linux-kernel - Re: sparse -Wptr-subtraction-blows: still needed?

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Date:	02 May 2007 15:19:11 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Triplett <josh@...edesktop.org>,
	linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@....linux.org.uk>
Subject: Re: sparse -Wptr-subtraction-blows: still needed?

Alan Cox <alan@...rguk.ukuu.org.uk> writes:
> 
> Good compilers even in the 1990's would defer the divide and try and
> propogate it out as a multiply the other side for constants, and they'll
> also use shifts when possible.

gcc has an algorithm that tends to generate a near perfect shift/add etc.
code sequence and also knows the obvious x / y ==> x*1/y

However it doesn't do that with -Os, prefering smaller code on x86
(idiv is fairly small compared to the expanded sequences for non power
of two dividends) and kernels are usually compiled with -Os these
days.

We've had a few cases in the past where this showed up as regression
against older kernels that still used -O2.

> Thus they'll turn
> 
> 	(ptr.element - base.element) < NELEM
> 
> into
> 	(ptr.char - base.char) < (constant) [NELEM *sizeof(element) ]
> 
> 
> at least for constant operations. Dunno if gcc is that clever

It is. However a few more complex transformations I would have liked
in the past are missing -- in particular 

x / (cond ? const1 : const2) ==> cond ? (x / const1) : (x / const2)

-Andi
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