[Bug build/18116] New: build failure on ppc64le: setcontext.S uses power6 mtfsf when not supported

["msebor at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18116

            Bug ID: 18116
           Summary: build failure on ppc64le: setcontext.S uses power6
                    mtfsf when not supported
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: build
          Assignee: unassigned at sourceware dot org
          Reporter: msebor at redhat dot com
                CC: carlos at redhat dot com

Created attachment 8185
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8185&action=edit
Proposed patch.

As discussed in gcc bug 65341, glibc fails to build with on ppc64le with gcc
5.0 configured without explicitly being configured for or targeting a power6 or
later cpu.  The reason for the failure is that after a recent change, gcc
implicitly defines the _ARCH_PWR6 preprocessor macro when compiling assembly
files but doesn't necessarily invoke the assembler in the corresponding power6
mode.  Glibc relies on the _ARCH_PWR6 macro to determine the mode the assembler
is put in by gcc and make use of power6 features such as the new-in-Power ISA
2.5 four-argument form of the mtfsf instruction, which according to comment 15
on the gcc bug is not reliable.

Since the gcc bug has been resolved as WONTFIX, the attached patch adjusts
glibc to put the assembler into power6 mode irrespective of _ARCH_PWR6.  For
some additional background on the code affected by the patch see glibc bug
10118.

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