[PATCH] D15240: X86: Don't emit SAHF/LAHF for 64-bit targets unless explicitly supported

[JF Bastien via llvm-commits]

jfb added a comment.

In http://reviews.llvm.org/D15240#332012, @hoffbrinkle wrote:

> In http://reviews.llvm.org/D15240#332004, @jfb wrote:
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> > Agreed, you'll need to compile with `-mattr=+sahf` to avoid that issue.
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> Well, right, but when not using that attribute, we shouldn't generate incorrect code.  llvm shouldn't issue a POPFQ unless it knows that it's not blowing away an updated EFLAGS.


Agreed. We're playing a game of whack-a-bug and a significant number of programs expect to run on machines with `lahf`/`sahf` so those uses won out on programs which get sad when `EFLAGS` are clobbered. That was the pre-D6629 status-quo, but the world is slightly better overall. It'll take another whack (and a big one at that) to fix the `EFLAGS` issue.


Repository:
  rL LLVM

http://reviews.llvm.org/D15240