Sanitizer on RenderTargetPool.cpp reports undefined behaviour

[Zefz]

Describe the bug
Using an undefined behaviour sanitizer with RenderTargetPool.cpp:167 reports an issue of unsigned integer undeflow:

if (UTILS_UNLIKELY(mDeepPurgeCountDown-- == 0)) {
        mDeepPurgeCountDown = POOL_ENTRY_MAX_AGE;
        uint32_t age = mCacheAge - POOL_ENTRY_MAX_AGE;
        // remove all entries that are older than CACHE_ENTRY_MAX_AGE
        auto last = std::remove_if(cache.begin(), cache.end(),
                [this, &driver, age](const Entry* entry) {
                    bool remove = entry->age <= age;
                    if (remove) {
                        destroyEntry(driver, entry);
                    }
                    return remove;
                });
        cache.erase(last, cache.end());
    }

In this case mDeepPurgeCountDown (an uint32_t) is zero when this block is reached when it becomes decremented and underflows.

Sanitizer report:

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /filament/filament/src/Camera.cpp:205:23 in
filament/filament/src/RenderTargetPool.cpp:167:9: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'uint32_t' (aka 'unsigned int')
#0 0xf7d904 in filament::RenderTargetPool::gc()
filament/filament/src/RenderTargetPool.cpp:167:9
#1 0xf203fa in filament::details::FRenderer::endFrame()
filament/filament/src/Renderer.cpp:371:9
#2 0xf21aa9 in filament::Renderer::endFrame()
filament/filament/src/Renderer.cpp:460:19

Expected behavior
I am quite sure this is not the desired behaviour. E.g something like this would solve the issue:
UTILS_UNLIKELY(mDeepPurgeCountDown == 0 || mDeepPurgeCountDown-- == 0)

Desktop (please complete the following information):

• OS: Linux Mint 18
• GPU: [NVIDIA GTX 1050]
• Backend: [OpenGL]

Additional context
Seems that this happens on shutdown, so it might not actually cause any runtime issues.

[romainguy]

Thanks for the report!

[pixelflinger]

I don't think there is problem here. mDeepPurgeCountDown is tested before being decremented. If it is 0, then it will indeed underflow, but it is immediately reset to POOL_ENTRY_MAX_AGE.