linux-kernel - Re: [PATCH] virtio_balloon: fix towards

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Date:	Mon, 18 Aug 2008 17:42:55 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anthony Liguori <aliguori@...ibm.com>
cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Avi Kivity <avi@...ranet.com>,
	virtualization@...ts.linux-foundation.org,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH] virtio_balloon: fix towards_target when deflating
 balloon



On Mon, 18 Aug 2008, Anthony Liguori wrote: <
> 
> This handles the case where v < vb->num_pages and ensures we get a 
> small, negative, s64 as the result.

That's just horrible code.

Maybe the compiler notices that you're doing something stupid, but 
basically, please don't do this.

> -	return v - vb->num_pages;
> +	if (v < vb->num_pages)
> +		return -(s64)(vb->num_pages - v);
> +	else
> +		return v - vb->num_pages;

What's wrong with just doing

	return (s64)v - vb->num_pages;

instead?

Casting 'v' to s64 guarantees that the subtraction will eb done in 64 
bits, and the compiler can just generate the trivial non-conditional code.

		Linus
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