Give uses within a loop a greater weight when doing representation inference.

Uses of a value are weighted by a factor of FLAG_loop_weight (default: 10) for
every loop they are in. This makes uses in inner loops "more important", which
should improve the result of the representation inference.

Review URL: https://chromiumcodereview.appspot.com/8277031

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10573 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/flag-definitions.h

@@ -166,6 +166,7 @@ DEFINE_bool(use_osr, true, "use on-stack replacement")
 DEFINE_bool(trace_osr, false, "trace on-stack replacement")
 DEFINE_int(stress_runs, 0, "number of stress runs")
 DEFINE_bool(optimize_closures, true, "optimize closures")
+DEFINE_int(loop_weight, 1, "loop weight for representation inference")
 
 // assembler-ia32.cc / assembler-arm.cc / assembler-x64.cc
 DEFINE_bool(debug_code, false,

src/hydrogen-instructions.cc

@@ -67,6 +67,14 @@ const char* Representation::Mnemonic() const {
 }
 
 
+int HValue::LoopWeight() const {
+  const int w = FLAG_loop_weight;
+  static const int weights[] = { 1, w, w*w, w*w*w, w*w*w*w };
+  return weights[Min(block()->LoopNestingDepth(),
+                     static_cast<int>(ARRAY_SIZE(weights)-1))];
+}
+
+
 void HValue::AssumeRepresentation(Representation r) {
   if (CheckFlag(kFlexibleRepresentation)) {
     ChangeRepresentation(r);
@@ -1139,7 +1147,7 @@ void HPhi::InitRealUses(int phi_id) {
     HValue* value = it.value();
     if (!value->IsPhi()) {
       Representation rep = value->RequiredInputRepresentation(it.index());
-      ++non_phi_uses_[rep.kind()];
+      non_phi_uses_[rep.kind()] += value->LoopWeight();
     }
   }
 }

src/hydrogen-instructions.h

@@ -569,6 +569,7 @@ class HValue: public ZoneObject {
 
   HBasicBlock* block() const { return block_; }
   void SetBlock(HBasicBlock* block);
+  int LoopWeight() const;
 
   int id() const { return id_; }
   void set_id(int id) { id_ = id; }

src/hydrogen.cc

@@ -1672,7 +1672,7 @@ Representation HInferRepresentation::TryChange(HValue* value) {
     Representation rep = use->RequiredInputRepresentation(it.index());
     if (rep.IsNone()) continue;
     if (use->IsPhi()) HPhi::cast(use)->AddIndirectUsesTo(&use_count[0]);
-    ++use_count[rep.kind()];
+    use_count[rep.kind()] += use->LoopWeight();
   }
   int tagged_count = use_count[Representation::kTagged];
   int double_count = use_count[Representation::kDouble];