On Thu, Dec 16, 2021 at 11:48 PM Mathieu Malaterre <malat@xxxxxxxxxx> wrote: > So if I understand the discussion correctly gcc is now adding by > default `--as-needed -latomic` for riscv arch to support c11/atomics. > What I fail to understand is why something identical was not applied > to powerpc/armel and mipsel ? > RISC-V is the only major target that does not support subword atomics. Some ports have hardware support for subword atomics. Some ports like PPC lack hardware support for subword atomics but have gcc patterns to implement them using full word atomic instructions. RISC-V has neither. So anything using subword atomics on RISC-V will fail to link unless -latomic is used, but will link fine on all other major targets. Unfortunately, there are many package maintainers that refuse to add -latomic to their packages because x86/arm doesn't need it. So we modified the compiler to add the necessary -latomic option. However, in the FSF tree, we only add -latomic when -pthread is used, because pthread definitely needs subword atomic support. Though convincing people to use the -pthread option is another issue, some package maintainers refuse to use it even though the gcc docs say it is required. It looks like you have a modified source tree that is adding the -latomic always which is one way to fix that problem. Adding gcc patterns to expand subword atomics into sequences of code using full word atomic instructions would be a better fix, but it is a complicated one. There are issues with the libstdc++ ABI changing, and issues of how exactly we test the patch, and issues with writing it and proving it is correct as the RISC-V memory consistency spec is complicated. This has been a known problem for over 4 years, and no one has volunteered to fix it yet. Jim
