> Le 25 févr. 2016 à 02:26, Jens Alfke <[email protected]> a écrit :
>
> Is there a way to control which section of the executable a specific NSString
> literal goes into? I’ve tried using the ‘section’ attribute, but it has no
> effect:
> __attribute__((section(“__TEXT,__foo”))) static NSString* const kFoo =
> @“Foobar”;
> The above doesn’t change where the string appears; there is no __TEXT,__foo
> section in the binary at all, and the string is still in the default
> __TEXT,__cstring section.
>
> I’m asking because the project I work on includes a lot of logging calls,
> which are left in the release build so that logging can be turned on for
> troubleshooting in the field, but for reasons of cache locality I’d like the
> resulting strings (and ideally the code) not to be mixed in with the rest of
> the executable.
>
> (Moving the code itself out of the way seems to be a non-starter. I’ve tried
> using some macro hackery to put the actual _log(…) call in a block, but
> adding a ‘section’ attribute to the block produces a compiler error saying
> that only functions and globals can have a section assigned.)
>
> —Jens
If your code is performance sensitive enough that you have to consider cache
locality of static strings, you may consider replacing your logging code by
dtrace USDT (User-Level Statically Defined Tracing).
The main benefit is that when disabled, they cost almost nothing (not even a
branch as traditional log does).
To do that, you just create a primes.d file that contains the list of probe you
want to define:
provider primes {
/* Start of the prime calculation */
probe primecalc__start(long prime);
/* End of the prime calculation */
probe primecalc__done(long prime, int isprime);
/* Exposes the size of the table of existing primes */
probe primecalc__tablesize(long tablesize);
};
You add that file to your Xcode project, and it will automatically generate the
corresponding header.
You can the include the header in your source:
--------
#include "primes.h"
long prime(long value) {
PRIMES_PRIMECALC_START(value);
….
PRIMES_PRIMECALC_DONE(result);
if (PRIMES_PRIMECALC_TABLESIZE_ENABLED()) {
// Do some computation useless when probe disabled.
PRIMES_PRIMECALC_TABLESIZE(size);
}
}
--------
Note that even if some code is conditionally protected by a if branch, the
linker replace it at link time by custom dtrace code, so you don’t have to pay
for it (dtrace is designed to cost nothing when disabled).
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