From: Richard Biener <richard.guenther@gmail.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: kugan <kugan.vivekanandarajah@linaro.org>,
"gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>
Subject: Re: [PR72835] Incorrect arithmetic optimization involving bitfield arguments
Date: Wed, 10 Aug 2016 10:28:00 -0000 [thread overview]
Message-ID: <CAFiYyc0bLsCOTU-OZ4OKKNyrsmpNx63E+jSCGCYiMpY7=-z9nQ@mail.gmail.com> (raw)
In-Reply-To: <20160810085703.GH14857@tucnak.redhat.com>
On Wed, Aug 10, 2016 at 10:57 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Wed, Aug 10, 2016 at 08:51:32AM +1000, kugan wrote:
>> I see it now. The problem is we are just looking at (-1) being in the ops
>> list for passing changed to rewrite_expr_tree in the case of multiplication
>> by negate. If we have combined (-1), as in the testcase, we will not have
>> the (-1) and will pass changed=false to rewrite_expr_tree.
>>
>> We should set changed based on what happens in try_special_add_to_ops.
>> Attached patch does this. Bootstrap and regression testing are ongoing. Is
>> this OK for trunk if there is no regression.
>
> I think the bug is elsewhere. In particular in
> undistribute_ops_list/zero_one_operation/decrement_power.
> All those look problematic in this regard, they change RHS of statements
> to something that holds a different value, while keeping the LHS.
> So, generally you should instead just add a new stmt next to the old one,
> and adjust data structures (replace the old SSA_NAME in some ->op with
> the new one). decrement_power might be a problem here, dunno if all the
> builtins are const in all cases that DSE would kill the old one,
> Richard, any preferences for that? reset flow sensitive info + reset debug
> stmt uses, or something different? Though, replacing the LHS with a new
> anonymous SSA_NAME might be needed too, in case it is before SSA_NAME of a
> user var that doesn't yet have any debug stmts.
I'd say replacing the LHS is the way to go, with calling the appropriate helper
on the old stmt to generate a debug stmt for it / its uses (would need
to look it
up here).
Richard.
>
> Jakub
next prev parent reply other threads:[~2016-08-10 10:28 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-09 13:43 kugan
2016-08-09 21:43 ` kugan
2016-08-09 21:46 ` Jakub Jelinek
2016-08-09 21:51 ` kugan
2016-08-09 21:55 ` Jakub Jelinek
2016-08-09 22:51 ` kugan
2016-08-10 1:46 ` kugan
2016-08-10 8:57 ` Jakub Jelinek
2016-08-10 9:14 ` kugan
2016-08-10 10:28 ` Richard Biener [this message]
2016-08-10 23:09 ` kugan
2016-08-19 8:19 ` Kugan Vivekanandarajah
2016-08-25 12:24 ` Richard Biener
2016-09-02 8:09 ` Kugan Vivekanandarajah
2016-09-14 11:38 ` Richard Biener
2016-09-18 21:58 ` kugan
2016-09-19 13:49 ` Richard Biener
2016-09-20 3:27 ` kugan
2016-09-20 12:01 ` Richard Biener
2016-08-09 21:50 ` Andrew Pinski
2016-08-09 21:53 ` kugan
2016-09-14 14:31 ` Georg-Johann Lay
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