[Bug target/111020] RFE: RISC-V: ability to cherry-pick additional instructions

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111020

palmer at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |palmer at gcc dot gnu.org

--- Comment #4 from palmer at gcc dot gnu.org ---
(In reply to Andrew Pinski from comment #3)
> (In reply to H. Peter Anvin from comment #2)
> > Named subsets are, inherently, designed to make sense toward mass-produced
> > products where the hardware and software are designed (mostly)
> > independently. However, what I mean with "very deep embedded use" is
> > hardware and software being co-designed.
> > 
> > The RISC-V ISA policy is that those are considered vendor-specific subsets
> > and are to be given an X* name; however, gcc obviously needs to be able to
> > understand the meaning of this X* name. At this point there is no way to do
> > without changing the source code in nontrivial ways.
> > 
> > Regardless of if it is done in source code or at runtime, by implementing a
> > fine-grained, preferably table-driven, approach to subsets in gcc then it
> > would be very simple for a hardware implementor to define their custom
> > X-subsets without a lot of surgery to the code, *and* it makes it possible
> > to take it one step further and allowing custom (or newly defined! - there
> > have been multiple instances already of new subsets of existing instructions
> > defined a posteori) instruction subsets to be defined in a configuration
> > file.
> 
> I am 100% disagree here. Because if you do this there would be a huge
> explosion of what is and is not considered a subset. THIS is why it should
> be defined at the ISA level instead. Why just CTZ for ZBB what next just
> bseti or bexti of ZBS?
> 
> defining the specific set during your development is different from a
> production compiler really. GCC should aim for production compiler quality
> even for highly embedded targets.

IMO adding some config file for custom subsets is going to make more headaches
than it fixes.  For a while we had args like "-mno-div", but that's kind of
hacky and we eventually ended up with Zmmul to handle it -- having an external
config file controlling this would expose a lot of interface surface we don't
have a sane way to test.

If vendors want a custom subset then they can make one, it'll just be called
"X${vendor}${subset}".  We've already got a few forks/subsets floating around,
look at the T-Head and Ventana stuff.  For a few instructions it's pretty
mechanical, aside from fixing whatever fallout comes from splitting off the
subset.

We do currently require (IIRC we still didn't write this down) some amount of
public commitment to hardware availability to take that code, but if that's the
problem we should try and figure something out.  It's certainly a pain for
vendors to keep in-development trees around, but we're trading that off with
upstream pain -- I've found these sorts of subsets drift around until the HW
actually ships, so we don't want to end up stuck keeping around subsets that
didn't ship.

Vendors also have the option of just implementing all the instructions (via
some trap or microcode or whatever), thus turning this into a performance
problem.  That sort of just trades one problem for another, but we've got some
examples of this as well (SiFive traps on a bunch of stuff, for example).