Path: csiph.com!xmission!news.snarked.org!news.linkpendium.com!news.linkpendium.com!news.iecc.com!.POSTED.news.iecc.com!nerds-end
From: Martin Ward <martin@gkc.org.uk>
Newsgroups: comp.compilers
Subject: Re: Optimization techniques and undefined behavior
Date: Thu, 2 May 2019 14:54:46 +0100
Organization: Compilers Central
Lines: 33
Sender: news@iecc.com
Approved: comp.compilers@iecc.com
Message-ID: <19-05-007@comp.compilers>
References: <19-05-003@comp.compilers> <19-04-021@comp.compilers> <19-04-023@comp.compilers> <19-04-037@comp.compilers> <19-04-039@comp.compilers> <19-04-042@comp.compilers> <19-04-045@comp.compilers> <19-04-049@comp.compilers>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="66462"; mail-complaints-to="abuse@iecc.com"
Keywords: optimize, arithmetic, errors
Posted-Date: 02 May 2019 12:09:37 EDT
X-submission-address: compilers@iecc.com
X-moderator-address: compilers-request@iecc.com
X-FAQ-and-archives: http://compilers.iecc.com
Xref: csiph.com comp.compilers:2243

On 01/05/19 12:40, Bart wrote:
>         for (int i=0; i<1000000000; ++i) {
>             y=x*3/3;
>             z+=y;
>             ++x;
>         }

Given that gcc can optimise y=x*3/3 to y=x
I would have expected the whole loop to be recognised
as computing the sum of an arithmetic sequence
and therefore optimised away completely.

This blog post:

https://xebia.com/blog/gcc-compiler-optimizations-dissection-of-a-benchmark/

discusses a program with the loop:

for( a = 0; a < arg1; a++ )
{
   sum += a;
}

and claims that gcc with -O3 will optimise
the loop to take only costant time.

But this is not the case on my machine with gcc 4.9.2

--
			Martin

Dr Martin Ward | Email: martin@gkc.org.uk | http://www.gkc.org.uk
G.K.Chesterton site: http://www.gkc.org.uk/gkc | Erdos number: 4