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bug#22689: [PATCH] Add logcount
From: |
Mark Oteiza |
Subject: |
bug#22689: [PATCH] Add logcount |
Date: |
Sat, 30 Sep 2017 09:16:36 -0400 |
User-agent: |
NeoMutt/20170912-48-0df7d3-dirty |
On 30/09/17 at 02:42pm, Eli Zaretskii wrote:
> > +#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)
I guess it comes from nowhere, that condition and the two helper
functions come from here
https://rosettacode.org/wiki/Population_count#C
> > +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?
Probably, I don't know--I'm out of my comfort zone here.
> > +#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?
True. Should I instead be doing
XINT (value) <= xxx_MAX &&
XINT (value) >= xxx_MIN
or might there be a better check? For negative values popcount
typically counts ones of the two's complement
- bug#22689: [PATCH] Add logcount, Mark Oteiza, 2017/09/29
- bug#22689: [PATCH] Add logcount, Eli Zaretskii, 2017/09/30
- bug#22689: [PATCH] Add logcount,
Mark Oteiza <=
- bug#22689: [PATCH] Add logcount, Eli Zaretskii, 2017/09/30
- bug#22689: [PATCH] Add logcount, Mark Oteiza, 2017/09/30
- bug#22689: [PATCH] Add logcount, Eli Zaretskii, 2017/09/30
- bug#22689: [PATCH] Add logcount, Mark Oteiza, 2017/09/30
- bug#22689: [PATCH] Add logcount, Eli Zaretskii, 2017/09/30
- bug#22689: [PATCH] Add logcount, Mark Oteiza, 2017/09/30
- bug#22689: [PATCH] Add logcount, Eli Zaretskii, 2017/09/30
- bug#22689: [PATCH] Add logcount, Mark Oteiza, 2017/09/30
- bug#22689: [PATCH] Add logcount, Benjamin Riefenstahl, 2017/09/30
- bug#22689: [PATCH] Add logcount, Mark Oteiza, 2017/09/30