64-unclean code in eval.c

[Aidan Kehoe]

 Ar an ceathrú lá déag de mí Meitheamh, scríobh Ben Wing: 

 > i'm staring at this code and i don't see what's wrong even on 64-bit 
 > platforms.  i'm pretty sure i wrote this code.  it is used for saving 
 > and restoring internal C variables of type `int' and it tries to put it 
 > into a Lisp int but it's careful to use a opaque pointer in the rare 
 > instance where the value won't fit into a Lisp int.  the code is careful 
 > with its casting so i don't see what the problem is.
 > 
 > if the problem is this line:

[It is.]

 >       val = (int) get_opaque_ptr (lval);
 > 
 > 
 > then something is too smart for its own good.

Yes, the compiler is. But hey, it’s just software. 

 > i would not recommend Aidan's patch because i think it's too obscure.  
 > instead, put in a comment to the effect that we know the value stored in 
 > the void * is a crypto-int since we put an int into it in the first 
 > place, and i'd use more clever casts to get rid of the problem.  for 
 > example, cast to (long) and then to (int).

On AMD64, every C int (32 value bits) will fit into a Lisp int (63 value
bits). So no value other than a Lisp int need ever be saved, and no value
other than a Lisp int need be restored. I believe long is also 32 bits
there; maybe a cast to long long and then a cast to int would work.  

 > btw how can this be an error under C++?  when i used to do devwork i 
 > always ran a C++ build as part of my tests, and unless something changed 
 > recently, this code has been around for long before then.

The change is the availablility of the AMD64 platform and its
pointer-bigger-than-int approach. If XEmacs had been running and regularly
compiled under DOS we probably wouldn’t be seeing these errors. 

 > and if it is indeed a problem under C++, use this:
 > 
 >       val = reinterpret_cast<int> get_opaque_ptr (lval);
 > 
 > 
 > that's the C++ way of saying "abandon hope all ye who enter here" :)

Good to know. 

-- 
On the quay of the little Black Sea port, where the rescued pair came once
more into contact with civilization, Dobrinton was bitten by a dog which was
assumed to be mad, though it may only have been indiscriminating. (Saki)