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

[Paul Eggert]

Pip Cet wrote:

> I'm already using a separate tag value for bignums, though it might
> help to use tag value 0 instead of a nonzero one (but then Qnil
> couldn't be all-zeroes in memory).

We'd have to measure it, as there is also a benefit when NILP and initialization 
to Qnil are fast.

> I've discovered that the sequence gcc (trunk) emits for a mask-and-compare is:
>      mov %rax, %rbx
>      and $7, %rbx
>      cmp $7, %rbx
> where I would use
>      lea 1(%rax), %ebx
>      test $7, %bl
>
> I can force the latter by testing ((char)((int)(XLI (x) + 8 -
> Lisp_String)) & 7) == 0 rather than the (equivalent, AFAICS) (XLI (x)
> & 7) == Lisp_String (sometimes gcc uses a longer sequence, though). At
> this point, I think gcc should be emitting the shorter two-insn
> sequence for both C expressions.

I agree, and filed a GCC bug report about it, here:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87104

> With this "improvement", the code size difference is 13184 bytes. I'm
> unable to measure a performance difference, since it is lost in the
> noise.

I tried a similar microoptimization on the Emacs trunk, and it made Emacs about 
0.4% faster on my platform. This seems like a clear win pretty much everywhere, 
regardless of what we do about eq vs eql, so I installed the attached patch into 
master.

> I've benchmarked the hash-cons approach, though the implementation I
> used was rather trivial, and simple bignum calculations appear to be
> about 20-50% slower. Again, I expect that number to increase once the
> extra copy in make_number is removed. (For floats, the slowdown is
> worse.)

Ouch. These figures are making me more inclined to stick with what we have in 
master, namely the Common Lisp and Scheme tradition that eq is a finer 
comparison operation than eql is, when these comparisons are applied to bignums 
and/or to floats.  Of course we can continue to guarantee the traditional eq 
semantics on fixnums.

0001-Improve-performance-of-CONSP-FIXNUMP-etc.txt
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