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

[Mehdi Amini]

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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