[Mesa-dev] IROUND() issue

[Brian Paul]

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.

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.


Comments?

-Brian