Detect truncating Phi uses of Phis with constant inputs

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index aadf87572df411eb5d8c9c9aeb1ce8b985010aea..8212e4cef61c82379b5a2f7d0a6fda9b7fe8969c 100644
--- a/src/hydrogen-instructions.cc
+++ b/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);
+      }
     }
   }
 }