I'm curious, what are you working on that requires writing inline assembly?
Never for performance. If I wanted to hand optimise code I'd be more likely to use SIMD intrinsics, play with C until the compiler does the right thing, or write the entire function in a separate asm file for better highlighting and easier handing of state at ABI boundary rather than mid-function like the carry flags mentioned above.
Those writing assembler typically/often think/know they can do better than the compiler. That means that isn’t necessarily a good thing.
(Similarly, veltas comment above about “play with C until the compiler does the right thing” is brittle. You don’t even need to change compiler flags to make it suddenly not do the right thing anymore (on the other hand, when compiling for a different version of the CPU architecture, the compiler can fix things, too)
It's brittle depending on your methods. If you understand a little about optimizers and give the compiler the hints it needs to do the right things, then that should work with any modern compiler, and is more portable (and easier) than hand-optimizing in assembly straight away.
gcc optimised it correctly though.
Generally playing with the C until the compiler does the right thing is slightly brittle in terms of performance but not in terms of functionality. Different compiler flags or a different architecture may give you worse performance, but the code will still work.
“Advanced chess is a form of chess in which each human player uses a computer chess engine to explore the possible results of candidate moves. With this computer assistance, the human player controls and decides the game.
Also called cyborg chess or centaur chess, advanced chess was introduced for the first time by grandmaster Garry Kasparov, with the aim of bringing together human and computer skills to achieve the following results:
- increasing the level of play to heights never before seen in chess;
- producing blunder-free games with the qualities and the beauty of both perfect tactical play and highly meaningful strategic plans;
- offering the public an overview of the mental processes of strong human chess players and powerful chess computers, and the combination of their forces.”
In general case you're right. Modern compilers are beasts.
And that was already after copious `assert_unchecked`s to have the compiler assume as many invariants as it could!
This is one of the remaining cases where the current compilers optimize rather poorly: when you have a tight loop around a huge switch-statement, with each case-statement performing a very small operation on common data.
In that case, a human writing assembler can often beat a compiler with a huge margin.
https://blog.reverberate.org/2025/02/10/tail-call-updates.ht...
LLVM codegen has been almost always sufficient, but for a routine that essentially amounts to adding two fixed-size bigints (e.g. 1024-bit ints represented as `[u64; 16]`), codegen was very very bad.
Writing a jump table by hand literally made the code 3× faster :)
It still happens in 2025