[PATCH] soft-fp: fix sticky bit in _FP_MUL_MEAT_2_120_240_double [BZ #34640]

[Adhemerval Zanella Netto]

On 25/09/26 07:39, Stian Halseth wrote:
> Hi Adhemerval
> 
> You're pushing this, right? I don't have commit access.
> 
> Best regards,
> Stian
> 
> 

I have pushed this on master, thanks!

> On Thu, 2026-09-17 at 19:47 +0200, Stian Halseth wrote:
>> On Thu, 2026-09-17 at 17:22 +0000, Adhemerval Zanella Netto wrote:
>>>
>>>
>>> On 16/09/26 13:17, Stian Halseth wrote:
>>>> When R##_f0 is assembled the _o240 chunk is shifted right by
>>>> (wfracbits - 1) - 96 bits, which is 19 for the quad format. 
>>>> Those
>>>> bits
>>>> are discarded without being folded into the sticky bit _y240,
>>>> which
>>>> is
>>>> computed only from _s240, _r240, _q240 and _p240, so a product
>>>> whose
>>>> only inexactness lies in the discarded bits is reported as exact.
>>>>
>>>> sparc64 is the only user of this macro.  There _Qp_mul does not
>>>> raise
>>>> FE_INEXACT, and in the directed rounding modes the result is one
>>>> ulp
>>>> short because the rounding step sees no sticky bit;
>>>> math/test-float64x-float128-mul reports 65 errors.
>>>>
>>>> Fold the discarded bits into _y240.
>>>>
>>>> Tested on sparc64: math/test-float64x-float128-mul goes from 65
>>>> errors
>>>> to none, the macro then agrees bit for bit with
>>>> _FP_MUL_MEAT_2_wide
>>>> on
>>>> 200000 random operand pairs, and full make check shows no test
>>>> that
>>>> passed before failing after.
>>>>
>>>> Signed-off-by: Stian Halseth <stian@itx.no>
>>>
>>> LGTM, thanks.  It would good to send a similar fix to gcc/libgcc,
>>> since
>>> they share the implementation.
>>
>> Thanks, will do!
>>>
>>> Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
>>>
>>>> ---
>>>>  soft-fp/op-2.h | 4 ++++
>>>>  1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/soft-fp/op-2.h b/soft-fp/op-2.h
>>>> index 8778af21..760ca9b0 100644
>>>> --- a/soft-fp/op-2.h
>>>> +++ b/soft-fp/op-2.h
>>>> @@ -515,6 +515,10 @@
>>>>        _v240 = _m240.i; \
>>>>        _w240 = _n240.i; \
>>>>        _x240 = _o240.i; \
>>>> +      /* The low bits of the _o240 chunk are shifted out when
>>>> R##_f0 \
>>>> + is assembled; fold them into the sticky bit.  */ \
>>>> +      if ((_x240 & ((((UDItype) 1) << ((wfracbits) - 97)) - 1))
>>>> !=
>>>> 0) \
>>>> + _y240 = 1; \
>>>>        R##_f1 = ((_t240 << (128 - (wfracbits - 1))) \
>>>>   | ((_u240 & 0xffffff) >> ((wfracbits - 1) - 104))); \
>>>>        R##_f0 = (((_u240 & 0xffffff) << (168 - (wfracbits - 1)))
>>>> \
>>>