On 07/12/2018 03:03, Matt Turner wrote: > On Wed, Dec 5, 2018 at 7:56 AM Samuel Iglesias Gonsálvez > <sigles...@igalia.com> wrote: >> >> Signed-off-by: Samuel Iglesias Gonsálvez <sigles...@igalia.com> >> --- >> src/util/Makefile.sources | 2 + >> src/util/double.c | 197 ++++++++++++++++++++++++++++++++++++++ >> src/util/double.h | 46 +++++++++ >> src/util/meson.build | 2 + >> 4 files changed, 247 insertions(+) >> create mode 100644 src/util/double.c >> create mode 100644 src/util/double.h > > Why do we need software routines for this? Couldn't we set/reset the > rounding mode (fegetround/fegetround) around a double -> float > conversion? >
Yes, that was my first idea. However, I found an issue with GCC that
forces me to do these software routines instead.
I implemented _mesa_double_to_float_rtz() as:
float
_mesa_double_to_float_rtz(double val)
{
int curr_method = fegetround();
float result;
fesetround(FE_TOWARDZERO);
result = val;
fesetround(curr_method);
return result;
}
If I add a printf, I got the proper value. However if I remove it, the
result value is using the default's rounding mode (FE_TONEAREST). I
think it is reordering instructions or optimizing something. I have set
#pragma STDC FENV_ACCESS ON but it doesn't work either.
Updated news: I give it another spin. I defined result as volatile
variable and that seems to work :) My GCC knowledge is limited, so if
someone brings a better idea to fix this, I will be delighted to test
it. If not, I will send a v2 version with the volatile and
fesetround()/fegetround() functions.
Thanks for the feedback!
Sam
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