Making C compilers for different architectures output great code from same source is really hard. e.g. "const" is not used by optimizers because it can be cast

Making C compilers for different architectures output great code from same source is really hard. e.g. "const" is not used by optimizers because it can be cast away. Interpreters, compression routines, etc. can always be sped up using assembly. If what your program does can be sped up using vector registers/instructions (e.g. DSP, image and video processing) then you want to do that because x4 and x8 speedups are common. Current autovectorisers are not very good. If it is not the most trivial example like "sum of contiguous array of floats", you'll want to write SIMD assembly or intrinsics or use something like Halide. In practice projects end up using nasm/yasm or creating a fancy macro assembler in a high level language. The choice to use assembly is economics, and it's all a matter of degree. How much performance is left on the table by the compiler? How many C lines of code take up 50% of the cpu time in your program? How rare is the person who is able to write fast assembly/SIMD code? How long does it takes to write correct and fast assembly/SIMD code for only the hot function for 4 different platforms (e.g. in-order ARM, Apple A10, AMD Jaguar, Haswell)? If you think "25%, 100k LoC, very rare, man-years" then you conclude it's not worth it. If you think "x8, 20 lines, only as rare as any other good senior engineer, 50 hours" then you conclude it's stupid to not do the inner loop in assembly. What are the numbers in practice? I don't know. In practice, all the products that have won in their market and can be sped up using SIMD have hand coded assembly or use something like Halide and none of them think the compiler is good enough.