Re: [committed] Convert xstormy16 to LRA

[Segher Boessenkool <segher@kernel.crashing.org>]

On Tue, May 02, 2023 at 10:11:27AM -0400, Paul Koning wrote:
> > On May 2, 2023, at 9:18 AM, Roger Sayle <roger@nextmovesoftware.com> wrote:
> > Yes, see the section -fsplit-wide-types in
> > https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
> 
> Thanks.  So I'm wondering why that would be a problem.
> 
> The obvious question is whether it interacts badly with MD file entries that describe wide operations, perhaps with constraints that require things like odd/even register pairs.  But I would assume all that gets handled.
> 
> Along the same lines, why would a target, or a user, not do early wide splitting all the time?  The documentation for that option gives no clue why it would ever be bad.

In <https://inbox.sourceware.org/gcc-patches/0c1cd0e5706087d51c0d981a313786990ddcad89.1562518763.git.segher@kernel.crashing.org/>
(the patch that created -fsplit-wide-types-early) I say "At least for
targets that do not have RTL patterns for operations on multi-register
modes it is a lot better to split patterns earlier, before combine and
all related passes."  If your target does in fact have patterns for
multi-reg modes it presumably wants those to be used preferably.

The patch did not change the default because that always is a lot of
pain.  The cost-benefit analysis did not work out here (for me!)


Segher