Support Julia 1.13's julia.atomicmodify pseudo-intrinsic

[vchuravy]

Julia 1.13 (JuliaLang/julia#57010) emits atomic modify operations on integer-typed pointer-free fields as a variadic pseudo-intrinsic

{old, new} = julia.atomicmodify.iN.pAS(ptr, op, ordering, syncscope, args...)

which atomically performs old = *ptr; new = op(old, args...); *ptr = new, where op's parameter i (i >= 1) is forwarded from call operand i + 3. It is only expanded to atomicrmw/cmpxchg loops by Julia's ExpandAtomicModify pass after GC lowering, so Enzyme must keep it intact in generated code. Previously these calls hit the generic call differentiation path, which rejects variadic calls with Number of arg operands != function parameters (EnzymeAD/Enzyme.jl#3086).

TypeAnalysis

Unify the pointee of ptr with both result elements and analyze op interprocedurally, relating the forwarded arguments with the modified memory. In particular this lets activity analysis prove atomic counter/lock manipulations inactive (Julia annotates real-world IR with enzyme_type already, but the interprocedural step recovers types for the modified value and forwarded operands).

AdjointGenerator

Handle the call akin to atomicrmw:

• fully inactive calls are replayed via the constant fallback,
• non-differentiable (integer/pointer) modifications within duplicated memory are replicated on the shadow location with the primal arguments, keeping counters and lock states of shadow objects consistent,
• in forward mode, ops recognized as linear in (old, value) (fadd/fsub/xchg through bitcasts) compute the tangent by applying the same op to the shadow location and shadow value,
• in reverse mode (with inactive result), fadd/fsub accumulate the adjoint of the value operand from a load of the shadow location with downgraded ordering (negated for fsub),
• everything else reports Active atomic modify not yet handled.

Note that on Julia 1.13 codegen only emits the intrinsic for integer element types (float fields still lower to inline cmpxchg loops), so the float paths are exercised by the included lit tests; they implement the intrinsic's full semantics for when emission widens.

Companion Enzyme.jl change (running ExpandAtomicModify in its post-AD legalization pipeline): EnzymeAD/Enzyme.jl branch vc/atomicmodify. Tested end-to-end on Julia 1.13.0-rc1: the previously crashing locks and threads test suites pass, including reverse mode over atomic counters in duplicated structs.

🤖 Generated with Claude Code

[LuuOW]

Technical audit: code patterns and logic verified.