Re: Conservative GC isn't safe

[Daniel Colascione <dancol@dancol.org>]

On 11/28/2016 10:00 AM, Eli Zaretskii wrote:
>> From: Daniel Colascione <dancol@dancol.org>
>> Date: Mon, 28 Nov 2016 09:51:37 -0800
>>
>> struct foo* f = something();
>> int* x = f->&field;
>> something_else(); // invalidate global memory
>> *x = 5; // f is dead here, but still in scope
>>
>> Even if you don't write this kind of code, the compiler is allowed to
>> generate it.
>
> But there's no such code in Emacs, and will never be.

I think you have too little faith in the ingenuity of compiler writers. 
Why can't the compiler generate this sort of code in cases we don't 
anticipate?

> Lisp objects we
> create are either temporaries that can be GC'ed, or values that cannot
> be GC'ed, in which case they are passed to some other code, either a
> callee or returned as a value.  The only ones that can be dead as
> above are the first variety, about which we don't care.

When this assumption stops holding, it's going to be very difficult to 
debug the resulting occasional crashes. Wouldn't it be easier to use the 
information *already in the memory tree* to make GC more conservative 
and understand interior pointers?