build: disable exceptions for highway on Windows (clang-cl)

[dylan-conway]

Summary

Fixes 9× clang-cl: warning: unknown argument ignored in clang-cl: '-fno-exceptions' [-Wunknown-argument] when building highway on Windows, and restores highway's no-exceptions build that was lost in the DirectBuild conversion.

#29584 converted highway from nested-cmake to DirectBuild and passed -fno-exceptions in the Windows cflags branch. clang-cl doesn't recognize the gcc spelling and was ignoring it, so highway was actually being compiled with exceptions enabled via globalFlags' /EHsc — a behavior change from nested-cmake, where highway's own CMakeLists.txt set /EHs-c- + -D_HAS_EXCEPTIONS=0 in its MSVC branch (vendor/highway/CMakeLists.txt:367-370).

This swaps the unrecognized -fno-exceptions for /EHs-c- -D_HAS_EXCEPTIONS=0, which clang-cl understands. The dep-specific cflags are appended after globalFlags in source.ts:1433, so /EHs-c- overrides /EHsc (later flag wins).

Separately: globalFlags itself has /EHsc with desc "Disable C++ exceptions" (flags.ts:189-191) — the flag enables exceptions but the desc describes /EHs-c-. That's been there since the original Windows CMake (bun 1.0.9) and affects every Windows TU; left for a follow-up.

Test plan

• bunx tsc --noEmit -p scripts/build/tsconfig.json
• ninja -C build/debug highway — all 9 objects compile with no warnings
• Verified /EHs-c- -D_HAS_EXCEPTIONS=0 appear after /EHsc in generated build.ninja
• Windows CI green

[robobun]

^{Updated 2:02 PM PT - Apr 23rd, 2026}

❌ @autofix-ci[bot], your commit 9f1f73f has 5 failures in Build #47577 (All Failures):

• test/js/web/websocket/websocket.test.js - code 1 on 🍎 13 aarch64
• test/js/bun/webview/webview-chrome.test.ts - code 1 on 🍎 13 aarch64
• test/js/bun/util/v8-heap-snapshot.test.ts - pid 1716 illegal instruction on 🪟 11 aarch64
• scripts/build/ci.ts - build failed on 🐧 aarch64 - build-bun
• test/js/bun/net/socket.test.ts - code 1 on 🪟 2019 x64
• test/js/bun/net/socket.test.ts - code 1 on 🪟 2019 x64-baseline

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🧪   To try this PR locally:

bunx bun-pr 29645

That installs a local version of the PR into your bun-29645 executable, so you can run:

bun-29645 --bun

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 31b9062 and 9f1f73f.

📒 Files selected for processing (1)

• scripts/build/deps/highway.ts

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Walkthrough

Windows build for the highway dependency now disables C++ exceptions using MSVC-style clang-cl flags (/EHs-c- and -D_HAS_EXCEPTIONS=0) instead of -fno-exceptions; non-Windows still uses -fno-exceptions and -fmath-errno. Inline comments explain clang-cl flag support and exception-flag precedence.

Changes

Cohort / File(s)	Summary
Highway Build Configuration scripts/build/deps/highway.ts	On Windows, replaced injection of -fno-exceptions with MSVC-style clang-cl flags /EHs-c- and -D_HAS_EXCEPTIONS=0. Updated inline documentation to note that -fno-exceptions/-fmath-errno are not accepted by clang-cl, that non-Windows still uses -fno-exceptions plus -fmath-errno, and that later exception flags override earlier ones.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: disabling exceptions for highway on Windows using clang-cl flags, which matches the core objective of the PR.
Description check	✅ Passed	The PR description is comprehensive and well-structured, covering the problem statement, solution details, test verification, and known follow-up items, though it deviates from the provided template structure.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[claude]

LGTM — behaviorally a no-op (clang-cl was already ignoring the flag), just silences the warning.

Extended reasoning...

Overview

One-line change in scripts/build/deps/highway.ts that drops -fno-exceptions from the Windows cflags branch (plus an updated comment). The flag was already being ignored by clang-cl with a -Wunknown-argument warning, so this changes no codegen — it just removes 9 build warnings. The non-Windows path is untouched.

Security risks

None. Build-script flag tweak for a vendored SIMD library; no runtime code, auth, crypto, or user-facing surface involved.

Level of scrutiny

Low. This is a mechanical follow-up to #29584's DirectBuild conversion, mirroring the existing -fmath-errno exclusion that was already in place for the same reason. Upstream highway's own CMakeLists also skips -fno-exceptions in its MSVC branch, so this matches vendor intent.

Other factors

The inline comment flags a pre-existing typo in scripts/build/flags.ts:189 (/EHsc vs the documented /EHs-c-) that the new code comment leans on for its rationale. That's worth fixing separately but doesn't affect the correctness of this change — before and after, highway on Windows receives the same effective flags from computeDepFlags(); the only delta is the dropped warning. No CODEOWNERS for this path, no prior reviewer comments, and tsc/ninja checks pass per the test plan.

[claude]

LGTM — thanks for following up with /EHs-c- -D_HAS_EXCEPTIONS=0; that addresses my earlier note and restores nested-cmake parity.

Extended reasoning...

Overview

Single-file change to scripts/build/deps/highway.ts: swaps the Windows cflags entry from ["-fno-exceptions"] (which clang-cl ignores with a warning) to ["/EHs-c-", "-D_HAS_EXCEPTIONS=0"], plus an updated explanatory comment. Non-Windows branch is byte-identical to before.

Prior feedback addressed

My earlier inline comment was on revision cc84321, which simply dropped the flag and leaned on globalFlags' /EHsc — which actually enables exceptions. The author pushed 31b9062 in response, now explicitly passing /EHs-c- -D_HAS_EXCEPTIONS=0 (matching upstream highway's MSVC branch) and rewrote the PR description to acknowledge the pre-existing flags.ts:189 /EHsc typo as a separate follow-up. That's the right scoping. I verified at source.ts:1433 that dep cflags are appended after computeDepFlags globals, so /EHs-c- correctly overrides /EHsc (later /EH wins per MSVC semantics).

Security risks

None. This is a compiler-flag tweak for a vendored SIMD library's build recipe — no runtime code, auth, crypto, or user-input handling involved.

Level of scrutiny

Low. Mechanical, one-line build-config change in a dependency build script. The intent (disable exceptions for highway on Windows) is clear, the spelling matches what upstream highway and MSVC docs prescribe, and the author verified the generated build.ninja ordering locally.

Other factors

The robobun CI comment shows Linux/macOS build-cpp/build-zig failures, but this diff only touches the cfg.windows ternary arm — the non-Windows arm is unchanged — so those cannot be caused by this change (and notably no Windows job is listed as failing). No bugs from the bug-hunting system. CodeRabbit had no actionable comments.