[Mesa-dev] IROUND() issue

[Jose Fonseca]

----- Original Message ----- 

> On Fri, May 18, 2012 at 11:28 AM, Brian Paul < brianp at vmware.com >
> wrote:

> > On 05/18/2012 10:11 AM, Jose Fonseca wrote:
> 

> > > ----- 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.
> > 
> 

> The instruction is "cvtss2si". 

Yep. Thanks.

> Even if you use SSE here, you depend
> on the rounding mode in the MXCSR register, which means you'll have
> to set that, because some applications change this mode to use a
> faster or more precise rounding mode. It's the parallel problem that
> you have with "fistp".

I think it is unrealistic for application developers to tamper with rounding mode, and expect OpenGL implementations (or any C library for that matter) to be unaffected. Realistically, the only place where this stuff can be safely done is in self-contained application code.

Furthermore, it would be a lot of work to reset it all the time, and the result slow. I think there's tones of more important stuff for us to worry about.


Jose