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Re: [PATCH] Preserve loops from CFG build until after RTL loop opts
- From: Richard Biener <rguenther at suse dot de>
- To: Tom de Vries <Tom_deVries at mentor dot com>
- Cc: gcc-patches at gcc dot gnu dot org, Jakub Jelinek <jakub at redhat dot com>
- Date: Mon, 29 Apr 2013 09:54:38 +0200 (CEST)
- Subject: Re: [PATCH] Preserve loops from CFG build until after RTL loop opts
- References: <alpine dot LNX dot 2 dot 00 dot 1304251607460 dot 7969 at zhemvz dot fhfr dot qr> <517A8EE8 dot 8010805 at mentor dot com> <517D155C dot 3080509 at mentor dot com>
On Sun, 28 Apr 2013, Tom de Vries wrote:
> On 26/04/13 16:27, Tom de Vries wrote:
> > On 25/04/13 16:19, Richard Biener wrote:
> > <SNIP>
> >> and compared to the previous patch changed the tree-ssa-tailmerge.c
> >> part to deal with merging of loop latch and loop preheader (even
> >> if that's a really bad idea) to not regress gcc.dg/pr50763.c.
> >> Any suggestion on how to improve that part welcome.
> > So I think this is really a cornercase, and we should disregard it if that makes
> > things simpler.
> > Rather than fixing up the loop structure, we could prevent tail-merge in these
> > cases.
> > The current fix tests for current_loops == NULL, and I'm not sure that can still
> > happen there, given that we have PROP_loops.
> I've found that it happens in these g++ test-cases:
> This seems rare enough to just bail out of tail-merge in those cases.
> > It's not evident to me that the test bb2->loop_father->latch == bb2 is
> > sufficient. Before calling tail_merge_optimize, we call loop_optimizer_finalize
> > in which we assert that LOOPS_MAY_HAVE_MULTIPLE_LATCHES from there on, so in
> > theory we might miss some latches.
> > But I guess that pre (having started out with simple latches) maintains simple
> > latches throughout, and that tail-merge does the same.
> I've added a comment related to this in the patch.
> Bootstrapped and reg-tested (ada inclusive) on x86_64.
> OK for trunk?
+ if (bb == NULL
+ /* Be conservative with loop structure. It's not evident that this
+ is sufficient. Before tail-merge, we've just called
+ loop_optimizer_finalize, and LOOPS_MAY_HAVE_MULTIPLE_LATCHES is
+ set, so there's no guarantee that the loop->latch value is still
+ But we assume that, since we've forced LOOPS_HAVE_SIMPLE_LATCHES
+ start of pre, we've kept that property intact throughout pre, and
+ keeping it throughout tail-merge using this test. */
+ || bb->loop_father->latch == bb)
A more complete test would be to use what the bb_loop_header_p predicate
does - skip latch _edges_. Not sure if that's easily possible in
the loop looking at succs
FOR_EACH_EDGE (e, ei, bb->succs)
int index = e->dest->index;
bitmap_set_bit (same->succs, index);
same_succ_edge_flags[index] = e->flags;
but we'd skip all edges for which
dominated_by_p (CDI_DOMINATORS, e->src, e->dest)
of course that's equal to skipping the whole basic-block if the
above is true.
I suppose the patch is ok as-is for now, but let's keep the above
in mind (I want to audit the whole bootstrap process for loops
that vanish and eventually re-appear, I just didn't get around
thinking about a proper way to efficiently instrument for that).