AFAICT the main difference to standard mode switching is that we (ab)use it
to set the rounding mode to the value it had initially, either at function
entry or after a call. That's different to regular mode switching which
assumes "static" rounding modes for different instructions.
Standard could e.g. be:
- insn1 demands frm1
- call1 demands frm4
- call2 demands frm5
Whereas we have:
- insn1 demands frm1
- call1 demands "frm at the start of the function"
- call2 demands "frm after call1 that could have called fesetround"
Weird, call2 can demand the frm as it existed after call1?!? I'm going
to try not to cry and return to my bubble :-)
Maybe demand was not really accurate. It's rather that we want call2 to
operate with the global rounding mode that call1 might have changed.
So the demand is not a specific rounding mode but rather the global one.
And as we might have changed the rounding mode without having restored it since
we need to jump through those hoops. Not defending, just describing the status
quo, I don't like it either ;) and yeah it's contrary to what we usually assume
of liveness etc.
I'd much rather see only a local backup of the rounding mode in the
mode-changing intrinsics. Like
backup FRM
set specific FRM
insn with specific FRM
restore FRM
so we'd always be sure the rounding mode is back to "normal"/"unknown" after an
intrinsic and wouldn't need to do anything for calls and exits.
Another argument would also be that we technically aren't allowed to change the
rounding mode without -frounding-math (as seen in a PR a while ago) because
passes might not do the right thing. Therefore, increasing the region of
non-default rounding mode, could lead to incorrect optimizations.
Anyway, I think Vineet's patches improve on what we have right now. I'd still
like to understand if this "abuse" of mode switching gets globally better
results than a very simple approach like above. If the other (non
mode-switching) LCMs cannot really optimize FRM reads and writes the simple
approach could indeed be worse.
--
Regards
Robin
