[BUG] Incorrect machine is detected while cross compiling

[jameshilliard]

Host compiler args are used instead of target args for target build when cross compiling:

>>> python-autobahn 25.12.2 Building
(cd /home/buildroot/buildroot/output/build/python-autobahn-25.12.2//; _PYTHON_HOST_PLATFORM="linux-aarch64" _PYTHON_PROJECT_BASE="/home/buildroot/buildroot/output/build/python3-3.13.11" _PYTHON_SYSCONFIGDATA_NAME="`{ [ -e /home/buildroot/buildroot/output/per-package/python-autobahn/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/python3.13//_sysconfigdata__linux_*.py ] && basename /home/buildroot/buildroot/output/per-package/python-autobahn/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/python3.13//_sysconfigdata__linux_*.py .py; } || true`" PATH="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin:/home/buildroot/buildroot/output/per-package/python-autobahn/host/sbin:/home/buildroot/.cargo/bin:/home/buildroot/bin:/home/buildroot/.local/bin:/home/buildroot/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/local/go/bin" GIT_DIR=. PATH="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin:/home/buildroot/buildroot/output/per-package/python-autobahn/host/sbin:/home/buildroot/.cargo/bin:/home/buildroot/bin:/home/buildroot/.local/bin:/home/buildroot/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/local/go/bin" AR="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc-ar" AS="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-as" LD="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-ld" NM="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc-nm" CC="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc" GCC="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc" CPP="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-cpp" CXX="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-g++" FC="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gfortran" F77="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gfortran" RANLIB="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc-ranlib" READELF="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-readelf" STRIP="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-strip" OBJCOPY="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-objcopy" OBJDUMP="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-objdump" AR_FOR_BUILD="/usr/bin/ar" AS_FOR_BUILD="/usr/bin/as" CC_FOR_BUILD="/usr/bin/gcc" GCC_FOR_BUILD="/usr/bin/gcc" CXX_FOR_BUILD="/usr/bin/g++" LD_FOR_BUILD="/usr/bin/ld" CPPFLAGS_FOR_BUILD="-I/home/buildroot/buildroot/output/per-package/python-autobahn/host/include" CFLAGS_FOR_BUILD="-O2 -I/home/buildroot/buildroot/output/per-package/python-autobahn/host/include" CXXFLAGS_FOR_BUILD="-O2 -I/home/buildroot/buildroot/output/per-package/python-autobahn/host/include" LDFLAGS_FOR_BUILD="-L/home/buildroot/buildroot/output/per-package/python-autobahn/host/lib -Wl,-rpath,/home/buildroot/buildroot/output/per-package/python-autobahn/host/lib" FCFLAGS_FOR_BUILD="" DEFAULT_ASSEMBLER="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-as" DEFAULT_LINKER="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-ld" CPPFLAGS="-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64" CFLAGS="-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2 -g0 -D_FORTIFY_SOURCE=1" CXXFLAGS="-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2 -g0 -D_FORTIFY_SOURCE=1" LDFLAGS="" FCFLAGS=" -O2 -g0" FFLAGS=" -O2 -g0" PKG_CONFIG="/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/pkg-config" STAGING_DIR="/home/buildroot/buildroot/output/per-package/python-autobahn/host/aarch64-buildroot-linux-gnu/sysroot" INTLTOOL_PERL=/usr/bin/perl PYTHONPATH="/home/buildroot/buildroot/output/per-package/python-autobahn/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/python3.13/" PYTHONNOUSERSITE=1  /home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/python3 -m build -n -w )
* Getting build dependencies for wheel...
* Building wheel...
======================================================================
Capturing FlatBuffers git version from deps/flatbuffers
======================================================================
  -> deps/flatbuffers not found or not a git repo
  -> Using existing version in _git_version.py
Building for Python with extension suffix: .cpython-313-aarch64-linux-gnu.so
Building CFFI module: /home/buildroot/buildroot/output/build/python-autobahn-25.12.2/src/autobahn/nvx/_utf8validator.py
generating ./_nvx_utf8validator.c
the current directory is '/home/buildroot/buildroot/output/build/python-autobahn-25.12.2/src/autobahn/nvx'
cc1: error: unknown value ‘native’ for ‘-march’
cc1: note: valid arguments are: armv8-a armv8.1-a armv8.2-a armv8.3-a armv8.4-a armv8.5-a armv8.6-a armv8.7-a armv8.8-a armv8.9-a armv8-r armv9-a armv9.1-a armv9.2-a armv9.3-a armv9.4-a armv9.5-a
warning: build_ext: building extension "_nvx_utf8validator" failed: command '/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc' failed with exit code 1

Building CFFI module: /home/buildroot/buildroot/output/build/python-autobahn-25.12.2/src/autobahn/nvx/_xormasker.py
generating ./_nvx_xormasker.c
the current directory is '/home/buildroot/buildroot/output/build/python-autobahn-25.12.2/src/autobahn/nvx'
cc1: error: unknown value ‘native’ for ‘-march’
cc1: note: valid arguments are: armv8-a armv8.1-a armv8.2-a armv8.3-a armv8.4-a armv8.5-a armv8.6-a armv8.7-a armv8.8-a armv8.9-a armv8-r armv9-a armv9.1-a armv9.2-a armv9.3-a armv9.4-a armv9.5-a
warning: build_ext: building extension "_nvx_xormasker" failed: command '/home/buildroot/buildroot/output/per-package/python-autobahn/host/bin/aarch64-linux-gcc' failed with exit code 1


======================================================================
Building FlatBuffers compiler (flatc)
======================================================================
WARNING: /home/buildroot/buildroot/output/build/python-autobahn-25.12.2/deps/flatbuffers not found
  -> Initialize git submodule: git submodule update --init
Successfully built autobahn-25.12.2-py3-none-any.whl

[oberstet]

yeah, that's a bug which bites in cross-compilation scenarios: -march=native is not valid or meaningful when cross-compiling

[oberstet]

also:

Summary of zlmdb vs autobahn-python cross-compilation issues:

Issue	zlmdb	autobahn-python
-march=native in compile flags	❌ No issue	✅ Issue #1834
Running target flatc on host	✅ Issue #107	✅ Issue #1832

So for zlmdb, only the flatc/reflection.bfbs issue (#107) needs fixing. The compile flags are already cross-compilation safe.

---

autobahn-python

Component	Optimization Impact	Why
flatc	Low	Build-time tool, runs once per schema change
NVX	High	Hot path! UTF-8 validation and XOR masking run on every WebSocket frame. SIMD instructions (SSE4.2, AVX) from -march=native can significantly improve throughput

NVX benefits from optimization because:

• Byte-by-byte operations (tight loops)
• Called on every single WebSocket message
• SIMD vectorization can process 16-64 bytes at once vs 1 byte
• Real, measurable throughput difference

zlmdb

Component	Optimization Impact	Why
flatc	Low	Same as above
LMDB	Low	I/O bound, not CPU bound. Performance dominated by disk/memory operations, not computation

LMDB's hot paths are:

• B-tree traversal (pointer chasing, memory latency bound)
• Page management (mmap operations)
• Memory copies (already optimized by memcpy)

LMDB is a database where data integrity trumps everything. The marginal performance gain from -march=native is:

1. Likely unmeasurable in real workloads
2. Not worth any theoretical risk of compiler optimization bugs
3. Could cause SIGILL crashes on older CPUs if wheels are misused

---

Conclusion: zlmdb's current approach (safe, portable flags) is the correct choice. The -march=native complexity in autobahn-python is justified only because NVX is genuinely CPU-bound hot-path code.