[Bug target/100736] ICE: unrecognizable insn

["segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100736

--- Comment #6 from Segher Boessenkool <segher at gcc dot gnu.org> ---
There are so many things here, it's hard to start.  Two big things:

Firstly, this is not floating point at all, so -ffinite-math-only should not
make any difference.  We currently abuse CCFP (in a non-safe way), this should
be fixed.

Secondly, -mcpu=power9 (to get isel) or -mcpu=power10 (to get setbc) are more
interesting.  What is the generated machine code for those?

More things:

crnot is needed to get the polarity of the result correct.  We could instead
do a xori 1 (or similar) on the eventual GPR.  If we do say a cror and a crnot
we should make this can be combined to a crnor (all 14 logic functions are
supported), but if the inversion is done in the GPR (with such a xori, say),
this is much harder to optimise.

If we are not interested in overflows, always one of LT GT EQ is set, so we
never need any crlogical insn here.

To be exact, this is whenever we have valid inputs: if there is output
overflow cr6.3 will be set as well, but still exactly one of cr6.0, cr6.1,
cr6.2 will be set.

But if there is an invalid *input* cr6 is set to 0b0001 always.  I don't think
we need to care here (otoh it isn't obvious how to best model it!)