[MacPorts] #62878: openmpi: fix builds for MacOS 10.8 and earlier, by disabling use of AVX instruction set; mark known_fail, for cases not easily fixable

[MacPorts]

#62878: openmpi: fix builds for MacOS 10.8 and earlier, by disabling use of AVX
instruction set; mark known_fail, for cases not easily fixable
----------------------+----------------------
  Reporter:  jmroot   |      Owner:  mascguy
      Type:  defect   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:
      Port:  openmpi  |
----------------------+----------------------

Comment (by kencu):

 success on 10.7 (with AVX enabled, as well):

 {{{
 $ port -v installed openmpi-gcc10
 The following ports are currently installed:
   openmpi-gcc10 @4.1.1_0+fortran (active) platform='darwin 11'
 archs='x86_64' date='2021-05-14T19:55:14-0700'
 }}}

 SO -- how....

 all the gcc versions on MacPorts (and all the macports-clang-N compilers)
 are built to use cctools assembler {{{${prefix}/bin/as}}}. For all older
 systems this is the cctools-supplied version (for newer systems it might
 be a symlink to the one provided by Xcode).

 {{{${prefix}/bin/as}}} has been set up to use a newer clang as the
 assembler if a newer clang is available, and it looks for macports-
 clang-9.0 ==> macports-clang-5.0 and then {{{clang}}} on 10.7+.

 {{{clang}}} on 10.7 and 10.8 is very old now, and it doesn't understand
 the newer assembly instructions. {{{macports-clang-9.0}}} is pretty
 recent, and easy to build for all systems all the way back to 10.5 Leopard
 Intel. So it's a pretty good choice for an assembler right now.

 It understands the failing instruction, and also understands the
 previously-failed AVX instructions.

 SO you can re-enable openmip-gcc10 on 10.7, and the AVX instructions on
 10.7 and 10.8, if you make sure that clang-9.0 is installed when openmpi-
 gcc10 (etc) is built.

 Something like this:



 {{{
     # For MacOS 10.8 and earlier, system-provided clang assembler doesn't
 support newer instructions
     if {${os.major} <= 12} {
         ui_debug "installed clang-9.0 to use as assembler"
         depends_build-append port:clang-9.0
     }
 }}}

 I'm not at this second 100% sure if it is also needed at runtime as an
 assembler. It might be, and for caution, I would suggest perhaps it just
 be a {{{depends_lib-append}}} so it's always available.

-- 
Ticket URL: <https://trac.macports.org/ticket/62878#comment:9>
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