On 09/22/2015 03:09 PM, Bernd Schmidt wrote:
On 09/22/2015 09:41 PM, Jeff Law wrote:
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
vectorization.
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.
As far as I can tell the ifunc is generated automatically (and the
functionality is documented as such), so the new target_clone doesn't
buy much. But one thing I didn't was that the existing support is only
available in C++, while Evgeny's patch works for C. That is probably an
argument that could be made for its inclusion.
In multi-versioning, you have a distinct source implementation for each
function. ie
__attribute__ ((target ("default")))
int foo ()
{
// The default version of foo.
return 0;
}
__attribute__ ((target ("sse4.2")))
int foo ()
{
// foo version for SSE4.2
return 1;
}
And so-on for each processor version you want to support.
In Evgeny's patch we'd have a single source implementation of foo which
gets compiled multiple times, once for each target specified by the
attribute.
I wasn't aware that multi-versioning was only implemented for C++, that
seems fairly lame. I hope I didn't approve that :-)
Jeff
