RFR: 8255579: x86: Use cmpq(Register,Address) in safepoint_poll

[Erik Österlund]

On Thu, 29 Oct 2020 09:52:11 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Looks good. I also got confused about this. When this instruction didn't work the way I thought it would, I thought it must be a weird style thing, that everything is right to left in the assembler, for consistency, so it would read from right to left. Then I convinced myself it made sense. But the instruction being incorrectly encoded also checks out. Sigh. Anyway, I ran some local testing that would just fall over immediately if this didn't work. And it seems to work.
>
> Thanks @fisk! Would you like someone else to look at this, or your review is enough?

Since the encodings of the two instructions are literally identical, I think this classifies as a trivial change. So I'm okay with one review.

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PR: https://git.openjdk.java.net/jdk/pull/924