Bug 169250 – LowerToAir should recognize subwidth variants of Equal(StrongCAS...)

• Status: NEW
• Component: JavaScriptCore

[Filip Pizlo]

We currently don't turn Equal(StrongCAS...) into a direct branch on the CAS result for Width8 and Width16.  We can do it.  We'd have to match patterns like:

Equal(SExt8(StrongCAS(@exp, ...)), SExt8(@exp))

and

Equal(SExt8(StrongCAS(BitAnd(@exp, 0xff), ...)), SExt8(@exp))

Where the right-hand SExt8 is matched with sharing.

It's not super obvious how profitable this optimization will be, particularly if we have any optimizations that turn strong CAS into weak CAS.