✔ about riscv atomic store. · general

Stream: general

Topic: ✔ about riscv atomic store.

yang yu (Jul 04 2022 at 23:20):

atomic store a value can fail.
https://msyksphinz-self.github.io/riscv-isadoc/html/rv64a.html#sc-d
should I use a loop to implement atomic store.

loop {
   sc.d rd,rs1,rs2
   if rd == 0   break;
}

Chris Fallin (Jul 05 2022 at 05:09):

hi @yang yu -- actually the semantics of Cranelift's atomic store are infallible, i.e., there is no "failure". I think you may be confusing it for a "store conditional" (sc in RISC-V or other classic RISCs like MIPS, ARM32) instruction, which fails if some other CPU touched the value since the paired "load-linked"

Chris Fallin (Jul 05 2022 at 05:10):

an atomic store should have SeqCst ordering, which means that it occurs in some order relative to other atomic loads/stores on the same address and all CPUs agree on that order, and it also imposes ordering edges on the surrounding program (ordinary loads/stores prior to an atomic store always come before, and are observed by, ordinary loads/stores after an atomic load that observes the atomic store)

Chris Fallin (Jul 05 2022 at 05:11):

You'll need to reason about the memory model -- I don't know in detail what RISC-V provides -- but probably what you'll need is an ordinary store instruction, preceded by and/or followed by a memory fence or barrier instruction of some kind

Chris Fallin (Jul 05 2022 at 05:13):

one easy way to see what should be generated is to use an atomic with SeqCst ordering in C++ or Rust, and compile it to assembly; then do whatever clang or rustc does

Chris Fallin (Jul 05 2022 at 05:15):

here are atomic loads and stores with SeqCst in Rust, compiled to RISC-V: https://godbolt.org/z/G5h4c3cxc

Compiler Explorer - Rust (rustc 1.58.0)

use std::sync::atomic::{AtomicU64, Ordering}; pub fn store(atomic: &AtomicU64, value: u64) { atomic.store(value, Ordering::SeqCst); } pub fn load(atomic: &AtomicU64) -> u64 { atomic.load(Ordering::SeqCst) }

yang yu (Jul 05 2022 at 08:45):

@Chris Fallin thanks , But I am not very understand this. May be I will find some course to learn about this.

Chris Fallin (Jul 05 2022 at 14:49):

@yang yu for now I think just emitting the same output as what you see from rustc (in the Godbolt compiler explorer link above) should be fine

yang yu (Jul 05 2022 at 21:43):

Notification Bot (Aug 10 2022 at 00:47):

yang yu has marked this topic as resolved.

Last updated: Dec 23 2024 at 12:05 UTC