1068960 - GVN: protecting foldsTo from discarded dependencies pessimizes redundance elimination

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement)

Description

[Dan Gohman [:sunfish]]

Attachment: gvn-reestablish-dependency.patch

As noted in bug 1029830 comment 34, bug 1029830 caused an AWFY regression on misc-bugs-608733-interpreter.js.

Bug 1049691 added a foldsTo method to fold loads when their dependency() is a dominating store.

This makes foldsTo depend on dependency() fields being in non-discarded blocks. To ensure this remains true as other patches in bug 1029830 enable GVN to discard blocks, bug 1029830 attachment 8476266 [details] [diff] [review] (gvn-clear-dependencies.patch) was added to clear problematic dependencies, protecting the foldsTo optimization.

However, this causes GVN to miss the opportunity to eliminate redundant loads when their dependencies happen to be in discarded blocks. For congruence checking, a dependence in a discarded block is not a problem.

The attached patch restores the dependence after the foldsTo call is done, before doing the redundancy check. It fixes the perf regression in misc-bugs-608733-interpreter.js. It admittedly looks a little silly clearing and restoring the dependency() field like this, but I'm open to suggestions for better ways to do this.

Comment 1

[Dan Gohman [:sunfish]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/060b6c65fb96

Comment 2

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/060b6c65fb96