counts leading zero bits of a 64bit int, undefined on zero input.
(has nothing to do with atomics, added to atomic.h so target specific
helper functions are together.)
there is a logarithmic generic implementation and another in terms of
a 32bit a_clz_32 on targets where that's available.
previously powerpc had a_cas defined in terms of its native ll/sc
style operations, but all other atomics were defined in terms of
a_cas. instead define a_ll and a_sc so the compiler can generate
optimized versions of all the atomic ops and perform better inlining
of a_cas.
extracting the result of the sc (stwcx.) instruction is rather awkward
because it's natively stored in a condition flag, which is not
representable in inline asm. but even with this limitation the new
code still seems significantly better.
rather than having each arch provide its own atomic.h, there is a new
shared atomic.h in src/internal which pulls arch-specific definitions
from arc/$(ARCH)/atomic_arch.h. the latter can be extremely minimal,
defining only a_cas or new ll/sc type primitives which the shared
atomic.h will use to construct everything else.
this commit avoids making heavy changes to the individual archs'
atomic implementations. definitions which are identical or
near-identical to what the new shared atomic.h would produce have been
removed, but otherwise the changes made are just hooking up the
arch-specific files to the new infrastructure. major changes to take
advantage of the new system will come in subsequent commits.
Szabolcs Nagy
Rich Felker