Detect truncating Phi uses of Phis with constant inputs

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3744 matching lines @@
         HValue* use = it.value();
         if (use->IsPhi()) {
           int id = HPhi::cast(use)->phi_id();
           if (connected_phis[i]->UnionIsChanged(*connected_phis[id]))
             change = true;
         }
       }
     }
   }
 
+  // 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();
   }
 
   // Use the phi reachability information from step 2 to
   // push information about values which can't be converted to integer
   // without deoptimization through the phi use-def chains, avoiding
   // unnecessary deoptimizations later.
   for (int i = 0; i < phi_count; ++i) {
     HPhi* phi = phi_list->at(i);
@@ skipping 8441 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal