Comment by MYEUHD
Comment by MYEUHD a day ago
I'm curious, what are you working on that requires writing inline assembly?
Comment by MYEUHD a day ago
I'm curious, what are you working on that requires writing inline assembly?
Generally inline assembly is much easier these days as a) the compiler can see into it and make optimizations b) you don’t have to worry about calling conventions
> the compiler can see into it and make optimizations
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 rare that I see compiler-generated assembly without obvious drawbacks in it. You don't have to be an expert to spot them. But frequently the compiler also finds improvements I wouldn't have thought of. We're in the centaur-chess moment of compilers.
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.
Of course you can often beat the compiler, humans still vectorize code better. And that interpreter/emulator switch-statement issue I mentioned in the other comment. There are probably a lot of other small niches.
In general case you're right. Modern compilers are beasts.
> “play with C until the compiler does the right thing” is brittle
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.
Might be an interpreter or an emulator. That’s where you often want to preserve registers or flags and have jump tables.
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.
I'm curious if that's still the case generally after things like musttail attributes to help the compiler emit good assembly for well structured interpreter loops:
https://blog.reverberate.org/2025/02/10/tail-call-updates.ht...
I'm not them but whenever I've used it it's been for arch specific features like adding a debug breakpoint, synchronization, using system registers, etc.
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.