[cfe-dev] __attribute__ ((target("XXX"))) semantic in clang and gcc

Joerg Sonnenberger via cfe-dev cfe-dev at lists.llvm.org
Wed Dec 9 02:13:52 PST 2015


On Tue, Dec 08, 2015 at 01:00:47PM -0800, David Blaikie wrote:
> > This kind of logic sounds to be exactly like the
> > thing an application should be controlling.
> >
> 
> That's an option - if we surface the raw ifunc stuff, which I imagine we
> would first before implementing the higher level function multiversioning.

ifunc again is a non-portable feature...

> But I'm not sure it adds a lot of value to have the application write that
> code - but sure, could take a bit of a survey and see if function
> multiversioning is used much/worth the compiler implementation complexity
> over just implementing ifunc and letting the user figure it out from there.

Most libraries that care just have function pointers they call and
initialise to the appropiate version. That's how they have been using
optimised assembler versions for specific targets for ages. I don't see
how this is different. More importantly, it gives users/developers
control over what happens, which is surprisingly often an issue.

Joerg


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