| Message ID | 585499fc52.Jo@hobbes.bass-software.com |
|---|---|
| State | New |
| Headers | show |
Hi John, > John Tytgat<John@bass-software.com> >> >> * config/arm/fp16.c (__gnu_f2h_internal): Fix inaccuracy. >> Approved and applied. Cheers Nick
Index: libgcc/config/arm/fp16.c =================================================================== --- libgcc/config/arm/fp16.c (revision 193830) +++ libgcc/config/arm/fp16.c (working copy) @@ -47,11 +47,9 @@ mantissa |= 0x00800000; if (aexp < -14) { - mask = 0x007fffff; - if (aexp < -25) - aexp = -26; - else if (aexp != -25) - mask >>= 24 + aexp; + mask = 0x00ffffff; + if (aexp >= -25) + mask >>= 25 + aexp; } else mask = 0x00001fff;
Ping ? Paul, seen that you've contributed fp16.c together with Sandra Loosemore, perhaps you can review this patch please ? John. In message <ab11eef452.Jo@hobbes.bass-software.com> John Tytgat <john@bass-software.com> wrote: > __gnu_f2h_internal in libgcc converts single-precision floating-point value > to half-precision value for ARM. I noticed there is a slight inaccuracy > for floating-point values 2^-25 + epsilon (with epsilon > 0 and < 2^-26). > Those should all be converted to 2^-24 in half-precision. > > And this because the mask used to implement the even-odd rounding for > aexp = -25 is wrong. Currently we have: > > /* Decimal point between bits 22 and 23. */ > mantissa |= 0x00800000; > if (aexp < -14) > { > mask = 0x007fffff; > if (aexp < -25) > aexp = -26; > else if (aexp != -25) > mask >>= 24 + aexp; > } > else > mask = 0x00001fff; > > But when aexp is 25 the mask should be 0xffffff instead of 0x7fffff as the > decimal dot in half-precision will be between bit 24 and 23 of the > above mentioned mantissa. Cfr. the even-odd rounding done: > > /* Round. */ > if (mantissa & mask) > { > increment = (mask + 1) >> 1; > if ((mantissa & mask) == increment) > increment = mantissa & (increment << 1); > mantissa += increment; > if (mantissa >= 0x01000000) > { > mantissa >>= 1; > aexp++; > } > } > > Attached patch solves this problem. I've left out the clamping of > aexp to -26 for values less than -25 as this it not necessary. After > the even-odd rounding all aexp values less than -24 will result in +0. or > -0. anyway. > > John Tytgat <John@bass-software.com> > > * config/arm/fp16.c (__gnu_f2h_internal): Fix inaccuracy. > > I've got a copyright assignment but no write access. > > John Tytgat.