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