  for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
+    HBasicBlock* block = it.Current();
+    if (!block->EndsWithIf()) continue;
+
+    // Find elements of the diamond pattern.
+    HIf* if_instruction = block->GetLastInstruction()->AsIf();
+    HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
+    HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
+    DCHECK_NE(true_block, false_block);
+    if (!IsSimpleBlock(true_block) ||
+        !IsSimpleBlock(false_block) ||
+        !BlocksMergeTogether(true_block, false_block)) {
+      continue;
+    }
+    HBasicBlock* merge_block = true_block->GetSingleSuccessor();
+
+    // If the branches are not empty, move instructions in front of the If.
+    // TODO(dbrazdil): This puts an instruction between If and its condition.
+    //                 Implement moving of conditions to first users if possible.
+    if (!true_block->IsSingleGoto()) {
+      true_block->MoveInstructionBefore(true_block->GetFirstInstruction(), if_instruction);
+    }
+    if (!false_block->IsSingleGoto()) {
+      false_block->MoveInstructionBefore(false_block->GetFirstInstruction(), if_instruction);
+    }
+    DCHECK(true_block->IsSingleGoto());
+    DCHECK(false_block->IsSingleGoto());
+
+    // Find the resulting true/false values.
+    size_t predecessor_index_true = merge_block->GetPredecessorIndexOf(true_block);
+    size_t predecessor_index_false = merge_block->GetPredecessorIndexOf(false_block);
+    DCHECK_NE(predecessor_index_true, predecessor_index_false);
+
+    HPhi* phi = GetSingleChangedPhi(merge_block, predecessor_index_true, predecessor_index_false);
+    if (phi == nullptr) {
+      continue;
+    }
+    HInstruction* true_value = phi->InputAt(predecessor_index_true);
+    HInstruction* false_value = phi->InputAt(predecessor_index_false);
+
+    // Create the Select instruction and insert it in front of the If.
+    HSelect* select = new (graph_->GetArena()) HSelect(if_instruction->InputAt(0),
+                                                       true_value,
+                                                       false_value,
+                                                       if_instruction->GetDexPc());
+    if (phi->GetType() == Primitive::kPrimNot) {
+      select->SetReferenceTypeInfo(phi->GetReferenceTypeInfo());
+    }
+    block->InsertInstructionBefore(select, if_instruction);
+
+    // Remove the true branch which removes the corresponding Phi input.
+    // If left only with the false branch, the Phi is automatically removed.
+    phi->ReplaceInput(select, predecessor_index_false);
+    bool only_two_predecessors = (merge_block->GetPredecessors().size() == 2u);
+    true_block->DisconnectAndDelete();
+    DCHECK_EQ(only_two_predecessors, phi->GetBlock() == nullptr);
+
+    // Merge remaining blocks which are now connected with Goto.
+    DCHECK_EQ(block->GetSingleSuccessor(), false_block);
+    block->MergeWith(false_block);
+    if (only_two_predecessors) {
+      DCHECK_EQ(block->GetSingleSuccessor(), merge_block);
+      block->MergeWith(merge_block);
+    }
+
+    // No need to update dominance information, as we are simplifying
+    // a simple diamond shape, where the join block is merged with the
+    // entry block. Any following blocks would have had the join block
+    // as a dominator, and `MergeWith` handles changing that to the
+    // entry block.
+  }
+}
+
+}  // namespace art