On Mon, Oct 24, 2016 at 02:44:37PM +0100, James Greenhalgh wrote:
>
> Hi,
>
> I'm adapting this patch from work started by Matthew Wahab.
>
> Conversions from double precision floats to the ARM __fp16 are required
> to round only once. A conversion function for double to __fp16 to
> support this on soft-fp targets. This and the following patch add this
> conversion function by reusing the exising float to __fp16 function
> config/arm/fp16.c:__gnu_f2h_internal.
>
> This patch generalizes __gnu_f2h_internal by adding a specification of
> the source format and reworking the code to make use of it. Initially,
> only the binary32 format is supported.
>
> Bootstrapped on an ARMv8-A machine with no issues, and cross-tested with
> a reasonable multi-lib range.
>
> 2016-10-24 James Greenhalgh <[email protected]>
> Matthew Wahab <[email protected]>
>
> * config/arm/fp16.c (struct format): New.
> (binary32): New.
> (__gnu_float2h_internal): New. Body moved from
> __gnu_f2h_internal and generalize.
> (_gnu_f2h_internal): Move body to function __gnu_float2h_internal.
> Call it with binary32.
Looking at it carefully, this patch has a bug in the way it handles
rounding for doubles. The relevant code looks like this:
if (aexp < -14)
{
mask = point | (point - 1);
/* Minimum exponent for half-precision is 2^-24. */
if (aexp >= -25)
mask >>= 25 + aexp;
}
else
mask = 0x00001fff;
/* Round. */
if (mantissa & mask)
{
increment = (mask + 1) >> 1;
if ((mantissa & mask) == increment)
increment = mantissa & (increment << 1);
mantissa += increment;
if (mantissa >= (point << 1))
{
mantissa >>= 1;
aexp++;
}
}
But clearly that mask is not going to be sufficient when we are deciding
whether we ought to round a double or not as there are many more bits for
us to look at.
I'll work on a new spin of this patch.
I'm still hopeful that both this and the AArch64 (and generic bits and
pieces) support for _Float16 will make it for GCC 7.
Thanks,
James
