[Bug middle-end/77270] New: Flag -mprftchw is shared with 3dnow for -march=k8

["nightstrike at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77270

            Bug ID: 77270
           Summary: Flag -mprftchw is shared with 3dnow for -march=k8
           Product: gcc
           Version: 6.1.0
               URL: https://gcc.gnu.org/ml/gcc/2016-05/msg00000.html
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nightstrike at gmail dot com
                CC: ubizjak at gmail dot com, venkataramanan.kumar at amd dot com
  Target Milestone: ---

See https://gcc.gnu.org/ml/gcc/2016-05/msg00000.html

Using -march=native on a k8 platform does not add -mprfchw to the options list,
while using -march=k8 on a different platform does add the switch.

From Venkataramanan:

>       3DNow! instruction extensions         = true
>       3DNow! instructions                   = true

It has 3Dnow support.  "prefetchw" is available with 3dnow.

>       misaligned SSE mode                    = false
>       3DNow! PREFETCH/PREFETCHW instructions = false

It does not have 3DNowprefetch enabling ISA flag -mprftchw is not correct for
-march=k8.

>       OS visible workaround                  = false
>       instruction based sampling             = false

I think  we need to file bug for this.  Need to check with Uros why the flag
-mprfchw is shared with 3dnow.

To work around this issue you can use -mno-prfchw when building with -march=k8.