Re: Making 'eq' == 'eql' in bignum branch

[Paul Eggert]

Herring, Davis wrote:
> this is far from the only place in Emacs where what is formally undefined behavior is 
> expected to "do the obvious thing"

Emacs does not rely on undefined behavior for integer arithmetic. Even in Emacs 
26, (abs most-negative-fixnum) yields most-negative-fixnum without doing 
anything undefined at the C level, because (- most-negative-fixnum) fits within 
machine limits and the tagging operation does not overflow.

Although there may be a few places left in Emacs that rely on undefined integer 
behavior, these are now considered bugs and occasionally I try to stamp them out 
by building with gcc -fsanitize=undefined. For example, in Emacs 26 (* 
most-positive-fixnum most-positive-fixnum) returns 1 (the overflowed value) 
without overflowing at the C level.

Emacs *does* rely on undefined behavior when it tags pointers, but that's a 
different kettle of fish.