[LLVMdev] [PATCH] [ADT] APFloat - Fix sign handling for FMA results that truncate to zero.

[Lang Hames]

Hi Mehdi,

Thanks for the feedback. I think the comment is fine - the problem is just
that the original conditional didn't exactly match it. With the addition of
the underflow check I believe the condition now matches the comment: The
sign is only tweaked if the result of the addition is exactly zero, rather
than a small result that truncates to zero.

CC'ing llvm-commits, where this email should have gone in the first place.
:)

Cheers,
Lang.

On Thu, Jan 1, 2015 at 10:56 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:

> Hi Lang,
>
> On Jan 1, 2015, at 5:57 PM, Lang Hames <lhames at gmail.com> wrote:
>
> Hi All,
>
> APFloat::fusedMultiplyAdd currently computes the wrong signed zero when
> small negative results are truncated back to zero in standard precision.
> The following snippet handles the signedness in fusedMultiplyAdd:
>
> /* If two numbers add (exactly) to zero, IEEE 754 decrees it is a
>    positive zero unless rounding to minus infinity, except that
>
>    adding two like-signed zeroes gives that zero.  */
> if (category == fcZero && sign != addend.sign)
>   sign = (rounding_mode == rmTowardNegative);
>
> The test "category == fcZero" tells us that the result was zero after
> rounding back down to standard precision, but since the addition is carried
> out in extended precision this doesn't guarantee that the result of the
> addition was exactly zero (so the comment text may not apply). The attached
> patch adds a check for underflow during truncation which ensures the
> correct signedness of the result.
>
>
>
> According to what you describe ("the comment text may not apply"), I feel
> that the comment might be updated as well.
>
> Best,
>
> Mehdi
>
>
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