From 8c41f93ace335eaa02d43f270f2f0e78ef914828 Mon Sep 17 00:00:00 2001
From: LJ <81748770+elle-j@users.noreply.github.com>
Date: Thu, 9 Apr 2026 07:51:37 +0200
Subject: [PATCH] Build LLVM for Windows using MSVC-compatible Clang (#497)

## Description

Switches the Windows LLVM build from Visual Studio to `clang-cl` +
`lld-link` (Clang's MSVC-compatible drivers) with Ninja.

This is in order to better match the Clang toolchain used on our Linux
and macOS variants, while keeping the MSVC ABI.

It also speeds up the build time by at least 2x on Windows.
---
 .github/workflows/release-llvm.yml            |  8 ++++++
 .github/workflows/test-llvm-builder.yml       | 12 +++++++++
 CHANGELOG.md                                  |  1 +
 .../src/platforms/x86_64_windows_msvc.rs      | 27 ++++++++++---------
 4 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/.github/workflows/release-llvm.yml b/.github/workflows/release-llvm.yml
index 3a5b9d0..4d44531 100644
--- a/.github/workflows/release-llvm.yml
+++ b/.github/workflows/release-llvm.yml
@@ -73,6 +73,7 @@ jobs:
       contents: write # for uploading assets to release
     steps:
       - uses: actions/checkout@v4
+
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           # without this it will override our rust flags
@@ -101,6 +102,13 @@ jobs:
         run: |
           choco install ninja
 
+      # Provides MSVC toolchain (ml64, headers, libs) used by clang-cl when building LLVM.
+      - name: Set Up MSVC Developer Environment
+        if: ${{ matrix.host == 'windows' }}
+        uses: ilammy/msvc-dev-cmd@v1
+        with:
+          arch: x64
+
       - name: Install LLVM Builder
         run: |
           cargo install --locked --force --path crates/llvm-builder
diff --git a/.github/workflows/test-llvm-builder.yml b/.github/workflows/test-llvm-builder.yml
index 9f118c7..6f6bdbf 100644
--- a/.github/workflows/test-llvm-builder.yml
+++ b/.github/workflows/test-llvm-builder.yml
@@ -40,6 +40,18 @@ jobs:
         run: |
           brew install ninja
 
+      - name: Install Dependencies
+        if: matrix.runner == 'windows-2022'
+        run: |
+          choco install ninja
+
+      # Provides MSVC toolchain (ml64, headers, libs) used by clang-cl when building LLVM.
+      - name: Set Up MSVC Developer Environment
+        if: matrix.runner == 'windows-2022'
+        uses: ilammy/msvc-dev-cmd@v1
+        with:
+          arch: x64
+
       - name: Test
         run: make test-llvm-builder
         env:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6776f51..780d25e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ This is a development pre-release.
 - Updated `inkwell` to `0.8.0`
 - Updated `polkavm-*` to `0.31.0`
 - Updated `EMSDK` to `5.0.0`
+- Switched building LLVM for Windows from Visual Studio to MSVC-compatible Clang (clang-cl) and Ninja. [#497](https://github.com/paritytech/revive/pull/497)
 
 ## v1.0.0
 
diff --git a/crates/llvm-builder/src/platforms/x86_64_windows_msvc.rs b/crates/llvm-builder/src/platforms/x86_64_windows_msvc.rs
index 7902acb..b96ac33 100644
--- a/crates/llvm-builder/src/platforms/x86_64_windows_msvc.rs
+++ b/crates/llvm-builder/src/platforms/x86_64_windows_msvc.rs
@@ -1,4 +1,8 @@
-//! The revive LLVM amd64 `windows-gnu` builder.
+//! The revive LLVM amd64 `windows-msvc` builder.
+//!
+//! Uses `clang-cl` and `lld-link` (Clang's MSVC-compatible drivers)
+//! to match the Clang toolchain used on our Linux and MacOS variants,
+//! while keeping the MSVC ABI and C++ runtime.
 
 use std::collections::HashSet;
 use std::process::Command;
@@ -27,6 +31,9 @@ pub fn build(
     sanitizer: Option<Sanitizer>,
 ) -> anyhow::Result<()> {
     crate::utils::check_presence("cmake")?;
+    crate::utils::check_presence("clang-cl")?;
+    crate::utils::check_presence("lld-link")?;
+    crate::utils::check_presence("ninja")?;
 
     let llvm_module_llvm =
         LLVMPath::llvm_module_llvm().and_then(crate::utils::path_windows_to_unix)?;
@@ -43,12 +50,16 @@ pub fn build(
                 "-B",
                 llvm_build_final.to_string_lossy().as_ref(),
                 "-G",
-                "Visual Studio 17 2022",
+                "Ninja",
                 format!(
                     "-DCMAKE_INSTALL_PREFIX='{}'",
                     llvm_target_final.to_string_lossy().as_ref(),
                 )
                 .as_str(),
+                format!("-DCMAKE_BUILD_TYPE='{build_type}'").as_str(),
+                "-DCMAKE_C_COMPILER='clang-cl'",
+                "-DCMAKE_CXX_COMPILER='clang-cl'",
+                "-DLLVM_USE_LINKER='lld-link'",
                 format!(
                     "-DLLVM_TARGETS_TO_BUILD='{}'",
                     targets
@@ -99,17 +110,7 @@ pub fn build(
         "LLVM building cmake",
     )?;
 
-    crate::utils::command(
-        Command::new("cmake").args([
-            "--build",
-            llvm_build_final.to_string_lossy().as_ref(),
-            "--target",
-            "install",
-            "--config",
-            build_type.to_string().as_str(),
-        ]),
-        "Building with msbuild",
-    )?;
+    crate::utils::ninja(llvm_build_final.as_ref())?;
 
     Ok(())
 }