[PR] fix(parquet/utils): make unpack32 NEON profiler-safe [arrow-go]

["zeroshade (via GitHub)" <gi...@apache.org>]

zeroshade opened a new pull request, #1300:
URL: https://github.com/apache/arrow-go/pull/1300

   ### Rationale for this change
   
   Addresses the `_unpack32_neon` part of #983 following the regeneration approach outlined in https://github.com/apache/arrow-go/issues/983#issuecomment-5332134569.
   
   The previous assembly repurposed Go's reserved registers (R18 platform, R27 REGTMP, R28 `g`) and adjusted SP through opaque `WORD` directives, so the assembler's pcsp tables were wrong and `sigprof` could observe a clobbered `g` — truncated/invalid tracebacks, and a crash with `CGO_ENABLED=0`.
   
   ### What changes are included in this PR?
   
   - Regenerate `bit_packing_neon_arm64.s` from `_lib/bit_packing_neon.c` with `-ffixed-x18 -ffixed-x27 -ffixed-x28 -fomit-frame-pointer` (per the issue comment, plus omitting the frame). The result is a **leaf function with zero stack usage and zero reserved-register usage**: constants load via `VMOVQ` pseudo-instructions and every branch is a real Go branch instruction, so unwind state is always valid.
   - Add `_lib/neon2goasm.py`, which converts the compiled ELF object into the Go assembly and **rejects any instruction touching SP or a reserved register**, plus a `make bit_packing_neon_arm64.s` rule (`C_FLAGS_NEON_GOASM`) so regeneration is reproducible.
   - Drop the unused `<string.h>` include so the C compiles freestanding for cross-generation.
   - Add `BenchmarkUnpack32` (from the issue) so the packed-decode fast path is exercised under `-cpuprofile`.
   - Document the new generation path in `_lib/README.md`.
   
   ### Verification
   
   - Semantic equivalence: instruction-by-instruction comparison of the assembled Go function against the source ELF object (branch topology index-normalized, `VMOVQ` constants resolved and compared against `.rodata`) — no differences besides the parameter/return glue.
   - `go test ./parquet/internal/utils` (native NEON), `-race`, `-tags noasm`, and `go test -tags assert ./parquet/...` all pass on Apple Silicon.
   - The issue's failure mode is fixed: `CGO_ENABLED=0 go test -bench BenchmarkUnpack32 -cpuprofile` no longer crashes, and profiles now show full caller chains:
   
   ```text
   10ms  utils._unpack32_neon
         utils.unpack32NEON
         utils.(*BitReader).GetBatch
         utils.BenchmarkUnpack32
         testing.(*B).runN
   ```
   
   - Throughput improved ~27% as a side effect of the newer codegen (387 → 280 ns/op on `BenchmarkUnpack32`, Apple M4).
   
   ### Are there any user-facing changes?
   
   No API changes. CPU profiles of workloads that hit the ARM64 packed decoder are now correct.
   
   Note: `unpack_bool_neon_arm64.s` has a smaller instance of the same pattern (one `WORD`-encoded `stp` with SP writeback); left for a follow-up to keep this reviewable.
   


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Re: [PR] fix(parquet/utils): make unpack32 NEON profiler-safe [arrow-go]

["zeroshade (via GitHub)" <gi...@apache.org>]

zeroshade merged PR #1300:
URL: https://github.com/apache/arrow-go/pull/1300


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