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.
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.
The SSE2 plan sounds good. Though, I'd need some help with the
implementation. My x86 asm skills are pretty weak.
-Brian
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