Use movw/movt on ARM to load constant roots

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
===================================================================
--- src/arm/macro-assembler-arm.cc	(revision 12826)
+++ src/arm/macro-assembler-arm.cc	(working copy)
@@ -422,6 +422,17 @@
 void MacroAssembler::LoadRoot(Register destination,
                               Heap::RootListIndex index,
                               Condition cond) {
+  if (CpuFeatures::IsSupported(MOVW_MOVT_IMMEDIATE_LOADS)) {
+    Handle<Object> root(isolate()->heap()->roots_array_start()[index]);
+    if (root->IsOddball() || root->IsMap() ||

[danno, 2012/10/29 15:05:39]

Can you phrase this test as checking against a black-list rather than whitelist?
i.e. only do the ldr on roots where this won't work, and everything else uses
movw/movt? This will make sure that roots added in the future will also benefit
without having to add them them to this whitelist. 

+        index == Heap::kEmptyFixedArrayRootIndex ||
+        index == Heap::kSingleCharacterStringCacheRootIndex) {
+      // The CPU supports fast immediate values, and this root will never
+      // change. We will load it as a relocatable immediate value.
+      mov(destination, Operand(root), LeaveCC, cond);
+      return;
+    }
+  }
   ldr(destination, MemOperand(kRootRegister, index << kPointerSizeLog2), cond);
 }