[PATCH] [RFC] Lame fix for PR20376

[Tim Northover]

On 22 July 2014 17:24, Philip Reames <listmail at philipreames.com> wrote:
>> We actually do have a last-ditch EFLAGS copy available (though with
>> worrying implications for the red-zone, if we ever start using it). It's
>> used by test/CodeGen/Generic/2011-07-07-ScheduleDAGCrash.ll for example.
>
> Tim, could expand on this point a bit?

I'm afraid I forget who, but someone pointed it out to me on IRC the other day.

The AMD64 ABI says that the compiler is free to use 128 bytes below
%rsp for local variables (to avoid stack adjustment in small leaf
functions, I believe). So OS context-switches and interrupts have to
preserve that data, for example.

When we do need to copy EFLAGS, we currently emit a pushf/popw or
pushw/popf pair, which could clobber a local variable if we actually
made use of the red zone. I don't think we do though (fortunately).
There are apparently "lahf" and "sahf" we could use instead, if it
came to it (at least for our purposes).

Cheers.

Tim.