Re: [committed] Convert xstormy16 to LRA

["Roger Sayle" <roger@nextmovesoftware.com>]

Jeff Law wrote:
> This patch converts the xstormy16 patch to LRA.  It introduces a code 
> quality regression in the shiftsi testcase, but it also fixes numerous 
> aborts/errors.  IMHO it's a good tradeoff.

I've investigated the shiftsi regression on xstormy16 and the underlying
cause
appears to be an interaction between lower-subreg's "subreg3" pass and the
new LRA.  Previously, reload was not phased by the "clobbers" that are 
introduced by the decompose_multiword_subregs function, but they appear
to interfere with LRA's register assignments.

combine's make_extra_copies introduces a new pseudo-to-pseudo move,
but when subreg3 inserts a naked clobber between the original and the
new move, LRA is recombine theses pseudos back to the same allocno.

The shiftsi.cc regression on xstormy16 is fixed by adding
-fno-split-wide-types.
In fact, if all the regression tests pass, I'd suggest that
flag_split_wide-types = false
should be the default on xstormy16 now that we've moved to LRA.  And if this
works for xstormy16, it might be useful to other targets for the LRA
transition;
it's a difference in behaviour between reload and LRA that could potentially
affect multiple targets.

For reference, xstormy16 has a post-reload define_insn_and_split for movsi
(i.e. a multi-word move).  If this insn was split during split1 (i.e. before
subreg3)
there wouldn't be a problem (no clobber), but alas the target's
xstormy16_split_move
function has several asserts insisting this only get called when
reload_completed.

I hope this is useful.
Cheers,
Roger
--