[llvm] [llvm-exegesis] Add explicit support for setting DF in X86 (PR #115644)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Are you intending to add REP test coverage?

Not currently. This is mostly to make it easier to run snippets while inferring which registers they use in a downstream project given LLVM models `DF` explicitly.

https://github.com/llvm/llvm-project/pull/115644