path: root/arch/powerpc/atomic_arch.h
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2018-09-14fix broken atomic store on powerpc[64]Rich Felker-8/+0
in our memory model, all atomics are supposed to be full barriers; stores are not release-only. this is important because store is used as an unlock operation in places where it needs to acquire the waiter count to determine if a futex wake is needed. at least in the malloc-internal locks, but possibly elsewhere, soft deadlocks from missing futex wake (breakable by poking the threads to restart the syscall, e.g. by attaching a tracer) were reported to occur. once the malloc lock is replaced with Jens Gustedt's new lock implementation (see commit 47d0bcd4762f223364e5b58d5a381aaa0cbd7c38), malloc will not be affected by the issue, but it's not clear that other uses won't be. reducing the strength of the ordering properties required from a_store would require a thorough analysis of how it's used. to fix the problem, I'm removing the powerpc[64]-specific a_store definition; now, the top-level atomic.h will implement a_store using a_barrier on both sides of the store. it's not clear to me yet whether there might be issues with the other atomics. it's possible that a_post_llsc needs to be replaced with a full barrier to guarantee the formal semanics we want, but either way I think the difference is unlikely to impact the way we use them.
2017-08-29add a_clz_64 helper functionSzabolcs Nagy-0/+7
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.
2016-01-22overhaul powerpc atomics for new atomics frameworkRich Felker-14/+38
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.
2016-01-21refactor internal atomic.hRich Felker-0/+15
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.