Performance regression in "Rewrite pass management with LLVM"

[DaGenix]

Please see #8782 for more details since I'm running the same test cases as there.

revision	speed of test.rc (MB/s)
6a649e6 (parent revision)	80.45
7354055	63.77

I'm not quite sure how to check all the optimizations that were enabled before and that are enabled now at --opt-level 3, so, I don't know if that changed. Maybe this isn't an issue and the answer is just that I need to manually enable some pass now since LLVM/Clang don't enable that pass at any optimization level by default. This seems like a pretty significant drop in performance, though, so I figured I'd open up an issue for it.

[alexcrichton]

This is disturbing, thanks for narrowing it down to this! I will investigate this tonight.

[thestinger]

@DaGenix: can you try with the -Z flag to disable SLP vectorization?

[alexcrichton]

Or maybe I will investigate it now!

The threshold for inlining was raised from 225 to 275 during that change, and turning it back down to 225 restores the old performance.

I will both revert the numbers for now, and provide a way of overriding this.

[thestinger]

The performance at --opt-level=2 is now the same as it was before and the inlining threshold can be set manually.