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Re: [libvirt] [PATCH] Rewrite the way mockable functions are handled.



On Thu, Jul 13, 2017 at 01:14:12PM +0200, Martin Kletzander wrote:
> On Wed, Jul 12, 2017 at 05:06:22PM +0100, Daniel P. Berrange wrote:
> > Currently any mockable functions are marked with attributes
> > noinline, noclone and weak. This prevents the compiler from
> > optimizing away the impl of these functions.
> > 
> > It has an unfortunate side effect with the libtool convenience
> > libraries, if executables directly link to them. For example
> > virlockd, virlogd both link to libvirt_util.la  When this is
> > done, the linker does not consider weak symbols as being
> > undefined, so it never copies them into the final executable.
> > 
> > In this new approach, we stop annotating the headers entirely.
> > Instead we create a weak function alias in the source.
> > 
> >   int fooImpl(void) {
> >      ..the real code..
> >   }
> > 
> >   int foo(void) __attribute__((noinline, noclone, weak, alias("fooImpl"))
> > 
> > If any functions in the same file call "foo", this prevents the
> > optimizer from inlining any part of fooImpl. When linking to the
> > libtool convenience static library, we also get all the symbols
> > present. Finally the test suite can just directly define a
> > 'foo' function in its source, removing the need to use LD_PRELOAD
> > (though removal of LD_PRELOADS is left for a future patch).
> > 
> 
> What are you using for tag navigation?  With this macro-defined
> functions I cannot easily jump to them (the main reason why I don't like
> macros defining functions).

Grep & historic knowledge :-)

>                              I would very much prefer if
> ATTRIBUTE_MOCKABLE just took a parameter like this:
> 
> #define ATTRIBUTE_MOCKABLE(func) __attribute__((noinline, noclone, weak, alias(#func "Impl"))
> 
> (written by hand, don't take mu word for it working out of the box) and
> the original function would be left untouched.

Yeah, that is certainly a valid alternative. I was not entirely happy
with the results I get here. As long as we don't mind repeating the
arg list in both places, your approach is more attractive, despite
the duplication.

> Also, this fails on OS X [1] and I don't really see why:
> 
> util/vircommand.c:988:1: error: only weak aliases are supported on darwin
> VIR_MOCKABLE(void,
> ^
> ./internal.h:252:60: note: expanded from macro 'VIR_MOCKABLE'
>      ret name(__VA_ARGS__) __attribute__((noinline, weak, __alias__(#name "Impl"))); \

It appears clang simply doesn't support 'alias' on Darwin. 

Docs suggest that we need to use 'weakref(#name  "Impl")' so I'll have
to test whether that's viable for our usage scenario or not.

Regards,
Daniel
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