chore: auto-detect TARGET_ARCH in cmake instead of hardcoding in preset#21621
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ludamad merged 2 commits intomr/m3-build-fixesfrom Mar 16, 2026
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chore: auto-detect TARGET_ARCH in cmake instead of hardcoding in preset#21621ludamad merged 2 commits intomr/m3-build-fixesfrom
ludamad merged 2 commits intomr/m3-build-fixesfrom
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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
Implements Charlie's suggestion from #21611:
TARGET_ARCHfrom thedefaultcmake preset — it was set toskylakebut silently ignored on ARM (due to theif(NOT ARM)guard), which is confusing.arch.cmake— whenTARGET_ARCHis not explicitly set, pickskylakeon x86_64 andgenericon ARM. This matches what the cross-compile presets already do (zig-amd64-linuxusesskylake,zig-arm64-linuxusesgeneric).The
genericARM target prevents CPU-specific instructions (e.g. SVE on Graviton) from being emitted, so cached binaries work on all ARM64 machines including Apple Silicon in devcontainers.Presets that explicitly set
TARGET_ARCH(likezig-amd64-linux,zig-arm64-linux,zig-amd64-windows) are unaffected — the auto-detection only kicks in whenTARGET_ARCHis not set.ClaudeBox log: https://claudebox.work/s/8d283e5d61970673?run=1