If we're building for sh4a, the compiler is already free to use
instructions only available on sh4a, so we can do the same and inline the
llsc atomics. If we're building for an older processor, we still do the
same runtime atomics selection as before.
conceptually, a_spin needs to be at least a compiler barrier, so the
compiler will not optimize out loops (and the load on each iteration)
while spinning. it should also be a memory barrier, or the spinning
thread might keep spinning without noticing stores from other threads,
thus delaying for longer than it should.
ideally, an optimal a_spin implementation that avoids unnecessary
cache/memory contention should be chosen for each arch, but for now,
the easiest thing is to perform a useless a_cas on the calling
thread's stack.
the a_cas_l, a_swap_l, a_swap_p, and a_store_l operations were
probably used a long time ago when only i386 and x86_64 were
supported. as other archs were added, support for them was
inconsistent, and they are obviously not in use at present. having
them around potentially confuses readers working on new ports, and the
type-punning hacks and inconsistent use of types in their definitions
is not a style I wish to perpetuate in the source tree, so removing
them seems appropriate.
linux, gcc, etc. all use "sh" as the name for the superh arch. there
was already some inconsistency internally in musl: the dynamic linker
was searching for "ld-musl-sh.path" as its path file despite its own
name being "ld-musl-superh.so.1". there was some sentiment in both
directions as to how to resolve the inconsistency, but overall "sh"
was favored.
Bobby Bingham
Rich Felker