Re: better patch for linux/bitops.h

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On 05/04/16 21:03, Jeffrey Walton wrote:
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?


The disagreement here is the priority between these points. In my very strong opinion, "no undefined behavior" per the C standard is way less important than the others; what matters is what gcc and the other compilers we care about do. The kernel relies on various versions of C-standard-undefined behavior *all over the place*; for one thing sizeof(void *) == sizeof(size_t) == sizeof(unsigned long)!! but they are well-defined in the subcontext we care about.

(And no, not all architectures provide a rotate instruction.)

	-hpa

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