On 9/10/19 1:36 AM, Uros Bizjak wrote: > On Mon, Sep 9, 2019 at 8:44 PM Jeff Law <l...@redhat.com> 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 <l...@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. > > 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. jeff
