Re: [PATCH V2] powerpc: properly check for feenableexcept() on FreeBSD

[Segher Boessenkool <segher@kernel.crashing.org>]

On Fri, May 13, 2022 at 12:34:05PM +0200, Piotr Kubaj wrote:
> On 22-05-13 10:59:59, Kewen.Lin wrote:
> > on 2022/5/13 04:16, Segher Boessenkool wrote:
> > > On Tue, May 03, 2022 at 12:21:12PM +0200, pkubaj@FreeBSD.org wrote:
> > >> FreeBSD/powerpc* has feenableexcept() defined in fenv.h header.
> > > 
> > > Declared, not defined.  These are required to be real functions (on all
> > > platforms that have these functions), not macros or inlines or whatever.
> > > 
> > 
> > Piotr's reply "FreeBSD doesn't have this function in libm, it's
> > implemented in /usr/include/fenv.h." from [1] made me feel like
> > it's a definition instead of declaration.  So I thought the check
> > should use AC_LINK_IFELSE instead, since one fenv.h which doesn't
> > have the definition can still pass the proposed AC_COMPILE_IFELSE
> > check.
> > 
> > I just did a further search, the powerpc fenv.h [2] does include
> > the definition of feenableexcept.  By comparison, the x86 fenv.h [3]
> > doesn't.  But I'm not sure if it's the same as what Piotr's
> > environments have.  Hope it's similar. :-)
> > 
> > [1] https://gcc.gnu.org/pipermail/gcc-patches/2022-April/593193.html
> > [2] https://github.com/freebsd/freebsd-src/blob/main/lib/msun/powerpc/fenv.h
> > [3] https://github.com/freebsd/freebsd-src/blob/main/lib/msun/x86/fenv.h
> 
> Yes, it's a definition and thanks for confirming that. As for why it's not in libm, I asked a developer about that:
> 03:04 <@adalava> It shouldn't be difficulted but I moved to other thing after months looking at FPE in kernel, bugs in context switch and msun test cases failing :-P
> 
> As far as I know, there are currently no plans to move it to libm on powerpc. riscv, arm and arm64 are in the same boat.
> 
> I will follow with a next patch that will check for feenableexcept() in fenv.h if libm check is unsuccessful.

FreeBSD's own documentation
(<https://www.freebsd.org/cgi/man.cgi?query=feenableexcept>) says it is
a function, and it suggests it is in the standard library even.  This
would be as expected, the same holds for the similar C standard
functions.

This <fenv.h> does not provide an external definition, only a static
inline (this is a good thing for a header file of course).

You can take the address of a function.  You also can interpose a real
function normally, but that may not be relevant for FreeBSD, no idea.
You can do neither with a static inline.

So this should probably be fixed?

Thanks for explaining the situation better!


Segher