Re: [PATCH] s390x: Add single instruction math functions

[Rich Felker <dalias@libc.org>]

On Tue, Jun 20, 2017 at 09:07:08PM -0400, David Edelsohn wrote:
> Rich,
> 
> Another option is a test recommended by a colleague
> 
> #if (__HTM__ || __ARCH__ > z196)
> 
> __HTM__ is defined in earlier releases of GCC and is enabled in zEC12,
> so it can be used as a proxy for the architecture in earlier compiler
> releases.
> 
> Would that be acceptable?

Indeed, __HTM__ gets defined for me on gcc 6.3 with -march=zEC12 but
not with -march=z196. If that's acceptable to you I think it's okay;
users could also build with "-march=z196 -D__ARCH__=???"  to get the
math insns on baseline z196. BTW what is the actual value for __ARCH__
indicating z196? I think I figured it out from the gcc source as 10.
The "z196" you used in the above example isn't really a macro, is it?
If so that's a bad namespace violation in gcc that needs to be
fixed...

Rich


> On Tue, Jun 20, 2017 at 8:49 PM, Rich Felker <dalias@libc.org> wrote:
> > On Sun, Jun 18, 2017 at 01:12:15PM -0400, David Edelsohn wrote:
> >> How can we move forward with this patch?
> >>
> >> I would prefer to avoid the __ARCH__ complexity until there is a clear
> >> user requirement.
> >>
> >> Thanks, David
> >
> > Rob Landley informed me that the s390x environment he's building with
> > mkroot (https://github.com/landley/mkroot) for testing under qemu
> > system level emulation is running a kernel built for z900. If qemu can
> > emulate newer machines, this may just be an oversight that can be
> > changes by reconfiguring, but it does indicate that z900 seems to be
> > supported by kernel, and that there's at least someone using the
> > baseline ISA level now.
> >
> > For what it's worth I agree that we've spent an inordinate amount of
> > time on this topic, and I apologize. I just don't want it to turn into
> > a regression.
> >
> > Rich
> >
> >
> >> On Wed, Jun 14, 2017 at 7:44 PM, Rich Felker <dalias@libc.org> wrote:
> >> > On Mon, Jun 12, 2017 at 09:28:52AM -0400, David Edelsohn wrote:
> >> >> >> The following IBM table of supported and tested systems
> >> >> >>
> >> >> >> https://www-03.ibm.com/systems/z/os/linux/resources/testedplatforms.html
> >> >> >>
> >> >> >> shows that RHEL 7 and SLES 12 require at least z196, and Ubuntu 16.04
> >> >> >> requires at least zEC12.
> >> >> >>
> >> >> >> I can't find any official hardware requirements description for Alpine
> >> >> >> Linux. I tend to doubt that user would run it on older hardware,
> >> >> >> especially hardware no longer supported by other, modern Linux
> >> >> >> distributions.
> >> >> >>
> >> >> >> Building musl libc on older hardware is a nice accomplishment, but
> >> >> >> investing effort and complexity to maintain support probably isn't
> >> >> >> useful to any musl libc user and probably isn't a productive use of
> >> >> >> developer resources.
> >> >> >>
> >> >> >> I will continue to inquire if there is a simple technique to accomplish this.
> >> >>
> >> >> Apparently GCC 7.1 added architecture macros.
> >> >>
> >> >> As Tuan referenced, Alpine Linux also requires z196 as the minimum
> >> >> architecture level.  I believe that it would be better for s390-musl
> >> >> to default to z196 ISA than musl to require GCC 7.1.
> >> >
> >> > I agree we shouldn't "require GCC 7.1", but using the macros does not
> >> > imply such a requirement. For example:
> >> >
> >> >         #if __ARCH__ >= 10
> >> >
> >> > would only use the asm on z196+ (if I got the number right) with GCC
> >> > 7.1+ (no asm on older compilers), whereas:
> >> >
> >> >         #if __ARCH__ >= 10 || !defined(__ARCH__)
> >> >
> >> > would use the asm on z196+ or on compilers too old to provide __ARCH__
> >> > (and building for a more minimal baseline ISA would not be supported
> >> > on such compilers unless you manually add -D__ARCH__=5 or whatever to
> >> > CFLAGS).
> >> >
> >> > I'm fine with waiting to add those pp conditionals until if/when
> >> > someone actually wants to use the lower baseline ISA, if you don't
> >> > want to do it now. I am hesitant to add new ISA-forcing logic to
> >> > configure, though (see the other reply on that). Would it be bad to
> >> > have the build fail with low default -march? If so, maybe the
> >> > configure logic could check for !defined(__ARCH__) and then do a
> >> > compile test to define __ARCH__ on its own, and we could use the above
> >> > logic?
> >> >
> >> > Rich