[Bug target/71805] incorrect code for test pr45752.c with -mcpu=power9

["meissner at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71805

Michael Meissner <meissner at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-07-11
     Ever confirmed|0                           |1

--- Comment #1 from Michael Meissner <meissner at gcc dot gnu.org> ---
Note, you need the option '--param tree-reassoc-width=1' to make this test
fail.  It does not fail if you use '--param tree-reassoc-width=2'.

You do not need -flto to make the test fail.  The simplest set of switches to
make it fail is: -O3 -mcpu=power9 --param tree-reassoc-width=1

Now, looking at the code, if you use an reassociation width other than 1, the
code generates NO Altivec, VSX, power8/power9 instructions.  It just generates
simple PowerPC code.

If the reassociation width is 1, the compiler decides to vectorize the code.

If I change the compiler to only use vperm and vpermr and not xxperm/xxpermr,
the code runs fine.

We probably need to validate that the simulator is executing xxperm correctly,
and/or the compiler is using the instruction correctly.