Re: [Emacs-diffs] master f18af6c: Audit use of lsh and fix glitches

[Paul Eggert]

Pip Cet wrote:

> Maybe we ought to warn (in debug builds) about eq being used to
> compare several integers, at least one of which is outside a narrow
> interval of guaranteed fixnums (and ditto for memq and hashtest_eq). I
> wonder whether Emacs would even build with MOST_POSITIVE_FIXNUM =
> 0x100000, but it might be worth it to try and run it that way...

We get some of that effect already in 32- vs 64-bit builds. Perhaps you're right 
and we would benefit from artificially small fixnums, or from debug builds that 
warn about eq misuse.

> That said, vc-hg.el doesn't contain any suspicious-looking instances
> of `eq', `memq', or `hash'
Unfortunately that doesn't seem to be quite right. I looked and found one 
incorrect (though quite unlikely to fail) use of eq. I changed it to eql as part 
of the attached patch, which I installed.

> As for `eq' and `eql', what I thought Stefan proposed was to keep a
> hash value in struct Lisp_Bignum, which is calculated lazily the first
> time the bignum is compared to another bignum with `eq', at which
> point we might as well point out to the user that portable code would
> require `eql'. That sounds to me like it would be much cheaper than a
> hash table, but if there are objections to it we can still implement
> it without guaranteeing it for the future in the documentation.

It might be helpful to have that in a debugging build.

I am still leaning more towards hash tables, though; at least, I want to time 
them to see how much they actually cost in typical practice. I suspect the cost 
is rather small, and if so it may well be worth it to avoid hassles like the 
eq-vs-eql glitch that I just now fixed in vc-hg.el.

0001-Make-vc-hg-safe-for-bignums.patch
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