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Date: Fri, 23 Jun 2017 15:31:26 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] s390x: Add single instruction math functions
On Tue, Jun 20, 2017 at 11:34:55PM -0400, David Edelsohn wrote:
> __ARCH__ = 9 for z196.
>
> Attached is the original patch to add the FP math instructions and a
> second patch using the test proposed by my colleague and following the
> template of arm/sqrt.c to include the generic C code for earlier
> architectures.
OK, I'm merging these two and committing them with a note on the
motivation. I also did basic testing of builds with and without
support for the math instructions and they built successfully, so I
think all is well. Thanks.
Rich
> On Tue, Jun 20, 2017 at 9:20 PM, Rich Felker <dalias@...c.org> wrote:
> > 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@...c.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@...c.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
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