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Comment by davidsiems | Hacker News Reader

I think the 'best' trick I've seen is using pointer tagging on an object's virtual function table pointer to squeeze in an extra flag during garbage collection.

Adding another variable was thrashing the cache, so instead the GC would tag the VFT pointer (making it unusable obviously) and then untag it before GC ended, fixing the object.

I wasn't sure if I should be horrified or applaud when I found out about this.

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I can't recall the exact details, but just days or maybe a week before gold master of one title I worked on, there was a case where an object's virtual table was getting munged somehow. I do remember I managed to figure out exactly what was happening. But the amount and type of work it would take to fix would have likely delayed shipping.

So I wrote some code that constructed a new object of the same type on the stack, then

  memcpy(&realObject, &stackObject, 
         (char * )&stackObject.m_firstDataMember - (char * )&stackObject));

It worked. I'm not proud of it, but I am amused by it. I'm sure it wasn't guaranteed to work by any standard, but in practice worked fine for us.

I once used a similar trick combining C++ placement new and multiple classes to mimic several possible values of refcounters to simulate reference counting without a data member (for instance the object is created at C1, AddRef news it in place to C2, then C3, then Release news it again back to C2). When you have only a few possible values of refcounters and the object reuse (for instance if these represent oft-used values) warrants it, this can be used to save memory...

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