QEmu translation is incorrect when using REX in combination with LAHF/SAHF
This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1828867
When translating code that is using LAHF and SAHF in combination with the REX prefix then qemu translates incorrectly. These two instructions only ever use the AH register. Contrary to other instructions where if you use REX + high bit offsets then it'll pull in rsp and a few other registers. On hardware the REX prefix doesn't effect behaviour of these instructions at all. QEMU incorrectly selects RSP as the register of choice here due to this combination of REX + AH register usage. I've attached a patch that is super terrible just so I can work around the issue locally and to sort of show off how it is to be "fixed"
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