Blog author (and dav1d/ffmpeg dev) here. My talk at VDD 2023 (https://www.youtube.com/watch?v=Z4DS3jiZhfo&t=9290s) did a comparison like the ones asked above. I compared an intrinsics implementation of the AV1 inverse transform with the hand-written assembly one found in dav1d. I analyzed the compiler-generated version (from intrinsics) versus the hand-written one in terms of instruction count (and cycle runtime, too) for different types of things a compiler does (data loads, stack spills, constant loading, actual multiply/add math, addition of result to predictor, etc.). Conclusion: modern compilers still can't do what we can do by hand, the difference is up to 2x - this is a huge difference. It's partially because compilers are not as clever as everyone likes to think, but also because humans are more clever and can choose to violate ABI rules if it's helpful, which a compiler cannot do. Is this hard? Yes. But at some scale, this is worth it. dav1d/FFmpeg are examples of such scale.