On Wed, 26 Sep 2018 at 21:06, David Brown <david@xxxxxxxxxxxxxxx> wrote: > On 26/09/18 18:11, Kalamatee wrote: > > On Wed, 26 Sep 2018 at 16:44, David Brown <david@xxxxxxxxxxxxxxx > > <mailto:david@xxxxxxxxxxxxxxx>> wrote: > > On 26/09/2018 17:24, Kalamatee wrote: > > > On Wed, 26 Sep 2018 at 16:00, David Brown <david@xxxxxxxxxxxxxxx > > <mailto:david@xxxxxxxxxxxxxxx> > > > <mailto:david@xxxxxxxxxxxxxxx <mailto:david@xxxxxxxxxxxxxxx>>> wrote: > > > > > > On 26/09/18 14:54, Kalamatee wrote: > > > > Hi > > > > > > > > I am wondering if there is a reason globals marked const > > volatile > > > are put > > > > into the data section and not read-only as const should imply? > > > > > > > > > > "const" says /you/ promise not to change the value from your > > code. > > > "volatile" says that something else might change its value > > unknown to > > > the compiler. > > > > > > That’s not ‘all’ it says. To be precise volatile tells the compiler that > > the object is subject to sudden change for reasons which cannot be > > predicted from a study of the program itself*, and forces every > > reference to such an object to be a genuine reference.* > > > My only interest is in getting the issue resolved "correctly". > > > > For now I guess I will have to add the section attribute to all constant > > volatiles to force the correct thing to happen. > > ... > > Have you looked at what happens if you use 0.0 instead of this "volatile > const", to see that it is in some way necessary? > > > As I say, I am not an expert on floating point errors. But my feeling > here is that you will get exactly the same results in all cases - which > may not be the results you think you should get. > If it helps, this is likely related to a bugfix that went into freebsd's libm a while back after they switched to clang. I can't remember the details, but I believe the bug was to do with clang incorrectly constant-folding floating point expressions in the wrong rounding mode, or perhaps how it interacted with fesetround() and the fenv access pragmas. The mirror here [1] shows the fix. - Rob 1: https://github.com/freebsd/freebsd/commit/e7b0a63c190c7ecb475b09f41387a75952540f49
