A recursive call to the code generator could (conceivably) clobber any...

src/codegen-ia32.cc

 // Copyright 2006-2008 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 2901 matching lines @@
   ASSERT(args->length() == 2);
 
   Label slow_case;
   Label end;
   Label not_a_flat_string;
   Label not_a_cons_string_either;
   Label try_again_with_new_string;
   Label ascii_string;
   Label got_char_code;
 
-  // Load the string into eax.
+  // Load the string into eax and the index into ebx.
   Load(args->at(0));
+  Load(args->at(1));
+  frame_->Pop(ebx);
   frame_->Pop(eax);
   // If the receiver is a smi return undefined.
   ASSERT(kSmiTag == 0);
   __ test(eax, Immediate(kSmiTagMask));
   __ j(zero, &slow_case, not_taken);
 
-  // Load the index into ebx.
-  Load(args->at(1));
-  frame_->Pop(ebx);
-
   // Check for negative or non-smi index.
   ASSERT(kSmiTag == 0);
   __ test(ebx, Immediate(kSmiTagMask | 0x80000000));
   __ j(not_zero, &slow_case, not_taken);
   // Get rid of the smi tag on the index.
   __ sar(ebx, kSmiTagSize);
 
   __ bind(&try_again_with_new_string);
   // Get the type of the heap object into edi.
   __ mov(edx, FieldOperand(eax, HeapObject::kMapOffset));
@@ skipping 2257 matching lines @@
 
   // Slow-case: Go through the JavaScript implementation.
   __ bind(&slow);
   __ InvokeBuiltin(Builtins::INSTANCE_OF, JUMP_FUNCTION);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal