Re: [Xen-devel] [PATCH 0/5] use mask operations instead of test_bit()

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Fri, 2015-10-02 at 06:40 +0200, Juergen Gross wrote:
> Instead of using test_bit() which is an atomic operation and limits
> the compiler's choices to do optimization, use logical ANDs with
> bitmasks where possible.
> 
That's a good idea, I think.

> The possible candidates have been detected by searching definitions
> of bitmasks in the form:
> 
> #define MASK  1 << _MASK
> 
Right.

> On x86 the resulting code is slightly smaller (about 2 bytes for each
> case, checked via disassembly in few examples).
> 
> I'm quite sure I didn't replace a test_bit() call required to be
> atomic, but I'd be grateful for a thorough review especially in the
> scheduler.
> 
I'll have a deep look.

One question, can we introduce a __test_bit() macro/inline function,
like Jan did with __set_bit?

I've quickly-&-dirtily tested this:

#define __test_bit(nr, addr) ({  \
    unsigned _flags = 1 << nr;   \
    addr & _flags; \
})

and the result (I've checked a couple of cases) seems the same to me.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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