From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <dhowells@redhat.com>
Received: from mx2.redhat.com (mx2.redhat.com [66.187.237.31])
	by ozlabs.org (Postfix) with ESMTP id 8EF9CDDF70
	for <linuxppc-dev@ozlabs.org>; Thu, 11 Dec 2008 00:07:21 +1100 (EST)
From: David Howells <dhowells@redhat.com>
In-Reply-To: <1795455197.20081210132912@emcraft.com>
References: <1795455197.20081210132912@emcraft.com>
	<200812092044.40649.yur@emcraft.com>
	<Pine.LNX.4.64.0812100944080.25126@vixen.sonytel.be>
	<503391615.20081210130113@emcraft.com>
	<20081210101745.GG28946@ZenIV.linux.org.uk>
To: Yuri Tikhonov <yur@emcraft.com>
Subject: Re: Re[2]: [PATCH] fork_init: fix division by zero
Date: Wed, 10 Dec 2008 13:06:44 +0000
Message-ID: <5545.1228914404@redhat.com>
Sender: dhowells@redhat.com
Cc: Wolfgang Denk <wd@denx.de>, Detlev Zundel <dzu@denx.de>,
	linux-kernel@vger.kernel.org, Milton Miller <miltonm@bga.com>,
	linuxppc-dev@ozlabs.org, Al Viro <viro@ZenIV.linux.org.uk>,
	Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>,
	Ilya Yanok <yanok@emcraft.com>
List-Id: Linux on PowerPC Developers Mail List <linuxppc-dev.ozlabs.org>
List-Unsubscribe: <https://ozlabs.org/mailman/options/linuxppc-dev>,
	<mailto:linuxppc-dev-request@ozlabs.org?subject=unsubscribe>
List-Archive: <http://ozlabs.org/pipermail/linuxppc-dev>
List-Post: <mailto:linuxppc-dev@ozlabs.org>
List-Help: <mailto:linuxppc-dev-request@ozlabs.org?subject=help>
List-Subscribe: <https://ozlabs.org/mailman/listinfo/linuxppc-dev>,
	<mailto:linuxppc-dev-request@ozlabs.org?subject=subscribe>

Yuri Tikhonov <yur@emcraft.com> wrote:

>  Here we believe in preprocessor: since all PAGE_SIZE, 8, and 
> THREAD_SIZE are the constants we expect it will calculate this.

The preprocessor shouldn't be calculating this.  I believe it will _only_
calculate expressions for #if.  In the situation you're referring to, it
should perform a substitution and nothing more.  The preprocessor doesn't
necessarily know how to handle the types involved.

In any case, there's an easy way to find out: you can ask the compiler to give
you the result of running the source through the preprocessor only.  For
instance, if you run this:

	#define PAGE_SIZE 4096
	#define THREAD_SIZE 8192
	unsigned long mempages;
	unsigned long jump(void)
	{
		unsigned long max_threads;
		max_threads = mempages * PAGE_SIZE / (8 * THREAD_SIZE);
		return max_threads;
	}

through "gcc -E", you get:

	# 1 "calc.c"
	# 1 "<built-in>"
	# 1 "<command line>"
	# 1 "calc.c"
	unsigned long mempages;
	unsigned long jump(void)
	{
	 unsigned long max_threads;
	 max_threads = mempages * 4096 / (8 * 8192);
	 return max_threads;
	}


>  In any case, adding braces as follows probably would be better:
> 
> +     max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));

I think you mean brackets, not braces '{}'.

>  Right ?

Definitely not.

I added this function to the above:

	unsigned long alt(void)
	{
		unsigned long max_threads;
		max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
		return max_threads;
	}

and ran it through "gcc -S -O2" for x86_64:

	jump:
		movq    mempages(%rip), %rax
		salq    $12, %rax
		shrq    $16, %rax
		ret
	alt:
		xorl    %eax, %eax
		ret

Note the difference?  In jump(), x86_64 first multiplies mempages by 4096, and
_then_ divides by 8*8192.

In alt(), it just returns 0 because the compiler realised that you're
multiplying by 0.

If you're going to bracket the expression, it must be:

		max_threads = (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);

which should be superfluous.

>  E.g. here is the result from this line as produced by cross-gcc 
> 4.2.2:
> 
>         lis     r9,0
>         rlwinm  r29,r29,2,16,29
>         stw     r29,0(r9)
> 
>  As you see - only rotate-left, i.e. multiplication to the constant.

Ummm...  On powerpc, I believe rotate-left would be a division as it does the
bit-numbering and the bit direction the opposite way to more familiar CPUs
such as x86.

David