From: Richard Guenther <richard.guenther@gmail.com>
To: Jeff Law <law@redhat.com>
Cc: Jiangning Liu <jiangning.liu@arm.com>, gcc@gcc.gnu.org
Subject: Re: A case that PRE optimization hurts performance
Date: Tue, 27 Sep 2011 18:59:00 -0000 [thread overview]
Message-ID: <CAFiYyc0BSfmQtKwcCu-3+4Wgwyq92mo56k8YB9sy8OCFfxQEVQ@mail.gmail.com> (raw)
In-Reply-To: <4E81E406.4090305@redhat.com>
On Tue, Sep 27, 2011 at 4:56 PM, Jeff Law <law@redhat.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 09/27/11 01:30, Richard Guenther wrote:
>
>>
>>> It knows something about prephitmp.6_1 and thus could simplify
>>> D.2734_9 = prephitmp_6.1 & D.2732_7; to D.2734_9 = D.2732_7; But
>>> admittedly I have no idea if DOM tries to simplify things other
>>> than comparisons within the jump threading machinery ... (the
>>> above block basically ends in if (D.2729_6 != 0 &&
>>> prephitmp_6.1), so I'd guess it's worth to simplify the (complex)
>>> conditional expression).
> Jump threading will enter simplified expressions into the hash tables
> for every statement in the block in an effort to utilize any
> information available to determine the result of the comparison.
> However, those simplifications aren't ever reflected back into the IL.
>
> The thing to remember is jump threading is tasked detecting cases
> where the control statement has a predetermined destination utilizing
> path sensitive information. Expanding it to do general path
> sensitive optimizations is possible, but at even greater cost.
Yeah, I realize that. It's just this case is the non-cfg form of
if (pretmp)
if (D.1234)
...
where jump-threading would probably handle the CFG variant just fine.
Oh well ;)
Richard.
next prev parent reply other threads:[~2011-09-27 15:01 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4e3762ed.030d440a.1143.2af2SMTPIN_ADDED@mx.google.com>
2011-08-02 9:23 ` Richard Guenther
2011-09-16 2:01 ` Jiangning Liu
[not found] ` <4e72add8.c9d0e30a.142a.ffffe60eSMTPIN_ADDED@mx.google.com>
2011-09-23 20:25 ` Richard Guenther
2011-09-26 8:34 ` Jiangning Liu
[not found] ` <4e802c3d.94ebd80a.4b1f.ffff8101SMTPIN_ADDED@mx.google.com>
2011-09-26 14:33 ` Richard Guenther
2011-09-27 7:30 ` Jeff Law
2011-09-27 11:15 ` Jiangning Liu
2011-09-27 13:44 ` Richard Guenther
2011-09-27 18:59 ` Jeff Law
2011-09-27 18:59 ` Richard Guenther [this message]
2011-09-27 21:56 ` Jeff Law
2011-08-02 2:37 Jiangning Liu
2011-08-15 8:40 ` Václav Zeman
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