Bug 60866 – Evaluation order broken for empty alternatives in subpatterns

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Gavin Barraclough]

for example:
    /(|a)/.exec("a");
should return ["",""], but instead returns ["a","a"].

This was broken by https://bugs.webkit.org/show_bug.cgi?id=51395, which will be reverted (this unsafe optimization does not appear significant to any benchmarks we are tracking, we should reimplement in a more restricted, safe fashion only if we can demonstrate an important workload benefitting from doing so).

[Gavin Barraclough]

This unsafe optimization is masking further problems in the JIT, reverting the optimization causes failures in fast/regex/parentheses.

These issues are fixed by 60860.

[Geoffrey Garen]

rs=me to roll out that optimization with a layout test for

/(|a)/.exec("a")
/(a|)/.exec("b")

[Gavin Barraclough]

Fixed in r86547.

[Zoltan Herczeg]

Perhaps a non-greedy conversion? (|a) == a?? (a|) == a?

[Gavin Barraclough]

Conversion to an appropriately greedy quantifier would be possible, but would limit the scope of this optimization.  We would consider such a patch if it were to provide a useful performance gain on an interesting workload.