Re: [PATCH v1] Mode-Switching: Add optional EMIT_AFTER hook

[Robin Dapp <rdapp.gcc@gmail.com>]

> Conceptually the rounding mode is just a property.  The call, in
> effect, should demand a "normal" rounding mode and set the rounding
> mode to unknown if I understand how this is supposed to work.  If my
> understanding is wrong, then maybe that's where we should start --
> with a good description of the problem ;-)

That's also what I what struggled with last time this was discussed.

Normally, mode switching is used to switch to a requested mode for
an insn or a call and potentially switch back afterwards.

For those riscv intrinsics that specify a variable, non-default rounding
mode we have two options:

- Save and restore before and after each mode-changing intrinsic
 fegetround old_rounding
 fesetround new_rounding 
 actual instruction
 fesetround old_rounding)

- Have mode switching do it for us (lazily) to avoid most of the
storing of the old rounding mode by storing an (e.g.) function-level
rounding-mode backup value.  The backup value is used to lazily
restore the currently valid rounding mode.

The problem with this now is that whenever fesetround gets called
our backup is outdated.  Therefore we need to update our backup after
each function call (as fesetround can of course be present anywhere)
and this is where most of the complications come from.

So in that case the callee _does_ impact the caller via the backup
clobbering.  That was one of my complaints about the whole procedure
last time.  Besides, I didn't see the need for those intrinsics
anyway and would much rather have explicit fesetround calls but well :)

Having said that, it looks like Pan's patch just tries to move some of
the dirty work from the backend to the mode-switching pass by making it
easier to do something after a call.  I believe I asked for that back in
one of the reviews even?

Regards
 Robin