Re: better patch for linux/bitops.h

From: Jeffrey Walton
Date: Thu May 05 2016 - 00:03:36 EST

On Wed, May 4, 2016 at 11:50 PM, Theodore Ts'o <tytso@xxxxxxx> 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?