fix: skip -march auto-detection when cross-compiling#21653
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fix: skip -march auto-detection when cross-compiling#21653AztecBot wants to merge 1 commit intomerge-train/spartanfrom
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The M3 devcontainer fix (#21611) auto-detects TARGET_ARCH based on host architecture, but this breaks cross-compilation from x86_64 to aarch64 targets: - arm64-android/ios: HOST is x86_64 → ARM=false → TARGET_ARCH=skylake → '-march=skylake' on aarch64 = 'unknown CPU' error - arm64-macos: CMAKE_SYSTEM_PROCESSOR=aarch64 → ARM=true → TARGET_ARCH=generic → '-march=generic' conflicts with '-mcpu=apple_a14' causing libdeflate NEON/AES errors Fix: only auto-detect TARGET_ARCH for native (non-cross) builds. Cross-compilation presets specify their target via -target/-mcpu flags on the compiler command line; adding -march based on HOST arch is wrong.
ludamad
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# fix: ARM64 devcontainer builds — skip `-march` on ARM and use explicit zig aarch64 target ## Summary Fixes SIGILL (Illegal Instruction) crashes and build failures on ARM64 Mac (M3/Apple Silicon) devcontainers caused by incorrect `-march` handling introduced in #21611. ## Problem PR #21611 originally fixed ARM64 devcontainer builds by using explicit `aarch64-linux-gnu.2.35` zig targets. During the merge, that approach was replaced with cmake-based auto-detection that sets `TARGET_ARCH=generic` on ARM and passes `-march=generic` to the compiler. This caused two distinct failures: ### 1. SIGILL crashes (`Illegal instruction`) The zig compiler wrappers still used `-target native-linux-gnu.2.35`, which auto-detects the host CPU. On CI (AWS Graviton with SVE extensions), this produces binaries containing SVE instructions. These cached binaries are then downloaded on Apple Silicon devcontainers (ARM64 without SVE), causing SIGILL when executed — e.g. `honk_solidity_key_gen` crashing during `barretenberg/sol` bootstrap. The `-march=generic` flag was supposed to override this, but `-march=generic` is **not a valid value on aarch64**. It's an x86 concept. LLVM/zig silently ignored it, so the native CPU detection still produced SVE instructions. ### 2. Build failures (`unknown CPU: 'armv8'`) Even attempting `-march=armv8-a` (a valid GCC/Clang aarch64 value) fails because zig uses its own CPU naming scheme (e.g. `generic`, `cortex_a72`, `apple_m3`), not GCC-style architecture strings. Zig interprets `-march=armv8-a` as CPU name `armv8`, which doesn't exist → `error: unknown CPU: 'armv8'`. **Bottom line:** The `-march` cmake approach fundamentally doesn't work with zig on ARM. Zig has its own architecture targeting via `-target`, which is the correct mechanism. ## What this PR changes ### 1. `arch.cmake` — Skip `-march` auto-detection on ARM Removed the ARM branch from the auto-detection. On x86_64, we still auto-detect `TARGET_ARCH=skylake`. On ARM, we don't set `TARGET_ARCH` at all, so no `-march` flag is passed — the zig wrappers handle architecture targeting instead. ### 2. `zig-cc.sh` / `zig-c++.sh` — Explicit aarch64 target on ARM Linux Restored the original fix from #21611 that was dropped during merge. On ARM64 Linux, the wrappers now use `-target aarch64-linux-gnu.2.35` instead of `-target native-linux-gnu.2.35`. This produces generic ARM64 code without CPU-specific extensions (SVE, etc.), ensuring cached binaries work on all ARM64 machines — Graviton, Apple Silicon, Ampere, etc. x86_64 behavior is unchanged (still uses `-target native`). ## Context: what happened after #21611 After #21611 merged with the cmake auto-detection approach, it triggered a cascade of follow-up PRs trying to fix the fallout: | PR | Status | Issue | |----|--------|-------| | #21621 | Merged | Introduced the auto-detect approach (replaced zig wrapper fix with cmake `-march`) | | #21356 | Merged | Added `NOT CMAKE_CROSSCOMPILING` guard for cross-compile failures | | #21637 | Open | Attempting to fix cross-compiles + restore `native_build_dir` | | #21660 | Open | Attempting to fix cross-compile targets | | #21632 | Open | Attempting to fix cross-compile targets | | #21662 | Open | Adding `CMAKE_SYSTEM_PROCESSOR` to ARM64 cross-compile presets | | #21653 | Open | Attempting to skip auto-detection when cross-compiling | | #21655 | Open | Attempting to skip auto-detection for cross-compilation targets | This PR supersedes the still-open PRs above by addressing the root cause: `-march` via cmake doesn't work with zig on ARM. The zig `-target` mechanism is the correct approach.
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Summary
The M3 devcontainer fix (#21611) auto-detects
TARGET_ARCHbased on the host CPU, but this breaks cross-compilation from x86_64 to aarch64 targets:ARM=false→TARGET_ARCH=skylake→-march=skylakeon aarch64 = "unknown CPU" errorCMAKE_SYSTEM_PROCESSOR=aarch64→ARM=true→TARGET_ARCH=generic→-march=genericconflicts with-mcpu=apple_a14causing libdeflate NEON/AES errorsFix: only auto-detect
TARGET_ARCHfor native (non-cross) builds by addingNOT CMAKE_CROSSCOMPILINGto the guard. Cross-compilation presets already specify their target via-target/-mcpucompiler flags.Detailed analysis: https://gist.github.com/AztecBot/1dd033cbfb1a98fb66670ff3485707da
Test plan
cmake --preset arm64-android— no-marchflag in build.ninjacmake --preset arm64-ios— no-marchflag in build.ninjacmake --preset arm64-macos— no-marchflag in build.ninjacmake --preset default—-march=skylakestill present (native build)ClaudeBox log: https://claudebox.work/s/5d64f104232cbbaa?run=1