We got a change request for the RISC-V psABI to define the atomic structure size and alignment. And looking at this, it turned out that gcc and clang are implementing this differently. Consider this testcase
rohan:2274$ cat tmp.c #include <stdio.h> struct s { int a; int b; int c;}; int main(void) { printf("size=%ld align=%ld\n", sizeof (struct s), _Alignof(struct s)); printf("size=%ld align=%ld\n", sizeof (_Atomic (struct s)), _Alignof(_Atomic (struct s))); return 0; } rohan:2275$ gcc tmp.c rohan:2276$ ./a.out size=12 align=4 size=12 align=4 rohan:2277$ clang tmp.c rohan:2278$ ./a.out size=12 align=4 size=16 align=16 rohan:2279$ This is with an x86 compiler. I get the same result with a RISC-V compiler. This is an ABI incompatibility between gcc and clang. gcc has code in build_qualified_type in tree.c that sets alignment for power-of-2 structs to the same size integer alignment, but we don't change alignment for non-power-of-2 structs. Clang is padding the size of non-power-of-2 structs to the next power-of-2 and giving them that alignment. Unfortunately, I don't know who to contact on the clang side, but we need to have a discussion here, and we probably need to fix one of the compilers to match the other one, as we should not have ABI incompatibilities like this between gcc and clang. The original RISC-V bug report is at https://github.com/riscv/riscv-elf-psabi-doc/pull/112 There is a pointer to a gist with a larger testcase with RISC-V results. Jim