llvm dev - Aug 2019 - [GlobalISel?] When are implicit def-dead flags supposed to be first set?

<div dir='auto'>I've noticed PreRA MachineSinking is largely
blocked on the arch I'm on due to most instructions writing to an implicit
flags register.<div dir="auto"><br></div><div
dir="auto">MachineSinking *is* allowed to move instructions where
all physregs are dead, but at the point where it's called in default pass
order, liveness hasn't been computed yet. Assuming I haven't missed a
pass somewhere, this must be a feature of SelectionDAG that is not yet
implemented?</div><div
dir="auto"><br></div><div dir="auto">I
believe they could be precomputed per pattern and set in the selector vm, partly
wondering if that is the plan.</div><div
dir="auto"><br></div><div
dir="auto">For now I've changed the two lines required to make
recomputeLivenessFlags work when in SSA form (or at least I can't see why it
won't work, which makes me wonder why the asserts were there in the first
place...) and added a custom pass, ju<span style="font-family:
sans-serif;">st wanted to make sure I'm on the right path here and
haven't missed something obvious. Thank
you.</span></div></div>