gcc 4.5.1 -fstrict-aliasing and store removal

[Vyacheslav Egorov <vegorov@xxxxxxxxxxxx>]

Hi,

I am experiencing the following problem when compiling V8[1] with gcc
4.5.1 with -O1 -fstrict-aliasing enabled: for some reason GCC removes
one of the stores to memory.

Store removal happens in the method Genesis::HookUpGlobalProxy [2]

void Genesis::HookUpGlobalProxy(Handle<GlobalObject> inner_global,
Handle<JSGlobalProxy> global_proxy) {
  // ...
  inner_global->set_global_context(*global_context());
  // ...
}

 GCC inlines method GlobalObject::set_global_context, which is defined
through macroses ACCESSORS[3] and WRITE_FIELD[4]

  void GlobalObject::set_global_context(Context* value, WriteBarrierMode mode) {
    WRITE_FIELD(this, offset, value);
    CONDITIONAL_WRITE_BARRIER(this, offset, mode);
  }

#define FIELD_ADDR(p, offset) \
  (reinterpret_cast<byte*>(p) + offset - kHeapObjectTag)

#define WRITE_FIELD(p, offset, value) \
  (*reinterpret_cast<Object**>(FIELD_ADDR(p, offset)) = value)

and removes memory store which should be generated from WRITE_FIELD.

I am trying to figure out why GCC decides that this memory store can
be safely removed.

Thanks.

[1] http://v8.googlecode.com
[2] http://code.google.com/p/v8/source/browse/trunk/src/bootstrapper.cc#615
[3] http://code.google.com/p/v8/source/browse/trunk/src/objects-inl.h#80
[4] http://code.google.com/p/v8/source/browse/trunk/src/objects-inl.h#743

--
Vyacheslav Egorov,
Software Engineer, Google Denmark ApS.