[ARM64] Optimize load followed by shift.

src/compiler/arm64/instruction-selector-arm64.cc

Index: src/compiler/arm64/instruction-selector-arm64.cc
diff --git a/src/compiler/arm64/instruction-selector-arm64.cc b/src/compiler/arm64/instruction-selector-arm64.cc
index 640452ffbf2a4f0af2ac56b14df0770c3c99e20b..14fdafd9844a0e4b080443d342628292aa6f47ec 100644
--- a/src/compiler/arm64/instruction-selector-arm64.cc
+++ b/src/compiler/arm64/instruction-selector-arm64.cc
@@ -161,6 +161,78 @@ void VisitRRO(InstructionSelector* selector, ArchOpcode opcode, Node* node,
                  g.UseOperand(node->InputAt(1), operand_mode));
 }
 
+struct ExtendingLoadMatcher {
+  ExtendingLoadMatcher(Node* node, InstructionSelector* selector)
+      : matches_(false), selector_(selector), base_(nullptr), immediate_(0) {
+    Initialize(node);
+  }
+
+  bool Matches() const { return matches_; }
+
+  Node* base() const {
+    DCHECK(Matches());
+    return base_;
+  }
+  int64_t immediate() const {
+    DCHECK(Matches());
+    return immediate_;
+  }
+  ArchOpcode opcode() const {
+    DCHECK(Matches());
+    return opcode_;
+  }
+
+ private:
+  bool matches_;
+  InstructionSelector* selector_;
+  Node* base_;
+  int64_t immediate_;
+  ArchOpcode opcode_;
+
+  void Initialize(Node* node) {
+    Int64BinopMatcher m(node);
+    // When loading a 64-bit value and shifting by 32, we should
+    // just load and sign-extend the interesting 4 bytes instead.
+    // This happens, for example, when we're loading and untagging SMIs.
+    DCHECK(m.IsWord64Sar());
+    if (m.left().IsLoad() && m.right().Is(32) &&
+        selector_->CanCover(m.node(), m.left().node())) {
+      Arm64OperandGenerator g(selector_);
+      Node* load = m.left().node();
+      Node* offset = load->InputAt(1);
+      base_ = load->InputAt(0);
+      opcode_ = kArm64Ldrsw;
+      if (g.IsIntegerConstant(offset)) {
+        immediate_ = g.GetIntegerConstantValue(offset) + 4;
+        matches_ = g.CanBeImmediate(immediate_, kLoadStoreImm32);
+      }
+    }
+  }
+};
+
+bool TryMatchExtendingLoad(InstructionSelector* selector, Node* node) {
+  ExtendingLoadMatcher m(node, selector);
+  return m.Matches();
+}
+
+bool TryEmitExtendingLoad(InstructionSelector* selector, Node* node) {
+  ExtendingLoadMatcher m(node, selector);
+  Arm64OperandGenerator g(selector);
+  if (m.Matches()) {
+    InstructionOperand inputs[2];
+    inputs[0] = g.UseRegister(m.base());
+    InstructionCode opcode =
+        m.opcode() | AddressingModeField::encode(kMode_MRI);
+    DCHECK(is_int32(m.immediate()));
+    inputs[1] = g.TempImmediate(static_cast<int32_t>(m.immediate()));
+    InstructionOperand outputs[] = {g.DefineAsRegister(node)};
+    selector->Emit(opcode, arraysize(outputs), outputs, arraysize(inputs),
+                   inputs);
+    return true;
+  }
+  return false;
+}
+
 bool TryMatchAnyShift(InstructionSelector* selector, Node* node,
                       Node* input_node, InstructionCode* opcode, bool try_ror) {
   Arm64OperandGenerator g(selector);
@@ -179,7 +251,10 @@ bool TryMatchAnyShift(InstructionSelector* selector, Node* node,
       *opcode |= AddressingModeField::encode(kMode_Operand2_R_LSR_I);
       return true;
     case IrOpcode::kWord32Sar:
+      *opcode |= AddressingModeField::encode(kMode_Operand2_R_ASR_I);
+      return true;
     case IrOpcode::kWord64Sar:
+      if (TryMatchExtendingLoad(selector, input_node)) return false;
       *opcode |= AddressingModeField::encode(kMode_Operand2_R_ASR_I);
       return true;
     case IrOpcode::kWord32Ror:
@@ -1130,6 +1205,7 @@ void InstructionSelector::VisitWord32Sar(Node* node) {
 
 
 void InstructionSelector::VisitWord64Sar(Node* node) {
+  if (TryEmitExtendingLoad(this, node)) return;
   VisitRRO(this, kArm64Asr, node, kShift64Imm);
 }