[llvm] r244503 - x86: Emit LAHF/SAHF instead of PUSHF/POPF

[JF Bastien via llvm-commits]

>
> I've bisected this to PR24535, please try to take a look at it if you get
> a free moment.
>

Looking...


> Given what Eli brought up, shouldn't we guard it's usage in 64-bit mode on
> a -mattr flag?  Materializing lahf or sahf without knowing apriori that a
> CPU can handle it seems problematic.
>

See this discussion:

http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150810/293071.html

We've been emitting lahf with impunity since 2012 without issues. We could
fix it, but is it worth fixing at this point if it's never been a problem?
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