fix: skip -march auto-detection for cross-compilation targets#21655
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fix: skip -march auto-detection for cross-compilation targets#21655AztecBot wants to merge 1 commit intomerge-train/spartanfrom
AztecBot wants to merge 1 commit intomerge-train/spartanfrom
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The M3 devcontainer fix (6845b72) added auto-detection of TARGET_ARCH in arch.cmake, but this breaks cross-compilation presets that don't explicitly set TARGET_ARCH or CMAKE_SYSTEM_PROCESSOR: - arm64-android: CMAKE_SYSTEM_NAME=Generic, no CMAKE_SYSTEM_PROCESSOR -> ARM not set -> defaults to skylake -> zig aarch64 compiler gets -march=skylake -> error: unknown CPU 'skylake' - arm64-macos: CMAKE_SYSTEM_PROCESSOR=aarch64 -> ARM=ON -> -march=generic -> strips crypto/AES extensions -> libdeflate NEON/AES intrinsic errors - arm64-ios/arm64-ios-sim: same as android (Generic, no processor set) Fix: add CMAKE_CROSSCOMPILING guard to skip auto-detection for cross builds. Zig already handles arch selection via -target/-mcpu flags, so -march from CMake is redundant and harmful for cross-compilation.
ludamad
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Mar 17, 2026
# 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
Fixes the CI failure on
merge-train/spartancaused by the M3 devcontainer fix (#21611, commit 6845b72).That commit added auto-detection of
TARGET_ARCHinarch.cmake, but this breaks cross-compilation presets that don't explicitly setTARGET_ARCHorCMAKE_SYSTEM_PROCESSOR:CMAKE_SYSTEM_NAME=Generic, noCMAKE_SYSTEM_PROCESSOR→ARMnot set → defaults toskylake→ zig aarch64 compiler gets-march=skylake→error: unknown CPU 'skylake'CMAKE_SYSTEM_PROCESSOR=aarch64→ARM=ON→-march=generic→ strips crypto/AES extensions → libdeflate NEON/AES intrinsic compilation errorsGeneric, no processor set)Fix
Add
CMAKE_CROSSCOMPILINGguard to skip auto-detection for cross builds. Zig already handles arch selection via-target/-mcpuflags, so the CMake-marchflag is redundant and harmful for cross-compilation.Verification
Ran
./bootstrap.sh ci-fulllocally. All three previously-failing targets now pass:bb-cpp-cross-arm64-android✅bb-cpp-cross-arm64-macos-objects✅bb-cpp-cross-arm64-ios✅ClaudeBox log: https://claudebox.work/s/080a1b45bdb9305a?run=1