PUSH/POPF* instructions push the flags register to save comparison results when you need to reuse the registers containing the original comparison arguments. They're well-pipelined, integrated with the stack engine and very fast on modern CPUs, and I've definitely seen the compiler emit them (more on i686 than x86_64 to be fair).

I won't speak to the quality of the generated code otherwise, but this bit of evidence by itself doesn't seem persuasive.

(edit: pbsd is right: per Agner Fog, these guys are slow. It's likely that the L/SAHF trick is the one I was remembering.)