os(atomic): add portable pointer-width CAS __os_atomic_cas_ptr#33
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Add a pointer-width compare-and-swap to the atomic layer, which until now exposed only 32-bit and 64-bit integer CAS. Native implementations in the GCC __atomic and legacy __sync builtin tiers (over intptr_t, whose codegen is more uniform than pointer-typed C11 atomics and is value-preserving for object pointers). A generic fallback synthesizes it from the tier's sized CAS (64-bit when pointers are 8 bytes, else 32-bit), the mutex-emulated atomic-pool mutex when there are no native atomics, or a plain store in the single-threaded build. This is foundational for lock-free/RCU data structures (Treiber stacks, epoch reclamation) used in the multi-core scaling work. No current caller; added as the substrate the RCU-for-shared-structures track builds on.
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What
Add a pointer-width compare-and-swap,
__os_atomic_cas_ptr, to the atomiclayer. Until now the layer exposed only 32-bit and 64-bit integer CAS.
How
__atomicand legacy__syncbuiltintiers, expressed over
intptr_t. Integer atomics have more uniform codegenthan pointer-typed C11 atomics and are value-preserving for object pointers.
the 64-bit CAS when pointers are 8 bytes, otherwise the 32-bit CAS; the
mutex-emulated atomic-pool mutex when no native atomics are configured; or a
plain store in the single-threaded build.
os_ext.hunder the matching tier guards.Status
No current caller. This lands the primitive as the substrate that lock-free
and RCU data structures (Treiber stacks, epoch reclamation) in the multi-core
scaling work will build on. It depends on real hardware atomics, which #32
made available on modern toolchains.
Verification
Builds clean on top of current master (meson, macOS clang, GCC
__atomictier);
__os_atomic_cas_ptris present as a native text symbol. On x86_64 itcompiles to a single
lock cmpxchg.