Thank you Mr. Wilson! Indeed, I feel that I stepped unknowingly into this well-known (to others) issue… Consider open-coding subword atomics or move to libgcc · Issue #12 · riscv/riscv-gcc (github.com)<https://github.com/riscv/riscv-gcc/issues/12> I’m going to tackle your last recommendation because I do need these routines. I had hoped that compiler built-ins (via stdatomic.h) would give me a jump start. I presume the same status is true for clang 12? They haven’t implemented these atomic built-ins either? Thanks! -- Aileen Honess https://brekersystems.com/support From: Jim Wilson<mailto:jimw@xxxxxxxxxx> Sent: Tuesday, August 24, 2021 2:30 PM To: Aileen Honess<mailto:aileen@xxxxxxxxxxxxxxxxx> Cc: gcc-help@xxxxxxxxxxx<mailto:gcc-help@xxxxxxxxxxx> Subject: Re: GCC-11.2.0 libatomic for bare-metal (riscv64-unknown-elf) On Sat, Aug 21, 2021 at 1:56 PM Aileen Honess <aileen@xxxxxxxxxxxxxxxxx<mailto:aileen@xxxxxxxxxxxxxxxxx>> wrote: When I configure and create a gcc cross-compiler for aarch64-unknown-elf, I get a valid and working libatomic. Critically, this contains implementations of the methods that underlie the gcc atomic builtins (__atomic_load(), etc.). I mean things like '__atomic_fetch_and_1()'. When I do similar for riscv64-unknown-elf, I do not get libatomic. The “configure.tgt” for libatomic says that the platform is UNSUPPORTED. The build continues, but if I use the atomic builtins, I’ll get undefined references to those underlying functions. libatomic fails to build for all *-elf targets. So it is failing to build for both aarch64-elf and riscv-elf. The difference here is that rv32 only has atomic support for 32-bit words, and rv64 only has atomic support for 32-bit words and 64-bit double words. Whereas aarch64 v8.1-a has atomic support for all integer types, so can emit code directly for operations like __atomic_fetch_and_1. If compiling for RISC-V or armv8.0 then you will get calls to functions for unsupported types. aarch64 apparently has a library somewhere to implement these functions for *-elf toolchains. RISC-V does not yet. We have long term plans to open code the char and short operations using lr.[wd] and sc.[wd] but we don't know when this work will be done. Meanwhile, you should avoid char/short atomic operations. Or you could write your own library routines. You could take the linux riscv libatomic and disassemble it to get the code you need. Jim
