[Bug tree-optimization/83651] [7/8 regression] 20% slowdown of linux kernel AES cipher

[aldyh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83651

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #8 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
I ran the testcase in comment #7 and can confirm that there is a 5.24%
performance regression from 6.3.1 to 7.2.1, and a 2.88% regression from 6.3.1
to 8.0.

I ran the test on my unloaded workstation, which is a:

model name      : Intel(R) Core(TM) i7-6600U CPU @ 2.60GHz

Average user time of 15 runs: a.out.6.3.1 is 3.820000
Average user time of 15 runs: a.out.7.2.1 is 4.020000
Average user time of 15 runs: a.out.8.0 is 3.931333

I can also confirm that everything runs much faster with -fno-code-hoisting,
but that may or may not be related:

Average user time of 15 runs: a.out.8.0 -fno-code-hoisting is 3.734667

These regressions seem pretty small, though.

Perhaps we could find what regressed from 6.3.1 to 7.2.1, and maybe that's
still causing problems in 8 (even though other factors may be causing it to be
faster in 8).

Confirmed.