Mere r4172 to trunk. The new version is 2.1.6.1.

src/ia32/codegen-ia32.cc

Index: src/ia32/codegen-ia32.cc
===================================================================
--- src/ia32/codegen-ia32.cc	(revision 4174)
+++ src/ia32/codegen-ia32.cc	(working copy)
@@ -3763,6 +3763,21 @@
     }
   }
 
+  // The update expression resets the type of the loop variable. So we
+  // set it to smi before compiling the test expression.
+  if (node->is_fast_smi_loop()) {
+    // Set number type of the loop variable to smi.
+    Slot* slot = node->loop_variable()->slot();
+    ASSERT(slot->type() == Slot::LOCAL);
+    frame_->SetTypeForLocalAt(slot->index(), NumberInfo::Smi());
+    if (FLAG_debug_code) {
+      frame_->PushLocalAt(slot->index());
+      Result var = frame_->Pop();
+      var.ToRegister();
+      __ AbortIfNotSmi(var.reg(), "Loop variable not a smi.");
+    }
+  }
+
   // Based on the condition analysis, compile the backward jump as
   // necessary.
   switch (info) {
@@ -6867,7 +6882,12 @@
       old_value = allocator_->Allocate();
       ASSERT(old_value.is_valid());
       __ mov(old_value.reg(), new_value.reg());
+
+      // The return value for postfix operations is the
+      // same as the input, and has the same number info.
+      old_value.set_number_info(new_value.number_info());
     }
+
     // Ensure the new value is writable.
     frame_->Spill(new_value.reg());
 
@@ -6931,6 +6951,13 @@
     }
     deferred->BindExit();
 
+    // The result of ++ or -- is an Integer32 if the
+    // input is a smi. Otherwise it is a number.
+    if (new_value.is_smi()) {
+      new_value.set_number_info(NumberInfo::Integer32());
+    } else {
+      new_value.set_number_info(NumberInfo::Number());
+    }
 
     // Postfix: store the old value in the allocated slot under the
     // reference.