Re: [RFC][PATCH 0/3] gcc work-around and math128

[Andy Lutomirski <luto@amacapital.net>]

On 04/24/2012 09:10 AM, Peter Zijlstra wrote:
> Hi all,
> 
> The SCHED_DEADLINE review resulted in the following three patches;
> 
> The first is a cleanup of various copies of the same GCC loop optimization
> work-around. I don't think this patch is too controversial, at worst I've
> picked a wrong name, but I wanted to get it out there in case people
> know more sites.
> 
> The second two implement a few u128 operations so we can do 128bit math.. I
> know a few people will die a little inside, but having nanosecond granularity
> time accounting leads to very big numbers very quickly and when you need to
> multiply them 64bit really isn't that much.

I played with some of this stuff awhile ago, and for timekeeping, it
seemed like a 64x32->96 bit multiply followed by a right shift was
enough, and that operation is a lot faster on 32-bit architectures than
a full 64x64->128 multiply.  Something like:

uint64_t mul_64_32_shift(uint64_t a, uint32_t mult, uint32_t shift)
{
  return (uint64_t)( ((__uint128_t)a * (__uint128_t)mult) >> shift );
}

or (untested, but compilable 32-bit gcc)

uint64_t mul_64_32_shift(uint64_t a, uint32_t mult, uint32_t shift)
{
  uint64_t part1 = ((a & 0xFFFFFFFFULL) * mult) >> shift;
  uint64_t part2 = ((a >> 32) * mult) << (32 - shift);
  return part1 + part2;
}

--Andy