[PATCH] [arm] Implement Armv8.1-M low overhead loops

[Roman Zhuykov]

Andrea Corallo writes:

> With this patch the first insn of the low loop overhead 'doloop_begin'
> is expanded by 'doloop_modify' in loop-doloop.c.  The same does not
> happen with SMS.

That certainly works correct as in your first patch, doloop_begin
pattern also have "!flag_modulo_sched" condition.

> My understanding is that to have it working in that
> case too the machine dependent reorg pass should add it later.  Am I
> correct on this?

IMHO, this is not needed is your case.  Currently, list of platforms
(actually, gcc/config subfolders) which have doloop_end is rather big:
aarch64*, arc, arm*, bfin, c6x, ia64, pdp11, pru, rs6000, s390, sh,
tilegx*, tilepro, v850 and xtensa.  I marked three of them with a star -
they actually have a fake pattern, which is applied only with SMS. 
Reorg_loops from hw-doloop.c (see also
https://gcc.gnu.org/ml/gcc-patches/2011-06/msg01593.html and
https://gcc.gnu.org/ml/gcc-patches/2011-07/msg00133.html) is used only
in arc, bfin, c6x, and xtensa.  Certainly some other platforms may have
additional loop reorg steps in target-specific part (e.q. pru), but not
all of them.  And that reorg is actually needed independently, whether
SMS is on or off.

Actually, the question was: what goes wrong if you remove that
"!flag_modulo_sched" condition from three new patterns?  I had actually
made one step forward, removed that "!flag_modulo_sched" parts in your
patch, and made the following simplest testing for such modified patch. 
I've build and then compared regtest results of two ARM cross-compilers:
first was built from clean trunk, second with patch.  Both compilers
were configured -with-march=armv8.1-m.main and had modified common.opt
to enable -fmodulo-sched and -fmodulo-sched-allow-regmoves by default. 
Regtest results are identical.
> Second version of the patch here addressing comments.

Thank you, now I see in second patch that aspect was solved.

> SMS is disabled in tests not to break them when SMS does loop versioning.

And I'm not really sure about this.  First of all, there are a lot of
scan-assembler-times tests which fail when modulo-scheduler is enabled,
probably the same happens when some unrolling parameters are not
default.  It seems that any non-default optimization which creates more
instruction copies can break scan-assembler-times check.  IMHO, it is
not necessary to workaround this in few particular tests.  Second, I'm
not sure how dg-skip-if directive works.  When one enables SMS setting
"Init(1)" directly into common.opt this won't be catched, would it?

Roman