RFR 8203628: Optimize (masked) byte memory comparisons on x86

[John Rose]

On May 22, 2018, at 8:10 AM, Aleksey Shipilev <shade at redhat.com> wrote:
> 
> We have found this during Shenandoah performance work. There, the GC barrier fastpath checks the
> thread-local byte field to see if barriers are enabled. It follows different shapes, e.g.:
> 
>  Set cr (CmpI (LoadB mem) imm))
>  Set cr (CmpI (LoadUB mem) imm))
>  Set cr (CmpI (AndI (LoadUB mem) imm) zero))


Nicely done.  You covered both B and UB evenly.

Just to verify:  There are already similar patterns for int32 and int64, right?

And I have to ask, "what about" int16?  Do we care?  Probably not.

— John
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