I hate to say it but that simply doesn't work: you can't write out of bounds to trick the compiler, it'll just ignore your out of bounds work. You can look at

I hate to say it but that simply doesn't work: you can't write out of bounds to trick the compiler, it'll just ignore your out of bounds work. You can look at the generated sass on godbolt: https://cuda.godbolt.org/z/19excTxM3 https://cuda.godbolt.org/z/19excTxM3 Note that there are 1024 FFMA instructions in the loop but you would expect 16*8*BK = 2048. This would suggest half the operations are skipped, which lines up with the half of writes that are out of bounds being omitted. After the compute loop when you're calculating the final result and storing it, you can see that the FFMAs referencing out of bounds indices write QNAN instead of any real results. Is it possible that the NANs are what are messing with your tests? Those are notoriously hard to deal with correctly, but you should assert that the result doesn't have any NANs whatsoever.