option -mprfchw on 2 different Opteron cpus

[NightStrike]

Reposting from here:
https://gcc.gnu.org/ml/gcc-help/2016-05/msg3.html

Not sure if this applies:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54210

If I compile on a k8 Opteron 248 with -march=native, I do not see
-mprfchw listed in the options in -fverbose-asm.  In the assembly, I
see this:

prefetcht0  (%rax)  # ivtmp.1160
prefetcht0  304(%rcx)   #
prefetcht0  (%rax)  # ivtmp.1160

If I compile on a bdver2 Opteron 6386 SE with -march=k8 (thus trying
to target the older system), I do see it listed in the options in
-fverbose-asm.  In the assembly, I see this:

prefetcht0  (%rax)  # ivtmp.1160
prefetcht0  304(%rcx)   #
prefetchw   (%rax)  # ivtmp.1160

(The third line is the only difference)

In both cases, I'm using gcc 4.9.3.  Which is correct for a k8 Opteron 248?

Also, FWIW:

1) The march=native version that uses prefetcht0 is very repeatably
faster by about 15% in the particular test case I'm looking at.

2) The compilers in both instances are not just the same version, they
are the same compiler binary installed on an NFS mount and shared to both
computers.

gcc-7-20160501 is now available

[gccadmin]

Snapshot gcc-7-20160501 is now available on
  ftp://gcc.gnu.org/pub/gcc/snapshots/7-20160501/
and on various mirrors, see http://gcc.gnu.org/mirrors.html for details.

This snapshot has been generated from the GCC 7 SVN branch
with the following options: svn://gcc.gnu.org/svn/gcc/trunk revision 235693

You'll find:

 gcc-7-20160501.tar.bz2   Complete GCC

  MD5=ee74a8c6469d0b56379a70e16a896895
  SHA1=d73b5d640b8d6cfbed4c344ddb1dc797660d

Diffs from 7-20160424 are available in the diffs/ subdirectory.

When a particular snapshot is ready for public consumption the LATEST-7
link is updated and a message is sent to the gcc list.  Please do not use
a snapshot before it has been announced that way.