[Mesa-dev] [PATCH 1/2] util/u_math: Implement a logbase2 function for unsigned long

[Karol Herbst]

On Wed, May 23, 2018 at 1:12 PM, Eric Engestrom
<eric.engestrom at intel.com> wrote:
> On Tuesday, 2018-05-22 23:06:16 +0200, Karol Herbst wrote:
>> From: Pierre Moreau <pierre.morrow at free.fr>
>>
>> Signed-off-by: Karol Herbst <kherbst at redhat.com>
>> ---
>>  src/gallium/auxiliary/util/u_math.h | 55 +++++++++++++++++++++++++++++
>>  src/util/bitscan.h                  | 11 ++++++
>>  2 files changed, 66 insertions(+)
>>
>> diff --git a/src/gallium/auxiliary/util/u_math.h b/src/gallium/auxiliary/util/u_math.h
>> index 46d02978fd6..504d67664b2 100644
>> --- a/src/gallium/auxiliary/util/u_math.h
>> +++ b/src/gallium/auxiliary/util/u_math.h
>> @@ -421,6 +421,23 @@ util_logbase2(unsigned n)
>>  #endif
>>  }
>>
>> +static inline uint64_t
>> +util_logbase2_64(uint64_t n)
>> +{
>> +#if defined(HAVE___BUILTIN_CLZLL)
>> +   return ((sizeof(uint64_t) * 8ll - 1ll) - __builtin_clzll(n | 1ll));
>> +#else
>> +   uint64_t pos = 0ll;
>> +   if (n >= 1ll<<32ll) { n >>= 32ll; pos += 32ll; }
>> +   if (n >= 1ll<<16ll) { n >>= 16ll; pos += 16ll; }
>> +   if (n >= 1ll<< 8ll) { n >>=  8ll; pos +=  8ll; }
>> +   if (n >= 1ll<< 4ll) { n >>=  4ll; pos +=  4ll; }
>> +   if (n >= 1ll<< 2ll) { n >>=  2ll; pos +=  2ll; }
>> +   if (n >= 1ll<< 1ll) {             pos +=  1ll; }
>                 ^^
> `ll` is being added a bit too much (although it's harmless); for
> instance, in this function I believe this one is the only one that does
> anything, and technically only on the first line (`1ll<<32`).
>
> They should be unsigned though, ie. `ull`; with that:
> Reviewed-by: Eric Engestrom <eric.engestrom at intel.com>
>

yeah... seems like I didn't look careful enough. I will simply remove
all "ll"s except the one from 1ll where I do a 1ull instead. It only
really matters there and the code is easier to read with the ll
removed.

>> +   return pos;
>> +#endif
>> +}
>> +
>>  /**
>>   * Returns the ceiling of log n base 2, and 0 when n == 0. Equivalently,
>>   * returns the smallest x such that n <= 2**x.
>> @@ -434,6 +451,15 @@ util_logbase2_ceil(unsigned n)
>>     return 1 + util_logbase2(n - 1);
>>  }
>>
>> +static inline uint64_t
>> +util_logbase2_ceil64(uint64_t n)
>> +{
>> +   if (n <= 1ll)
>> +      return 0ll;
>> +
>> +   return 1ll + util_logbase2_64(n - 1ll);
>> +}
>> +
>>  /**
>>   * Returns the smallest power of two >= x
>>   */
>> @@ -465,6 +491,35 @@ util_next_power_of_two(unsigned x)
>>  #endif
>>  }
>>
>> +static inline uint64_t
>> +util_next_power_of_two64(uint64_t x)
>> +{
>> +#if defined(HAVE___BUILTIN_CLZLL)
>> +   if (x <= 1ll)
>> +       return 1ll;
>> +
>> +   return (1ll << ((sizeof(uint64_t) * 8ll) - __builtin_clzll(x - 1ll)));
>> +#else
>> +   uint64_t val = x;
>> +
>> +   if (x <= 1ll)
>> +      return 1ll;
>> +
>> +   if (util_is_power_of_two_or_zero64(x))
>> +      return x;
>> +
>> +   val--;
>> +   val = (val >> 1ll)  | val;
>> +   val = (val >> 2ll)  | val;
>> +   val = (val >> 4ll)  | val;
>> +   val = (val >> 8ll)  | val;
>> +   val = (val >> 16ll) | val;
>> +   val = (val >> 32ll) | val;
>> +   val++;
>> +   return val;
>> +#endif
>> +}
>> +
>>
>>  /**
>>   * Return number of bits set in n.
>> diff --git a/src/util/bitscan.h b/src/util/bitscan.h
>> index 5cc75f0beba..310b4ed2e37 100644
>> --- a/src/util/bitscan.h
>> +++ b/src/util/bitscan.h
>> @@ -123,6 +123,17 @@ util_is_power_of_two_or_zero(unsigned v)
>>     return (v & (v - 1)) == 0;
>>  }
>>
>> +/* Determine if an uint64_t value is a power of two.
>> + *
>> + * \note
>> + * Zero is treated as a power of two.
>> + */
>> +static inline bool
>> +util_is_power_of_two_or_zero64(uint64_t v)
>> +{
>> +   return (v & (v - 1ll)) == 0ll;
>> +}
>> +
>>  /* Determine if an unsigned value is a power of two.
>>   *
>>   * \note
>> --
>> 2.17.0
>>
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