[Bug target/81616] Update -mtune=generic for the current Intel and AMD processors

[andrewm.roberts at sky dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81616

--- Comment #5 from Andrew Roberts <andrewm.roberts at sky dot com> ---
I've been testing on a Ryzen system and also comparing with Haswell and
Skylake. From my testing -mtune=znver1 does not perform well and never has,
including as of last snapshot:
gcc version 8.0.0 20171119 (experimental) (GCC)

-mtune=generic seems a better option for all three systems as a default for
-march=native

This is only with one test case (attached), but I've seen the same across many
other tests.

See the attached testcase (matix.c) and performance logs 
Ryzen - znver1-tunebug.txt
Haswell - znver1-tunebug2.txt
Skylake - znver1-tunebug3.txt