[LLVMdev] [RFC] Storing default function attributes on the module

[Eric Christopher]

On Wed, Mar 11, 2015 at 1:08 PM Rafael Espíndola <rafael.espindola at gmail.com>
wrote:

> > 4. Having `llc` mutate the IR itself -- the obvious solution, which
> >    Akira posted a patch for a few months ago -- does the job just as
> >    well as `llvm-attr-mutate` but with a much cleaner interface.  It
> >    fails to distinguish between target defaults and explicit
> >    attributes, but when combined with `llvm-extract`, it gives you full
> >    control over the codegen options for each function.
> >
> > Remind me again why we don't just do #4?  It seems like the simplest way
> > to keep `llc` viable in the short term.
> >
> >
> >
> > llc should be able to override the default values for options without
> > mutating anything that’s explicitly specified at a function (or module)
> > level.
> >
> > For example, if I have a .ll file with specialized functions with
> different
> > CPUs specified, I want to be able to recompile that file with -mcpu= on
> the
> > llc command line in such a way that those functions don’t get changed,
> but
> > any functions that don’t have an explicit CPU on them do. Consider
> trying to
> > test something like function multi versioning.
>
> Sure, but that can be implemented with 4, no? Something like:
>
> * In llc, if given -mcpu:
>   * Iterate over all functions:
>      * If a function has a an explicit cpu, leave it alone.
>      * If a function has no explicit cpu, set it to the -mcpu command line
> value
>
>
The problem with some of this is that we could see a lot of default cpus on
functions. I.e. it may be set most of the time. That's still up in the air
though.


-eric


> Cheers,
> Rafael
>
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