c11: --as-needed -latomic on riscv but not others

[Segher Boessenkool]

On Fri, Dec 17, 2021 at 12:00:47PM -0800, Jim Wilson via Gcc-help wrote:
> 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.

That was true until ISA 2.06 (POWER7, eleven years ago).

> 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.

The libatomic implementation requires pthread, I think?

> 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.

This might well generate worse code on average.  That is very target-
dependent of course.

> 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.

Good luck with it!  It is pretty hard work, and thankless.  For one
thing it isn't very visible.  For another a much better performance gain
will result from implementing it in hardware, and that will catch up, if
it takes another >4y to implement in software ;-)


Segher