79895be401
emitAtomicCmpxchg: LLVMBuildAtomicCmpXchg (success/failure orderings,
singleThread=0) returns a {T, i1} pair; LLVMSetWeak for the weak variant. The
sx ?T result (null = SUCCESS) is built as { extractvalue 0 (actual value),
xor(extractvalue 1 (success), true) } -- has_value = NOT success. Integer-only
(recognizer guard), so never a pointer/niche optional.
examples/1702 green: successful CAS returns null (value updated), failing CAS
returns the actual value (unchanged), weak retry loop increments a counter
(100 -> 105). LLVM IR shows `cmpxchg ... acq_rel acquire` and `cmpxchg weak`.
Unit test `emit: atomic cmpxchg (strong + weak)` locks `cmpxchg` + the weak
marker. Suite green (718/0).
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