From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-195471-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 107220 invoked by alias); 6 Mar 2018 16:09:14 -0000
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
Received: (qmail 107211 invoked by uid 89); 6 Mar 2018 16:09:13 -0000
Authentication-Results: sourceware.org; auth=none
X-Virus-Found: No
X-Spam-SWARE-Status: No, score=-2.3 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_PASS autolearn=ham version=3.3.2 spammy=prathamesh.kulkarni@linaro.org, prathameshkulkarnilinaroorg, H*i:sk:CAHFci2, H*f:sk:CAHFci2
X-HELO: mail-lf0-f44.google.com
Received: from mail-lf0-f44.google.com (HELO mail-lf0-f44.google.com) (209.85.215.44) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Tue, 06 Mar 2018 16:09:12 +0000
Received: by mail-lf0-f44.google.com with SMTP id q69so29264915lfi.10        for <gcc@gcc.gnu.org>; Tue, 06 Mar 2018 08:09:11 -0800 (PST)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;        d=1e100.net; s=20161025;        h=x-gm-message-state:mime-version:in-reply-to:references:from:date         :message-id:subject:to:cc;        bh=Io0kPXMBDAI6V+gXyvDjoNldTy7ZU0UE/5GrA5TZVZM=;        b=Zr/zOBExR1lqbbnpYCNCdCEd+pLDzyoDuyQDnXblX8wqq5zaM9toplBtV8uzQMeK2T         EPepG7bmNLmPtP/+IZeKRC6uNLzhyzHSb6horjHryFRkZHK6/vr60UyDWLbx2HEFKCxa         PPW8Ufwee+FzpjonBU7q16/yGVNJepl7xT2JknILnoSs+rYZotb8Y+RcY0QAnHRJYL2b         mYSleELcR9f9dvXhS4gfIgZUqY3o9TEnyWeSuMBj4Sqxxrt9QBuDZRnUaxOTa00dTOp8         VuZ44CyCDkKfgzThMFokU+jNqWXz+OFQRhls1xUJ2NyUCKIqrayqOeRYM6CL3S5CiF4S         u8Ng==
X-Gm-Message-State: APf1xPDZ6mzId2SvqXIPC7MmjMuRyeA+1OrfFPONHlmB0XRGXUGouLV3	9QcR0blDj2v8VjP34p31BCQHRZXD1OzKq0rpug4=
X-Google-Smtp-Source: AG47ELt5rDbLYxcA6MRkhWQAvGLQYM2h07r5AEt+Jtn2XKk7EOllqTI94VOtEzfgUYoerlR+rWRgUYgEotnn3NwUx9c=
X-Received: by 10.46.82.16 with SMTP id g16mr12882507ljb.67.1520352550124; Tue, 06 Mar 2018 08:09:10 -0800 (PST)
MIME-Version: 1.0
Received: by 10.46.104.4 with HTTP; Tue, 6 Mar 2018 08:09:09 -0800 (PST)
In-Reply-To: <CAHFci2_vzxxtSjzSba7z-HZQJx29zJCWHBTTaQABeEytCa0yCw@mail.gmail.com>
References: <CAAgBjMkcuch04W94DB06Vv7oYZQPLk2VRN5ALd_0Vt=CmzZ=-w@mail.gmail.com> <CAFiYyc0ZwKqyfX2hyFS+s7tB=gaEJ5OS1zq6BciDFLnn=ynM3w@mail.gmail.com> <CAHFci2_vzxxtSjzSba7z-HZQJx29zJCWHBTTaQABeEytCa0yCw@mail.gmail.com>
From: Richard Biener <richard.guenther@gmail.com>
Date: Tue, 06 Mar 2018 16:09:00 -0000
Message-ID: <CAFiYyc1AMoT-hTWcgw6ceaVWJ=qo1WCAfRK3VHwrZWh-zDOaUw@mail.gmail.com>
Subject: Re: eliminate dead stores across functions
To: "Bin.Cheng" <amker.cheng@gmail.com>
Cc: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>, GCC Development <gcc@gcc.gnu.org>
Content-Type: text/plain; charset="UTF-8"
X-IsSubscribed: yes
X-SW-Source: 2018-03/txt/msg00072.txt.bz2

On Tue, Mar 6, 2018 at 4:50 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Tue, Mar 6, 2018 at 2:28 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Tue, Mar 6, 2018 at 1:00 PM, Prathamesh Kulkarni
>> <prathamesh.kulkarni@linaro.org> wrote:
>>> Hi,
>>> For the following test-case,
>>>
>>> int a;
>>>
>>> __attribute__((noinline))
>>> static void foo()
>>> {
>>>   a = 3;
>>> }
>>>
>>> int main()
>>> {
>>>   a = 4;
>>>   foo ();
>>>   return a;
>>> }
>>>
>>> I assume it's safe to remove "a = 4"  since 'a' would be overwritten
>>> by call to foo ?
>>> IIUC, ipa-reference pass does mod/ref analysis to compute side-effects
>>> of function call,
>>> so could we perhaps use ipa_reference_get_not_written_global() in dse
>>> pass to check if a global variable will be killed on call to a
>>> function ? If not, I suppose we could write a similar ipa pass that
>>> computes the set of killed global variables per function but I am not
>>> sure if that's the correct approach.
>>
>> Do you think the situation happens often enough to make this worthwhile?
> There is one probably more useful case.  Program may use global flags
> controlling
> how it does (heavy) computation.  Such flags are only set couple of
> times in execution
> time.  It would be useful if we can (IPA) propagate flags into computation heavy
> functions by versioning (if necessary).  For example:
>
> int flag = 1;
> void foo ()
> {
>   //heavy computation wrto to flag
> }
> void main()
> {
>   flag = 2;
>   foo();
>   flag = 1;
>   foo();
> }

Yeah, libquantum does this.  There's also related example
from some SPEC fortran testcase:

vodi foo()
{
  static int initialized;
  static T data;
  if (!initialized)
    {
       data.x = 1;
       initialized = 1;
    }
...
}

where we want to constant propagate from data.x.  IIRC I tried
to work on this, not sure if I solved it yet...

Richard.

> Of course this may only be useful for LTO.

> Thanks,
> bin
>>
>> ipa-reference doesn't compute must-def, only may-def and may-use IIRC.
>>
>> Richard.
>>
>>> Thanks,
>>> Prathamesh