This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: don't force debug insns after their PREV_INSNs
- From: Alexandre Oliva <aoliva at redhat dot com>
- To: Bernd Schmidt <bernds at codesourcery dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Mon, 09 Apr 2012 03:50:41 -0300
- Subject: Re: don't force debug insns after their PREV_INSNs
- References: <ortyc7km50.fsf@livre.localdomain> <4DE927AB.3030905@codesourcery.com>
On Jun 3, 2011, Bernd Schmidt <bernds@codesourcery.com> wrote:
> On 06/03/2011 04:47 PM, Alexandre Oliva wrote:
>> https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=677681 can be
>> âfixedâ by disabling the artificial dependency of a debug insn on its
>> previous insn.
>>
>> Debug insns will often depend on their prevs anyway, in a use/def
>> relationship, but if the def was (re)moved or the use was reset, this
>> artificial dep helped keep the debug insn âin placeâ.
>>
>> Being a very imperfect heuristic, it's not clear that it helps more than
>> it harms. Thoughts? Regstrapped on x86_64-linux-gnu and
>> i686-linux-gnu.
> Can you explain a little more clearly what the problem is? The RH
> bugzilla isn't really clear.
The problem here is that a nondebug insn may be moved ahead of a useful
debug insn and clobber one of its inputs, rendering it useless, when
there's no good reason for the debug insn to be kept in place, other
than an accidental dependency on the previous insn when it happens to be
unrelated with the debug insn.
Removing the extraneous dependency, that was thought to be a way to
reduce movement of debug insns, improves on this problem. It's not
clear that this artificial dependency really does any good, since odds
are that that previous insn may be pulled ahead anyway, in which case so
will debug insn (unless that would fail other of its deps, of course)
Retested. Ok?
for gcc/ChangeLog
from Alexandre Oliva <aoliva@redhat.com>
* sched-deps.c (sched_analyze_insn): Don't force debug insns
to follow their original predecessors.
Index: gcc/sched-deps.c
===================================================================
--- gcc/sched-deps.c.orig 2012-02-25 09:45:31.749795611 -0200
+++ gcc/sched-deps.c 2012-04-08 02:10:39.710573253 -0300
@@ -2988,18 +2988,6 @@ sched_analyze_insn (struct deps_desc *de
reg_last->uses = alloc_INSN_LIST (insn, reg_last->uses);
}
CLEAR_REG_SET (reg_pending_uses);
-
- /* Quite often, a debug insn will refer to stuff in the
- previous instruction, but the reason we want this
- dependency here is to make sure the scheduler doesn't
- gratuitously move a debug insn ahead. This could dirty
- DF flags and cause additional analysis that wouldn't have
- occurred in compilation without debug insns, and such
- additional analysis can modify the generated code. */
- prev = PREV_INSN (insn);
-
- if (prev && NONDEBUG_INSN_P (prev))
- add_dependence (insn, prev, REG_DEP_ANTI);
}
else
{
--
Alexandre Oliva, freedom fighter http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/ FSF Latin America board member
Free Software Evangelist Red Hat Brazil Compiler Engineer