From: Kyrill Tkachov <kyrylo.tkachov@arm.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH][combine] PR rtl-optimization/68381: Only restrict pure simplification in mult-extend subst case, allow other substitutions
Date: Thu, 19 Nov 2015 15:00:00 -0000 [thread overview]
Message-ID: <564DE415.1090600@arm.com> (raw)
In-Reply-To: <20151119144102.GB32343@gate.crashing.org>
On 19/11/15 14:41, Segher Boessenkool wrote:
> On Thu, Nov 19, 2015 at 01:38:53PM +0000, Kyrill Tkachov wrote:
>>> That is troublesome. Could you look deeper?
>> Yes.
> Thanks.
>
>> So the bad case is when we're in subst and returning a CLOBBER of zero
>> and 'from' is (reg/v:SI 80 [ x ]) and 'to' is (zero_extend:SI (reg:HI 0 x0
>> [ x ])).
>> The call to subst comes from try_combine at line 3403:
>>
>> if (added_sets_1)
>> {
>> rtx t = i1pat;
>> if (i0_feeds_i1_n)
>> t = subst (t, i0dest, i0src_copy ? i0src_copy : i0src, 0, 0, 0);
>>
>> XVECEXP (newpat, 0, --total_sets) = t;
>> }
>>
>> It uses t after calling subst on it without checking that it didn't return
>> a clobber.
>> If I change that snippet to check for CLOBBER:
>> if (added_sets_1)
>> {
>> rtx t = i1pat;
>> if (i0_feeds_i1_n)
>> t = subst (t, i0dest, i0src_copy ? i0src_copy : i0src, 0, 0, 0);
>>
>> if (GET_CODE (t) == CLOBBER)
>> {
>> undo_all ();
>> return 0;
>> }
>> XVECEXP (newpat, 0, --total_sets) = t;
>> }
>>
>> The testcase gets fixed.
>> But shouldn't the XVECEXP (newpat, 0, --total_sets) = t; line create an
>> uncrecognizable rtx
>> that would then get rejected by combine or something?
> Yes. recog_for_combine_1 checks for a PARALLEL with such a CLOBBER
> right at the start; and of course having the clobber elsewhere will
> just not match.
>
>> If we don't check for clobber there and perform the "XVECEXP = ..."
>> the resulting newpat looks like:
>> (parallel [
>> (set (reg:CC 66 cc)
>> (compare:CC (const_int 0 [0])
>> (const_int 0 [0])))
>> (nil)
>> (clobber:DI (const_int 0 [0]))
>> ])
>>
>> ah, so the clobber is put in a parallel with another insn and is thus
>> accepted by recog?
> No, recog_for_combine should refuse it because of that third arm.
> The second arm (the nil) looks very wrong, where is that coming
> from? That isn't valid RTL.
Well, it came from a bit earlier before the subst call (around line 3390):
/* If the actions of the earlier insns must be kept
in addition to substituting them into the latest one,
we must make a new PARALLEL for the latest insn
to hold additional the SETs. */
<snip>
rtx old = newpat;
total_sets = 1 + extra_sets;
newpat = gen_rtx_PARALLEL (VOIDmode, rtvec_alloc (total_sets));
XVECEXP (newpat, 0, 0) = old;
extra_sets is 2, so we have a parallel with 3 slots so after the subst call
we put the clobber into --total_sets, that is slot 2:
XVECEXP (newpat, 0, --total_sets) = t;
A bit further below we fill slot 1 with another rtx, so all 3 parts of the PARALLEL
are filled.
I'll look further into why recog_for_combine doesn't kill the whole thing.
Kyrill
>
>> So, should we check 't' after subst for clobbers as above? Or should this
>> be fixed in
>> some other place?
> There is a bug somewhere, so that will need fixing. Workarounds are
> last resort, and even then we really need to know what is going on.
>
>> Thanks. In light of the above I think this patch happens to avoid
>> the issue highlighted above but we should fix the above code separately?
> Yes, if your patch creates better code we want it (and fix the regression),
> but you exposed a separate bug as well :-)
>
>
> Segher
>
next prev parent reply other threads:[~2015-11-19 15:00 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-19 10:26 Kyrill Tkachov
2015-11-19 10:57 ` Segher Boessenkool
2015-11-19 13:39 ` Kyrill Tkachov
2015-11-19 14:41 ` Segher Boessenkool
2015-11-19 15:00 ` Kyrill Tkachov [this message]
2015-11-19 15:20 ` Kyrill Tkachov
2015-11-24 0:40 ` Segher Boessenkool
2015-11-26 9:55 ` Kyrill Tkachov
2015-11-26 13:02 ` Segher Boessenkool
2015-11-26 13:32 ` Segher Boessenkool
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