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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list