ARM: Improve code aging sequence.

The code aging sequence contains a load which is unecessary for optimised
function. This has been replaced by a nop.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/19379003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15697 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/codegen-arm.cc

@@ -838,7 +838,7 @@ static byte* GetNoCodeAgeSequence(uint32_t* length) {
     CodePatcher patcher(byte_sequence, kNoCodeAgeSequenceLength);
     PredictableCodeSizeScope scope(patcher.masm(), *length);
     patcher.masm()->stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
-    patcher.masm()->LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+    patcher.masm()->nop(ip.code());
     patcher.masm()->add(fp, sp, Operand(2 * kPointerSize));
     initialized = true;
   }

src/arm/full-codegen-arm.cc

@@ -169,9 +169,7 @@ void FullCodeGenerator::Generate() {
     // The following three instructions must remain together and unmodified
     // for code aging to work properly.
     __ stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
-    // Load undefined value here, so the value is ready for the loop
-    // below.
-    __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+    __ nop(ip.code());
     // Adjust FP to point to saved FP.
     __ add(fp, sp, Operand(2 * kPointerSize));
   }
@@ -181,8 +179,11 @@ void FullCodeGenerator::Generate() {
     int locals_count = info->scope()->num_stack_slots();
     // Generators allocate locals, if any, in context slots.
     ASSERT(!info->function()->is_generator() || locals_count == 0);
-    for (int i = 0; i < locals_count; i++) {
-      __ push(ip);
+    if (locals_count > 0) {
+      __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+      for (int i = 0; i < locals_count; i++) {
+        __ push(ip);
+      }
     }
   }
 

src/arm/lithium-codegen-arm.cc

@@ -161,9 +161,7 @@ bool LCodeGen::GeneratePrologue() {
       // The following three instructions must remain together and unmodified
       // for code aging to work properly.
       __ stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
-      // Load undefined value here, so the value is ready for the loop
-      // below.
-      __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+      __ nop(ip.code());
       // Adjust FP to point to saved FP.
       __ add(fp, sp, Operand(2 * kPointerSize));
     }