[cfe-dev] Linking ProfileRT when using -nodefaultlibs

[Alexey Samsonov]

On Mon, Nov 10, 2014 at 5:49 PM, Justin Bogner <mail at justinbogner.com>
wrote:

> Alexey Samsonov <vonosmas at gmail.com> writes:
> > On Mon, Nov 10, 2014 at 3:12 PM, Eric Fiselier <eric at efcs.ca> wrote:
> >
> >     >Interesting. This issue doesn't arise with sanitizers, because we
> put
> >     sanitizer runtimes *before* the user inputs, and force it into the
> >     executable by wrapping it in "-whole-archive". I wonder if
> >     >we can do the same trick for profile runtime. I don't really know
> that,
> >     but it seems we have both GCDA profiling and new InstrProfiling stuff
> >     there, so pulling all of them into the executable
> >     >is not desirable, is it?
> >
> >     I can't answer if we *should* do it put the profile runtime first,
> but I
> >     tested doing so and the change worked for me.
> >
> > What do you mean? We can't put it first, because user libraries depend
> on it.
> > We fail when we put it last either, because
> > of this specific case, where user provides "-lc" + "-nostdlib".
> >
> > We can workaround this by forcing to link in libprofile deps even if
> -nostdlib
> > is specified (assuming the user passing
> > this weird combination knows what (s)he is doing).
> >
> > Or we can design a magic flag "--profile-lib", or smth. like this, which
> > would be matched and replaced by the path to actual static profile
> runtime in
> > the driver. Then you'd be able
> > to write ("clang++ -nostdlib a.o b.o --profile-lib -lc -lpthread").
>
> Can we just teach clang where to find compiler-rt libraries and spell
> this like so?
>
>     clang++ -nostdlib a.o b.o -lclang_rt.profile -lc -lpthread
>
> I don't think I'd mind that so much, as it makes sense for clang to know
> where to find it's own runtime libraries.
>

All compiler-rt libs are target-specific, so that would have to be
-lclang_rt.profile-x86_64.
Forcing user to write triple-dependent linker invocation is not nice...
However,
what we *might* do, is to restructure the way we layout resource directory,
and
make triple-specific folders. That is, replace this:
  lib/clang/3.6.0/lib/linux/libclang_rt.profile-x86_64.a
with this:
  lib/clang/3.6.0/lib/x86_64-linux-gnu/libclang_rt.profile.a
... and then setup correct -L flag depending on the target triple. FWIW, I
do something
similar in the internal compiler setup.
This is a rather intrusive change, though.

Also, this can work for profile or builtins runtimes, but won't work for
sanitizers,
as they require complex changes to the linker invocation.

-- 
Alexey Samsonov
vonosmas at gmail.com
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