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

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Message-ID: <20070502003557.GK4095@ftp.linux.org.uk>
Date:	Wed, 2 May 2007 01:35:57 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Josh Triplett <josh@...edesktop.org>, linux-sparse@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: sparse -Wptr-subtraction-blows: still needed?

On Tue, May 01, 2007 at 05:24:54PM -0700, Linus Torvalds wrote:
> The only thing that was gcc-specific about it is that gcc goes to some 
> extreme lengths to turn the constant-sized division into a sequence of 
> shifts/multiples/subtracts, and can often turn a division into something 
> like ten cheaper operations instead.
> 
> But that optimization was also what made gcc take such a long time if the 
> constant division is very common (ie a header file with an inline 
> function, whether that function is actually _used_ or not apparently 
> didn't matter to gcc)

There used to be a Dumb Algorithm(tm) in there - basically, gcc went through
all decompositions of constant it was multiplying at and it did it in the
dumbest way possible (== without keeping track of the things like "oh, we'd
already looked at the best way to multiply by 69, no need to redo that
again and again)").  _That_ is what blew the build time to hell for a while.
And yes, that particular idiocy got fixed (they added a moderately-sized
LRU of triplets <constant, price, best way to multiply on it>, which killed
recalculation from scratch for each pointer subtraction and got complexity
of dealing with individual subtraction into tolerable range).

That's separate from runtime issues, of course.
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