Subject: Re: bug#22689: [PATCH] Add logcount

From: Eli Zaretskii <[email-masked]>
Date: Sat, 30 Sep 2017 14:42:51 +0300

> Date: Fri, 29 Sep 2017 13:41:34 -0400
> From: Mark Oteiza <[email-masked]>
> 
> I made an attempt implementing this:

Thanks.  I'm not an expert on these ops, but otherwise the code looks
OK to me, modulo the minor comments below.

This will eventually need a NEWS entry and some docs in the ELisp
manual.

> +#if defined (__POPCNT__) && defined (__GNUC__) && (__GNUC__> 4 || (__GNUC__== 4 && __GNUC_MINOR__> 1))
> +#define HAVE_BUILTIN_POPCOUNTLL
> +#endif

Where does __POPCNT__ definition come from?  I don't see it in my
GCC's "gcc -dM" output.

As for the rest of the condition, I think you should use GNUC_PREREQ,
like this:

 #if GNUC_PREREQ (4, 1, 0)

> +static uint64_t
> +bitcount64 (uint64_t b)
> +{
> +  b -= (b >> 1) & 0x5555555555555555;
> +  b = (b & 0x3333333333333333) + ((b >> 2) & 0x3333333333333333);
> +  b = (b + (b >> 4)) & 0x0f0f0f0f0f0f0f0f;
> +  return (b * 0x0101010101010101) >> 56;

Shouldn't these constants have a ULL suffix, to make sure they are not
truncated to a 32-bit int?

> +#else  /* HAVE_BUILTIN_POPCOUNTLL */
> +  if (XINT (value) <= UINT_MAX)
> +    XSETINT (res, bitcount32 (XUINT (value)));
> +  else if (XINT (value) <= ULONG_MAX)
> +    XSETINT (res, bitcount64 (XUINT (value)));

The comparisons against Uxxx_MAX seem to assume that VALUE is
unsigned, but that's not guaranteed, right?