• oliver@apple.com's avatar
    fourthTier: Structure transition table keys don't have to ref their StringImpl's · 36e01a77
    oliver@apple.com authored
    https://bugs.webkit.org/show_bug.cgi?id=115525
    
    Reviewed by Geoffrey Garen.
    
    The structure transition table basically maps string to structure. The string is
    always also stored, and ref'd, in the structure in Structure::m_nameInPrevious.
    m_nameInPrevious is never mutated, and never cleared. The string cannot die unless
    the structure dies. If the structure dies, then that entry in the transition map
    becomes a zombie anyway and we will detect this separately.
    
    So, we don't need to use RefPtr<StringImpl>. We can just use StringImpl*.
    
    This also fixes a goof where we were getting the StringImpl's hash rather than
    using a pointer hash. Not only is the latter faster, but it prevents my change
    from leading to crashes: with my change we can have zombie keys, not just zombie
    values. They will exist only until the next map mutation, which will clear them.
    Lookups will work fine because the lookup routine will reject zombies. But it
    does mean that the HashMap will have to deal with dangling StringImpl*'s; all it
    takes to make this work is to ensure that the HashMap itself never dereferences
    them. Using a pointer hash rather than StringImpl::existingHash() accomplishes
    this.
    
    This also ensures that we don't accidentally call ref() or deref() from the
    compilation thread, if the compilation thread inspects the transition table.
    
    And no, we wouldn't have been able to use the HashMap<RefPtr<...>, ...>
    specialization, because the transition table is actually
    HashMap<pair<RefPtr<StringImpl>, unsigned>, ...>: hence that specialization
    doesn't kick in. We could have written a new specialization or something, but
    that seemed like a lot of work given that we don't need the table to be ref'ing
    the strings anyways.
    
    * runtime/Structure.cpp:
    (JSC::StructureTransitionTable::add):
    * runtime/StructureTransitionTable.h:
    (StructureTransitionTable):
    (Hash):
    (JSC::StructureTransitionTable::Hash::hash):
    
    git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153141 268f45cc-cd09-0410-ab3c-d52691b4dbfc
    36e01a77
ChangeLog 286 KB