Hello, On Tue, 31 Oct 2023, Peter Zijlstra wrote: (I can't say anything about the WRITE_ONCE/rcu code, just about the below codegen part) > Welcome is not the right word. What bugs me most is that this was never > raised when this code was written :/ > > Mostly my problem is that GCC generates such utter shite when you > mention volatile. See, the below patch changes the perfectly fine and > non-broken: > > 0148 1d8: 49 83 06 01 addq $0x1,(%r14) What is non-broken here that is ... > into: > > 0148 1d8: 49 8b 06 mov (%r14),%rax > 014b 1db: 48 83 c0 01 add $0x1,%rax > 014f 1df: 49 89 06 mov %rax,(%r14) ... broken here? (Sure code size and additional register use, but I don't think you mean this with broken). > For absolutely no reason :-( The reason is simple (and should be obvious): to adhere to the abstract machine regarding volatile. When x is volatile then x++ consists of a read and a write, in this order. The easiest way to ensure this is to actually generate a read and a write instruction. Anything else is an optimization, and for each such optimization you need to actively find an argument why this optimization is correct to start with (and then if it's an optimization at all). In this case the argument needs to somehow involve arguing that an rmw instruction on x86 is in fact completely equivalent to the separate instructions, from read cycle to write cycle over all pipeline stages, on all implementations of x86. I.e. that a rmw instruction is spec'ed to be equivalent. You most probably can make that argument in this specific case, I'll give you that. But why bother to start with, in a piece of software that is already fairly complex (the compiler)? It's much easier to just not do much anything with volatile accesses at all and be guaranteed correct. Even more so as the software author, when using volatile, most likely is much more interested in correct code (even from a abstract machine perspective) than micro optimizations. > At least clang doesn't do this, it stays: > > 0403 413: 49 ff 45 00 incq 0x0(%r13) > > irrespective of the volatile. And, are you 100% sure that this is correct? Even for x86 CPU pipeline implementations that you aren't intimately knowing about? ;-) But all that seems to be a side-track anyway, what's your real worry with the code sequence generated by GCC? Ciao, Michael.
