Regarding usage of UTILS_LIKELY/UTILS_UNLIKELY

[adamwych]

Hello. When browsing through the code I noticed a lot of UTILS_LIKELY and UTILS_UNLIKELY calls near branches. As I understood (from the docs), they tell the compiler whether a branch is... likely or unlikely - but often those calls seem to be inverted.

For example, here, when creating a material instance (https://github.com/google/filament/blob/main/filament/src/details/Engine.cpp#L808):

FMaterialInstance* FEngine::createMaterialInstance(const FMaterial* material,
        const FMaterialInstance* other, const char* name) noexcept {
    FMaterialInstance* p = mHeapAllocator.make<FMaterialInstance>(*this, other, name);
    if (UTILS_UNLIKELY(p)) { // should never happen
        auto pos = mMaterialInstances.emplace(material, "MaterialInstance");
        pos.first->second.insert(p);
    }
    return p;
}

This if would evaluate to something like:

if (__builtin_expect (p, false)) {
  // ...
}

The branch here is marked as unlikely (as in, we expect it to be 0), but I think it should be the opposite - it's expected to happen basically always? Did I understand it incorrectly or is this a mistake?

@pixelflinger I saw that your commit added it, can you explain?

[pixelflinger]

Oh that definitely looks like a mistake.