[Mesa-dev] [PATCH] Add an accelerated version of F_TO_I for x86_64

[Roland Scheidegger]

Looking all good, though it got me thinking about these numerous
float-to-int rounding functions in mesa in various places a bit more.
There is, for instance, a _mesa_round_to_even() function, which claims
that the c99 lrintf() and friends functions can't be used because the
environment might use a different rounding mode. This is of course true
though I wonder why it would matter there and not here for instance,
since of course cvtss2si is affected by that as well.
Also, I'm wondering about the IROUND fallback. Richard Sandiford
discovered a problem in llvmpipe (using lp_build_iround()) which was
using the same method (that is, (int)(val + 0.5) for unsigned numbers,
similar but more complex for signed numbers). This works ok for nearly
all numbers (though it is definitely not round to nearest even) except
numbers between [-]2^23 and [-]2^24-1, in which case it will always
return the next higher even number for odd numbers. So
IROUND((float)val) != val even for numbers which can be represented as
floats exactly. I'd guess though mesa probably doesn't use IROUND for
cases where this would matter (most likely some conversion of z24
numbers), worst case there would probably not just be the inaccuracy but
if you'd have clamped to z24 max (as float, 2^24-1) then done the IROUND
you'd get back 2^24. And FWIW some gallium util math function seem to
have this problem as well, though again I don't know if it would matter
(the gallium util code will only use them if c99 isn't available).
But anyway I guess that's slightly off-topic...

Roland


Am 23.07.2014 05:15, schrieb Jason Ekstrand:
> According to a quick micro-benchmark, this new version is 20% faster on my
> Haswell laptop.
> 
> v2: Removed the XXX note about x86_64 from the comment
> 
> Signed-off-by: Jason Ekstrand <jason.ekstrand at intel.com>
> ---
>  src/mesa/main/imports.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/src/mesa/main/imports.h b/src/mesa/main/imports.h
> index af780b2..c8ae7f2 100644
> --- a/src/mesa/main/imports.h
> +++ b/src/mesa/main/imports.h
> @@ -277,7 +277,6 @@ static inline int IROUND_POS(float f)
>  
>  /**
>   * Convert float to int using a fast method.  The rounding mode may vary.
> - * XXX We could use an x86-64/SSE2 version here.
>   */
>  static inline int F_TO_I(float f)
>  {
> @@ -285,6 +284,10 @@ static inline int F_TO_I(float f)
>     int r;
>     __asm__ ("fistpl %0" : "=m" (r) : "t" (f) : "st");
>     return r;
> +#elif defined(USE_X86_64_ASM) && defined(__GNUC__)
> +   int r;
> +   __asm__ ("cvtss2si %1, %0" : "=r" (r) : "xm" (f));
> +   return r;
>  #elif defined(USE_X86_ASM) && defined(_MSC_VER)
>     int r;
>     _asm {
>