[Mesa-dev] IROUND() issue

[Jose Fonseca]

----- Original Message -----
> 
> A while back I noticed that the piglit roundmode-pixelstore and
> roundmode-getinteger tests pass on my 64-bit Fedora system but fail
> on
> a 32-bit Ubuntu system.  Both glGetIntegerv() and glPixelStoref()
>  use
> the IROUND() function to convert floats to ints.
> 
> The implementation if IROUND() that uses the x86 fistp instruction is
> protected with:
> 
> #if defined(USE_X86_ASM) && defined(__GNUC__) && defined(__i386__)
> 
> but that evaluates to 0 on x86-64 (neither USE_X86_ASM nor __i386__
> are defined) so we use the C fallback:
> 
> #define IROUND(f)  ((int) (((f) >= 0.0F) ? ((f) + 0.5F) : ((f) -
> 0.5F)))
> 
> The C version of IROUND() does what we want for the piglit tests but
> not the x86 version.  I think the default x86 rounding mode is
> FE_UPWARD so that explains the failures.
> 
> 
> So I think I'd like to do the following:
> 
> 1. Enable the x86 fistp-based functions in imports.h for x86-64.

It's illegal/inneficient to use x87 on x86-64. We should use the appropriate SSE intrisinsic instead.

> 2. Rename IROUND() to IROUND_FAST() and define it as float->int
> conversion by whatever method is fastest.
> 
> 3. Define IROUND() as round to nearest int.  For the x86 fistp
> implementation this would involve setting/restoring the rounding
> mode.
> 
> 4. Do an inspection of current IROUND() calls and convert some to
> IROUND_FAST() where we can get away with it.

It sounds a sensible plan, but I wonder if the multiple iround versions are really worth it: SSE2 is guaranteed in x64, so we could just start requiring SSE2 on x86, and only use the C for other platforms.

Jose