Avoid using a register for constant external array indices.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 4b045385a40a9bd5eb809510db8825d21393847d..33143b462bffcef4fbcb2f60e0dece22b0a98378 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -2482,40 +2482,56 @@ void LCodeGen::DoLoadKeyedFastElement(LLoadKeyedFastElement* instr) {
 void LCodeGen::DoLoadKeyedSpecializedArrayElement(
     LLoadKeyedSpecializedArrayElement* instr) {
   Register external_pointer = ToRegister(instr->external_pointer());
-  Register key = ToRegister(instr->key());
+  Register key = no_reg;
   ExternalArrayType array_type = instr->array_type();
-  if (array_type == kExternalFloatArray) {
-    CpuFeatures::Scope scope(VFP3);
-    DwVfpRegister result(ToDoubleRegister(instr->result()));
-    __ add(scratch0(), external_pointer, Operand(key, LSL, 2));
-    __ vldr(result.low(), scratch0(), 0);
-    __ vcvt_f64_f32(result, result.low());
-  } else if (array_type == kExternalDoubleArray) {
+  bool key_is_constant = instr->key()->IsConstantOperand();
+  int constant_key = 0;
+  if (key_is_constant) {
+    constant_key = ToInteger32(LConstantOperand::cast(instr->key()));
+    if (constant_key & 0xF0000000) {
+      Abort("array index constant value too big.");
+    }
+  } else {
+    key = ToRegister(instr->key());
+  }
+  int shift_size = ExternalArrayTypeToShiftSize(array_type);
+
+  if (array_type == kExternalFloatArray || array_type == kExternalDoubleArray) {
     CpuFeatures::Scope scope(VFP3);
     DwVfpRegister result(ToDoubleRegister(instr->result()));
-    __ add(scratch0(), external_pointer, Operand(key, LSL, 3));
-    __ vldr(result, scratch0(), 0);
+    Operand operand(key_is_constant ? Operand(constant_key * (1 << shift_size))
+                                    : Operand(key, LSL, shift_size));
+    __ add(scratch0(), external_pointer, operand);
+    if (array_type == kExternalFloatArray) {
+      __ vldr(result.low(), scratch0(), 0);
+      __ vcvt_f64_f32(result, result.low());
+    } else  {  // i.e. array_type == kExternalDoubleArray
+      __ vldr(result, scratch0(), 0);
+    }
   } else {
     Register result(ToRegister(instr->result()));
+    MemOperand mem_operand(key_is_constant
+        ? MemOperand(external_pointer, constant_key * (1 << shift_size))
+        : MemOperand(external_pointer, key, LSL, shift_size));
     switch (array_type) {
       case kExternalByteArray:
-        __ ldrsb(result, MemOperand(external_pointer, key));
+        __ ldrsb(result, mem_operand);
         break;
       case kExternalUnsignedByteArray:
       case kExternalPixelArray:
-        __ ldrb(result, MemOperand(external_pointer, key));
+        __ ldrb(result, mem_operand);
         break;
       case kExternalShortArray:
-        __ ldrsh(result, MemOperand(external_pointer, key, LSL, 1));
+        __ ldrsh(result, mem_operand);
         break;
       case kExternalUnsignedShortArray:
-        __ ldrh(result, MemOperand(external_pointer, key, LSL, 1));
+        __ ldrh(result, mem_operand);
         break;
       case kExternalIntArray:
-        __ ldr(result, MemOperand(external_pointer, key, LSL, 2));
+        __ ldr(result, mem_operand);
         break;
       case kExternalUnsignedIntArray:
-        __ ldr(result, MemOperand(external_pointer, key, LSL, 2));
+        __ ldr(result, mem_operand);
         __ cmp(result, Operand(0x80000000));
         // TODO(danno): we could be more clever here, perhaps having a special
         // version of the stub that detects if the overflow case actually
@@ -3245,39 +3261,53 @@ void LCodeGen::DoStoreKeyedSpecializedArrayElement(
     LStoreKeyedSpecializedArrayElement* instr) {
 
   Register external_pointer = ToRegister(instr->external_pointer());
-  Register key = ToRegister(instr->key());
+  Register key = no_reg;
   ExternalArrayType array_type = instr->array_type();
+  bool key_is_constant = instr->key()->IsConstantOperand();
+  int constant_key = 0;
+  if (key_is_constant) {
+    constant_key = ToInteger32(LConstantOperand::cast(instr->key()));
+    if (constant_key & 0xF0000000) {
+      Abort("array index constant value too big.");
+    }
+  } else {
+    key = ToRegister(instr->key());
+  }
+  int shift_size = ExternalArrayTypeToShiftSize(array_type);
 
-  if (array_type == kExternalFloatArray) {
+  if (array_type == kExternalFloatArray || array_type == kExternalDoubleArray) {
     CpuFeatures::Scope scope(VFP3);
     DwVfpRegister value(ToDoubleRegister(instr->value()));
-    __ add(scratch0(), external_pointer, Operand(key, LSL, 2));
-    __ vcvt_f32_f64(double_scratch0().low(), value);
-    __ vstr(double_scratch0().low(), scratch0(), 0);
-  } else if (array_type == kExternalDoubleArray) {
-    CpuFeatures::Scope scope(VFP3);
-    DwVfpRegister value(ToDoubleRegister(instr->value()));
-    __ add(scratch0(), external_pointer, Operand(key, LSL, 3));
-    __ vstr(value, scratch0(), 0);
+    Operand operand(key_is_constant ? Operand(constant_key * (1 << shift_size))
+                                    : Operand(key, LSL, shift_size));
+    __ add(scratch0(), external_pointer, operand);
+    if (array_type == kExternalFloatArray) {
+      __ vcvt_f32_f64(double_scratch0().low(), value);
+      __ vstr(double_scratch0().low(), scratch0(), 0);
+    } else {  // i.e. array_type == kExternalDoubleArray
+      __ vstr(value, scratch0(), 0);
+    }
   } else {
     Register value(ToRegister(instr->value()));
+    MemOperand mem_operand(key_is_constant
+        ? MemOperand(external_pointer, constant_key * (1 << shift_size))
+        : MemOperand(external_pointer, key, LSL, shift_size));
     switch (array_type) {
       case kExternalPixelArray:
         // Clamp the value to [0..255].
         __ Usat(value, 8, Operand(value));
-        __ strb(value, MemOperand(external_pointer, key));
-        break;
+        // Fall through to the next case for the store instruction:
       case kExternalByteArray:
       case kExternalUnsignedByteArray:
-        __ strb(value, MemOperand(external_pointer, key));
+        __ strb(value, mem_operand);
         break;
       case kExternalShortArray:
       case kExternalUnsignedShortArray:
-        __ strh(value, MemOperand(external_pointer, key, LSL, 1));
+        __ strh(value, mem_operand);
         break;
       case kExternalIntArray:
       case kExternalUnsignedIntArray:
-        __ str(value, MemOperand(external_pointer, key, LSL, 2));
+        __ str(value, mem_operand);
         break;
       case kExternalFloatArray:
       case kExternalDoubleArray: