[PATCH] New attribute to create target clones

Jeff Law law@redhat.com
Tue Sep 22 20:24:00 GMT 2015

On 09/21/2015 07:25 AM, Bernd Schmidt wrote:
> On 08/27/2015 01:18 PM, Evgeny Stupachenko wrote:
>> Based on RFC:
>> https://gcc.gnu.org/ml/gcc-patches/2015-08/msg01322.html
>> The patch implement an extension to Function Multiversioning that
>> allows to clone a function for multiple targets.
>> __attribute__((target_clones("avx","arch=slm","default")))
>> int foo ()
>> ...
>> Will create 3 clones of foo(). One optimized with -mavx, one optimized
>> with -march=slm, and one with default optimizations.
>> And will create ifunc resolver that calls appropriate clone (same as
>> in Function Multiversioning).
> The general question is - do we want this, given that it seems to
> introduce no functionality that can't be had with the existing
> multiversioning? You could always compile the same source file to
> multiple objects with different defines for the target optimization, or
> include a file containing the multiversioned function multiple times
> with changing #defines.
Essentially it allows us to more easily support 
per-microarchitecture-optimized versions of functions.   You list just 
have to list the microarchitectures and the compiler handles the rest. 
Very simple, very easy.  I'd think it'd be particularly helpful for 

You could emulate this with compiling the same source multiple times 
with different flags/defines and wire up on ifunc by hand.  But Evgeny's 
approach is vastly simpler.

We are still waiting on Jan to chime in as to whether or not any of this 
breaks the rules WRT nodes in the symbol table.  Given I mucked up the 
MPX code by not knowing those rules, I'm hesitant to approve this patch 
without input from Jan.


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