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

Wed Jan 20 20:19:23 PST 2016

jfb added a comment.

In http://reviews.llvm.org/D15240#331998, @hoffbrinkle wrote:

> @jfb, this brings back the issue we solved with http://reviews.llvm.org/D6629.  We can't issue PUSHFQ/POPFQ as the POPFQ can corrupt EFLAGS by loading a stale value.

Agreed, you'll need to compile with `-mattr=+sahf` to avoid that issue.

We could also teach LLVM to be smarter about flags, but that's quite a bit more work that we've already poured into this!

Repository:
  rL LLVM

http://reviews.llvm.org/D15240