C2: Did something just happen to unrolling?

Andrew Haley aph-open at littlepinkcloud.com
Tue Apr 12 15:40:41 UTC 2022

I'm working on a patch for AArch64 vector code, and I just rebased my patch on
mainline. The performance regression is shocking. On some machines I don't
see much difference, but on others I see almost 2*.

The cause of the difference seems to be that there is now much less unrolling,
so much that "before" I see an 8-lane vector op unrolled 8 times, and now
it's only unrolled twice.

So, does anyone here reading this have any idea what happened in the last month
or two? Did someone change the unrolling heuristics?


Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671

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