linux-kernel - Re: better patch for linux/bitops.h

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Date:	Thu, 5 May 2016 00:03:29 -0400
From:	Jeffrey Walton <noloader@...il.com>
To:	"Theodore Ts'o" <tytso@....edu>,
	Jeffrey Walton <noloader@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, John Denker <jsd@...n.com>,
	linux-kernel@...r.kernel.org,
	Stephan Mueller <smueller@...onox.de>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Andi Kleen <andi@...stfloor.org>,
	Sandy Harris <sandyinchina@...il.com>,
	cryptography@...edaemon.net, linux-crypto@...r.kernel.org
Subject: Re: better patch for linux/bitops.h

On Wed, May 4, 2016 at 11:50 PM, Theodore Ts'o <tytso@....edu> wrote:
> ...
> But instead of arguing over what works and doesn't, let's just create
> the the test set and just try it on a wide range of compilers and
> architectures, hmmm?

What are the requirements? Here's a short list:

  * No undefined behavior
    - important because the compiler writers use the C standard
  * Compiles to native "rotate IMMEDIATE" if the rotate amount is a
"constant expression" and the machine provides it
    - translates to a native rotate instruction if available
    - "rotate IMM" can be 3 times faster than "rotate REG"
    - do any architectures *not* provide a rotate?
  * Compiles to native "rotate REGISTER" if the rotate is variable and
the machine provides it
    - do any architectures *not* provide a rotate?
  * Constant time
    - important to high-integrity code
    - Non-security code paths probably don't care

Maybe the first thing to do is provide a different rotates for the
constant-time requirement when its in effect?

Jeff