Re: [libvirt PATCH] meson: stop CLang doing inter-procedural analysis

The virNumaNodeIsAvailable function is stubbed out when building without libnuma, such that it just returns a constant value. When CLang is optimizing, it does inter-procedural analysis across function calls. When it sees that the call to virNumaNodeIsAvailable returns a fixed constant, it elides the conditional check for errors in the callers such as virNumaNodesetIsAvailable. This is a valid optimization as the C standard declares that there must only be one implementation of each function in a binary. This is normally the case, but ELF allows for function overrides when linking or at runtime with LD_PRELOAD, which is technically outside the mandated C language behaviour. So while CLang's optimization works fine at runtime, it breaks in our test suite which aims to mock the virNumaNodeIsAvailable function so that it has specific semantics regardless of whether libnuma is built or not. The return value check optimization though means our mock override won't have the right effect. The mock will be invoked, but its return value is not used. Potentially the same problem could be exhibited with GCC if certain combinations of optimizations are enabled, though thus far we've not seen it. To be robust on both CLang and GCC we need to make it more explicit that we want to be able to replace functions and thus optimization of calls must be limited. Currently we rely on 'noinline' which does succesfully prevent inlining of the function, but it cannot stop the eliding of checks based on the constant return value. Thus we need a bigger hammer. There are a couple of options to disable this optimization: * Annotate a symbol as 'weak'. This is tells the compiler that the symbol is intended to be overridable at linktime or runtime, and thus it will avoid doing inter-procedural analysis for optimizations. This was tried previously but have to be reverted as it had unintended consequences when linking .a files into our final .so, resulting in all the weak symbol impls being lost. See commit 407a281a8e2b6c5078ba1148535663ea64fd9314 * Annotate a symbol with 'noipa'. This tells the compiler to avoid inter-procedural analysis for calls to just this function. This wold be ideal match for our scenario, but unfortunately it is only implemented for GCC currently: https://reviews.llvm.org/D101011 * The '-fsemantic-interposition' argument tells the optimizer that any functions may be replaced with alternative implementations that have different semantics. It thus blocks any optimizations across function calls. This is quite a harsh block on the optimizer, but it appears to be the only one that is viable with CLang. Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; --- meson.build | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)