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

[David Edelsohn <dje.gcc@gmail.com>]

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?

Thanks, David

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