[Bug tree-optimization/123606] 5-8% slowdown of mcf on x86 caused by r16-6679-g3ebe697f32197e (was sped up by r16-5258-g1d8e2d51e5c5cb)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123606

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> It's likely that having a latch vs. not might trigger some of the many weird
> conditions we have on threading around loops.

No differences in -fopt-info (trunk vs. trunk with r16-6679 reverted), but
threading doesn't emit opt-info messages (which is IMO good).

On a Zen2 machine I can barely measure a runtime difference (161s vs. 158s
at best, which is below 2%).  With that perf produces only useless data for
me.  There's very few actual assembler differences, even less meaningful
ones.  I can see one larger BB ordering difference, so possibly the
effect is on profile / bb-reorder effects with the reduced churn possibly
preserving more correct data that turns out to be "bad".

Having a CFG-only dump might prove useful (possibly indicating stmt count
stats).