Numba-Enzyme differentiates functions that allocate arrays via the Numba
runtime (NRT). NRT_MemInfo_alloc* is registered as an allocation (shadow
handler), but its paired release, NRT_decref, was not recognized as a
deallocation. Enzyme then duplicated the (inactive-marked) refcount op
across the augmented-forward and reverse passes and separately freed the
allocation, multi-freeing it and corrupting the heap.

Recognizing NRT_decref in isDeallocationFunction lets Enzyme cache the
allocation and emit a single, correctly-placed free, matching how it
handles malloc / jl_alloc_array.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Removed comments explaining NRT_decref handling.

Isolates the isDeallocationFunction(NRT_decref) recognition with a
built-in-recognized allocation (malloc) whose only release is NRT_decref
-- the malloc/free-nouse pattern of alloctomallocnouse.ll with free
replaced by NRT_decref. With the recognition the pair is treated exactly
as malloc/free (empty augmented pass; reverse re-allocates a shadow and
frees it once); before it, differentiation fails with "No augmented
forward pass found for NRT_decref".

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Jbd9RehxJeX7BvDvSiPiQ

The test used opaque-pointer (`ptr`) CHECK lines, which only match the IR
Enzyme emits on LLVM >= 17. On LLVM 15 (typed pointers by default) and
LLVM 16 (typed forced via `-opaque-pointers=0` in the lit config) the
output uses `double*`/`i8*`, so FileCheck failed the "Enzyme CI LLVM 15/16"
jobs while >= 17 passed.

Match pointers loosely so the single CHECK set works under both forms: the
augmented function is checked for emptiness without pinning its parameter
types, and the shadow malloc/free lines use `{{.*}}` in place of the pointer
type. Also add the old-PM RUN line gated `%llvmver < 16` (mirroring
alloctomallocnouse.ll, the sibling this test is modeled on) for parity.
The assertion is unchanged: recognizing NRT_decref as a deallocation makes
the augmented pass empty and frees the shadow once, with no NRT_decref
duplicated into the derivative.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Jbd9RehxJeX7BvDvSiPiQ

Replace the synthetic malloc stand-in with the real Numba runtime functions:
an array is allocated with NRT_MemInfo_alloc_aligned and released with
NRT_decref (the IR Numba emits). The allocation is registered self-contained
with __enzyme_allocation_like (in Numba-Enzyme this is a downstream C-API
shadow handler), run via the preserve-nvvm pass; its shadow-free is registered
as @free -- NOT NRT_decref -- so recognition of the SOURCE NRT_decref as a
deallocation depends solely on isDeallocationFunction (the change under test).

With the recognition the augmented pass is empty and the reverse pass
re-allocates a shadow via NRT_MemInfo_alloc_aligned and frees it once, with no
NRT_decref duplicated into the derivative; renaming the source NRT_decref makes
differentiation fail ("No augmented forward pass found for NRT_decref").
Pointer spellings stay matched loosely and the input uses typed pointers, so
the single new-PM RUN line works across LLVM 15-20.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Jbd9RehxJeX7BvDvSiPiQ