ci(llvm): reduce Windows LLVM tool count while keeping llvm-config (retry of reverted tool-disable)

[nebasuke]

Context

On Windows hosted runners, lld-link is slow per-executable. With ccache at 100% hit on compiles, the dominant wall-clock in build-llvm is linking the ~200 LLVM tool binaries (opt.exe, llc.exe, llvm-objdump.exe, etc.). solx consumes LLVM purely as a library via inkwell FFI, so none of these tool binaries are invoked at runtime.

An attempt to disable them via LLVM_BUILD_TOOLS=Off + LLVM_INCLUDE_TOOLS=Off (commit 3106d1e in #360) was reverted because it broke the Rust test step.

Evidence the approach works (and what broke)

Test run #24771100328 at commit 3106d1e:

• Build LLVM step on Windows: 42 min → 9:44 (4.3×). The tool-disable mechanism worked exactly as hypothesised — ~200 fewer lld-link invocations.
• Run tests step failed with:

  error: No suitable version of LLVM was found system-wide or pointed
  error: could not compile `llvm-sys` (lib) due to 1 previous error
  

Root cause: the llvm-sys Rust crate (pulled in by inkwell) runs ${LLVM_SYS_211_PREFIX}/bin/llvm-config at Rust crate-build time to discover include paths, library paths, and link flags. llvm-config is an LLVM "tool" under llvm/tools/llvm-config/, so LLVM_INCLUDE_TOOLS=Off drops its CMakeLists from the configure and the binary isn't produced.

llvm-config is the only tool binary solx needs (and only at Rust build time, not runtime). Every other tool we disabled is safely unused.

Proposed approaches

Option A — surgical override

Keep LLVM_INCLUDE_TOOLS=On (so tools/ subdirectory is configured) but LLVM_BUILD_TOOLS=Off (so tools aren't in the ALL target by default), then force just llvm-config:

-DLLVM_BUILD_TOOLS='Off'
-DLLVM_INCLUDE_TOOLS='On'
-DLLVM_TOOL_LLVM_CONFIG_BUILD='On'

LLVM's cmake uses the LLVM_TOOL_<name>_BUILD pattern to allow per-tool overrides of LLVM_BUILD_TOOLS. Expectation: llvm-config.exe is built, the ~200 other tool binaries are not.

Risks to validate in CI:

1. LLVM_INCLUDE_TOOLS=On re-emits configure/compile rules for all tools. If LLVM's cmake uses EXCLUDE_FROM_ALL correctly, ninja only builds llvm-config in practice. If not, the speedup partially erodes. Needs measurement.
2. Some tools may be tangled in install targets (e.g. via llvm_install_symlink), which could force them in via install-time dependencies. Watch the final ninja install step for hidden work.

Option B — LLVM_DISTRIBUTION_COMPONENTS whitelist

Most surgical. Tell cmake: "when building the distribution target, include only these components." Shape roughly:

-DLLVM_INCLUDE_TOOLS='On'
-DLLVM_BUILD_TOOLS='Off'
-DLLVM_DISTRIBUTION_COMPONENTS='llvm-libraries;llvm-headers;cmake-exports;llvm-config;mlir-libraries;mlir-headers;lld-libraries'

Requires switching solx-dev llvm build's final step from ninja install to ninja install-distribution. That's a ~one-line change in solx-dev/src/llvm/build.rs or equivalent. Clean but touches Rust code.

Option C — scope to Windows only (same as the reverted commit)

Whichever of A or B lands should retain the Windows-only gate — other platforms weren't link-bound and had no issues with tools enabled.

Expected impact

Based on the successful half of run #24771100328:

Metric	Before	With tool-disable	Speedup
Windows Build LLVM step	~42 min	~9:44	4.3×
Windows whole job	~1h 19min	~15–20 min est.	~4×

Applied per Windows CI run on submodule bumps + any path where ccache is the only cache in play.

Acceptance criteria

• Windows Build LLVM step completes in under 15 min on a warm-ccache run.
• Run tests step on Windows completes successfully (verifies llvm-sys found llvm-config).
• No regression on Linux/macOS legs (confirmed by wall-clock comparison against the last good run).
• Show ccache stats still reports near-100% hit rate on the warm path.

Out of scope

• Linux/macOS link-phase optimisations — Linux/macOS weren't link-bound in the ci(cache): re-add ccache as artifact-cache fallback #360 measurements.
• Broader ninja install-distribution rework on all platforms.

Related

• ci(cache): re-add ccache as artifact-cache fallback #360 — added ccache; included the first attempt at Windows tool-disable (commit 3106d1e, reverted).
• ci: speed up LLVM builds with cmake flag tuning #245 — earlier LLVM cmake flag tuning (link-jobs cap, SPLIT_DWARF, OPTIMIZED_TABLEGEN).