Implement basic array prefetching hints in Hydrogen.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
===================================================================
--- src/ia32/lithium-ia32.cc	(revision 13123)
+++ src/ia32/lithium-ia32.cc	(working copy)
@@ -422,6 +422,10 @@
   } else {
     stream->Add("]");
   }
+  if (prefetch_distance() != NULL) {
+    stream->Add(" ");
+    prefetch_distance()->PrintTo(stream);
+  }
 }
 
 
@@ -1958,6 +1962,23 @@
 }
 
 
+static bool ShouldPrefetch(HValue* distance, ElementsKind elements_kind) {
+  bool should_prefetch = true;
+
+  if (distance->IsConstant()) {
+    HConstant* prefetch_distance = HConstant::cast(distance);
+    ASSERT(prefetch_distance->HasInteger32Value());
+    int32_t distance_value = prefetch_distance->Integer32Value();
+    if (distance_value < 0) distance_value = -distance_value;
+    // Omit the nearby accesses which are supposed to hit the cache.
+    should_prefetch =
+        (distance_value >= (64 >> ElementsKindToShiftSize(elements_kind)));
+  }
+
+  return should_prefetch;
+}
+
+
 LInstruction* LChunkBuilder::DoLoadKeyed(HLoadKeyed* instr) {
   ASSERT(instr->key()->representation().IsInteger32() ||
          instr->key()->representation().IsTagged());
@@ -1969,9 +1990,17 @@
       : UseRegisterOrConstantAtStart(instr->key());
   LLoadKeyed* result = NULL;
 
+  LOperand* prefetch = NULL;
+  LOperand* temp = NULL;
+  if (instr->prefetch_distance() != graph()->GetConstantUndefined() &&
+      ShouldPrefetch(instr->prefetch_distance(), elements_kind)) {
+    prefetch = UseRegisterOrConstant(instr->prefetch_distance());
+    temp = TempRegister();
+  }
+
   if (!instr->is_external()) {
     LOperand* obj = UseRegisterAtStart(instr->elements());
-    result = new(zone()) LLoadKeyed(obj, key);
+    result = new(zone()) LLoadKeyed(obj, key, prefetch, temp);
   } else {
     ASSERT(
         (instr->representation().IsInteger32() &&
@@ -1981,7 +2010,7 @@
          ((elements_kind == EXTERNAL_FLOAT_ELEMENTS) ||
           (elements_kind == EXTERNAL_DOUBLE_ELEMENTS))));
     LOperand* external_pointer = UseRegister(instr->elements());
-    result = new(zone()) LLoadKeyed(external_pointer, key);
+    result = new(zone()) LLoadKeyed(external_pointer, key, prefetch, temp);
   }
 
   DefineAsRegister(result);