[Bug target/108248] Some insns in the risc-v backend do not have mappings to functional units

[andrew at sifive dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108248

--- Comment #3 from Andrew Waterman <andrew at sifive dot com> ---
Yikes.  Thanks for the explanation, Jeff.

(cc Kito Cheng: at some point, we should revisit the pipeline modeling of Zb*
instructions for sifive-7.  The short version is that all Zb* instructions can
execute on the B pipe.  The A pipe supports only a subset: rev8, bext[i],
addu.w, slliu.w, sext.*, zext.*, orn, xnor, andn.  So, to do an accurate job,
we'll need finer granularity than simply describing them all as "bitmanip".)