llvm dev - Jan 2015 - [LLVMdev] [PATCH] [ADT] APFloat - Fix sign handling for FMA results that truncate to zero.

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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OK I see now, your change enforce the “(exactly)” in the comment.

The commit LGTM.

Some comments:

I would still love to see a comment in the unit test file (as it is done earlier
in the file for the other tests).

Also the unit test does not test the rmTowardNegative case. It seems to be never
tested by the validation. I changed

sign = (rounding_mode == rmTowardNegative);

for 

sign = 0; 

and ninja check-all is still passing.

Mehdi

> On Jan 2, 2015, at 4:59 PM, Lang Hames <lhames at gmail.com> wrote:
> 
> 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
<mailto:mehdi.amini at apple.com>> wrote:
> Hi Lang,
> 
>> On Jan 1, 2015, at 5:57 PM, Lang Hames <lhames at gmail.com
<mailto: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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Committed with additional tests and comments as requested in r225123.

Thanks for the review Mehdi.

- Lang.

On Fri, Jan 2, 2015 at 7:26 PM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
> OK I see now, your change enforce the “(exactly)” in the comment.
>
> The commit LGTM.
>
> Some comments:
>
> I would still love to see a comment in the unit test file (as it is done
> earlier in the file for the other tests).
>
> Also the unit test does not test the rmTowardNegative case. It seems to
> be never tested by the validation. I changed
>
> sign = (rounding_mode == rmTowardNegative);
>
> for
>
> sign = 0;
>
> and ninja check-all is still passing.
>
> Mehdi
>
>
> On Jan 2, 2015, at 4:59 PM, Lang Hames <lhames at gmail.com> wrote:
>
> 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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