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Re: [PATCH] Simplify sinh (x) / cosh (x)

On 9/10/19 1:36 AM, Uros Bizjak wrote:
> On Mon, Sep 9, 2019 at 8:44 PM Jeff Law <> wrote:
>> On 9/4/19 12:16 PM, Rafael Tsuha wrote:
>>> Hi, Jeff
>>> Em seg, 29 de abr de 2019 às 18:22, Jeff Law <> escreveu:
>>>> On 1/22/19 12:31 PM, Rafael Tsuha wrote:
>>>>> This patch simplifies the expression sinh (x) / cosh (x) to tanh (x).
>>>>> This rule is mathematically valid.
>>>>> There's a slight difference in the result when applying this
>>>>> optimization with x in the interval 0 < x <= 1e-4951. With the
>>>>> optimization, the result using long double is -0 and without the
>>>>> optimization, the result is +0.
>>>> That's an indication something has gone wrong.
>>>> If I'm reading this correctly it sounds like tanh in that range is
>>>> returning -0?  If so, that just seems like the wrong output from tanh
>>>> since IIUC for a positive input tanh will always have a positive output.
>>> I reverted the patch sent to solve bug 88556 and found out that
>>> tanhl(0) started returning -0 after this patch.
>>> patch we reverted:
>>> (
>>> In the line 44480 of this patch, it checks the sign bit of the input
>>> and if it's false the expression is multiplied by -1. In the way it's
>>> being calculated, this should be done only if the input is a number
>>> greater than zero.
>>> If we follow the code starting at line 44468, replacing op1 with 0, we
>>> can see that e2 equals 0 at line 44482, flags will be false and
>>> finally the e2 = -e2 operation will be executed generating the -0
>>> result.
>>> I have implemented a testcase to reproduce the bug:
>>> this code was compiled with -Ofast when we tested it.
>>> Should I file a bug about this? And for fixing, Is it a good solution
>>> to emit an instruction to return zero immediately if the input equals
>>> zero?
>> So if I'm understanding Uros's patch correctly, it's supposed to only be
>> used for -ffast-math where we don't necessarily honor signed zeros.
> True. The full patch is at [1], where it is evident that all these
> expanders are protected by flag_unsafe_math_optimizations. As
> explained in the patch sumbission, the equations are ported from [2],
> so barring some unwanted bug in the porting, they should be equal. I
> didn't analyse the correctness of the original equations.
It (your patch) looked fine to me given the -ffast-math constraint.

I think the question we need to go back and answer is why the proposed
patch to improve sinh/cosh -> tanh is using those expanders in an
unexpected way.


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