Detect truncating Phi uses of Phis with constant inputs

Review URL: https://codereview.chromium.org/14260013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14525 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -1958,6 +1958,10 @@ void HPhi::DeleteFromGraph() {
 void HPhi::InitRealUses(int phi_id) {
   // Initialize real uses.
   phi_id_ = phi_id;
+  // Compute a conservative approximation of truncating uses before inferring
+  // representations. The proper, exact computation will be done later, when
+  // inserting representation changes.
+  SetFlag(kTruncatingToInt32);
   for (HUseIterator it(uses()); !it.Done(); it.Advance()) {
     HValue* value = it.value();
     if (!value->IsPhi()) {
@@ -1967,6 +1971,9 @@ void HPhi::InitRealUses(int phi_id) {
         PrintF("#%d Phi is used by real #%d %s as %s\n",
                id(), value->id(), value->Mnemonic(), rep.Mnemonic());
       }
+      if (!value->IsSimulate() && !value->CheckFlag(kTruncatingToInt32)) {
+        ClearFlag(kTruncatingToInt32);
+      }
     }
   }
 }

src/hydrogen.cc

@@ -3763,7 +3763,39 @@ void HInferRepresentation::Analyze() {
     }
   }
 
+  // Set truncation flags for groups of connected phis. This is a conservative
+  // approximation; the flag will be properly re-computed after representations
+  // have been determined.
+  if (phi_count > 0) {
+    BitVector* done = new(zone()) BitVector(phi_count, graph_->zone());
+    for (int i = 0; i < phi_count; ++i) {
+      if (done->Contains(i)) continue;
+
+      // Check if all uses of all connected phis in this group are truncating.
+      bool all_uses_everywhere_truncating = true;
+      for (BitVector::Iterator it(connected_phis.at(i));
+           !it.Done();
+           it.Advance()) {
+        int index = it.Current();
+        all_uses_everywhere_truncating &=
+            phi_list->at(index)->CheckFlag(HInstruction::kTruncatingToInt32);
+        done->Add(index);
+      }
+      if (all_uses_everywhere_truncating) {
+        continue;  // Great, nothing to do.
+      }
+      // Clear truncation flag of this group of connected phis.
+      for (BitVector::Iterator it(connected_phis.at(i));
+           !it.Done();
+           it.Advance()) {
+        int index = it.Current();
+        phi_list->at(index)->ClearFlag(HInstruction::kTruncatingToInt32);
+      }
+    }
+  }
+
   // Simplify constant phi inputs where possible.
+  // This step uses kTruncatingToInt32 flags of phis.
   for (int i = 0; i < phi_count; ++i) {
     phi_list->at(i)->SimplifyConstantInputs();
   }