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

[Jeff Law <law@redhat.com>]

On 9/4/19 12:16 PM, Rafael Tsuha wrote:
> Hi, Jeff
> 
> Em seg, 29 de abr de 2019 às 18:22, Jeff Law <law@redhat.com> 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:
> (https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/config/i386/i386.c?r1=267325&r2=267324&pathrev=267325)
> 
> 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:
> https://paste.debian.net/1098800/
> 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.

Are you applying the sinh/cosh -> tanh transformation only with
-ffast-math (it's been so long I simply can't remember).

jeff