linux-kernel - Re: Why is the kfree() argument const?

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Date:	Fri, 18 Jan 2008 00:30:29 -0800
From:	Vadim Lobanov <vlobanov@...akeasy.net>
To:	Giacomo Catenazzi <cate@...eee.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Schwartz <davids@...master.com>,
	Johannes Weiner <hannes@...urebad.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	clameter@....com, penberg@...helsinki.fi
Subject: Re: Why is the kfree() argument const?

On Thursday 17 January 2008 11:51:49 pm Giacomo Catenazzi wrote:
> Linus Torvalds wrote:
> > No, I'm saying that "const" has absolutely *zero* meaning on writes to an
> > object through _other_ pointers (or direct access) to the object.
>
> Hints: "restrict" is the C99 keyword for such requirement (or better
> "const restrict")

The restrict keyword controls aliasing, to be exact. And I'm skeptical that 
inserting const there would do anything at all.

> BTW I think C use non const free as a BIG warning about not to be
> to "smart" on optimization.

I must ask what relationship you think the const keyword has to compiler 
optimizations. I know of none, and I've yet to see that keyword cause any 
difference in the resulting assembly. It forces you to make your code clean 
and well-structured, but that's about it.

Of course, it would be an interesting experiment to potentially redefine the 
const keyword to have stronger semantics, such as having the compiler assume 
that a function taking a const pointer argument will not modify the memory 
the pointer points to, and thus saving itself a memory load in the caller 
after the function executes, as long as the data is not global. I imagine 
that this would lead to some simple and measurable optimizations, all the 
while (this is where I get into hand-waving territory) breaking a minimum 
amount of code in current existence.

But that is emphatically not how C is currently defined, and you're basically 
inventing an entirely new language... C2009 perhaps? :-)

-- Vadim Lobanov
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