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It's possible that `pushfq`+`popfq` get turned into a `mov` from the flag register into a the output register due to microcode-level optimisations.

That is not the case. On a Skylake chip, a pushfd + popfd roundtrip costs 24 cycles.

Just out of curiosity, how do you know this? Is it some kind of measurement you made yourself, written on Intel docs somewhere or something else?

I've been wondering about how much instructions cost lately and would like to be able to know this without the need of asking others.

Agner's instruction listings [1] and InstLatX64 [2] are good sources for a variety of chips. The 24 cycle figure above can be seen at https://github.com/InstLatx64/InstLatx64/blob/ee13abcfb1e2bb...

There's also the intel optimization manuals.