Update instructions for building LLVM intrinsics

Author: Nicholas Rodrigues Lordello

.github/workflows/rust.yml

@@ -25,6 +25,7 @@ jobs:
         cargo test --release
     - name: Test LLVM Intrinsics
       run: |
+        sudo apt update && sudo apt install clang-16
         cargo clippy --features llvm-intrinsics --all-targets -- -D warnings
         cargo test --features llvm-intrinsics
         cargo test --features llvm-intrinsics --release

README.md

@@ -99,6 +99,24 @@ in order to actually take advantage of the the optimized assembly:
 RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build
 ```
 
+Note that **the `clang` version must match the `rustc` LLVM version**. If not,
+it is possible to encounter errors when running the `ethnum-intrinsics` build
+script. You can verify the LLVM version used by `rustc` with:
+
+```sh
+rustc --version --verbose | grep LLVM
+```
+
+In particular, this affects macOS which ships its own `clang` binary. The
+`ethnum-intrinsics` build script accepts a `CLANG` environment variable to
+specity a specific `clang` executable path to use. Using the major LLVM
+version from the command above:
+
+```
+brew install llvm@${LLVM_VERSION}
+CLANG=/opt/homebrew/opt/llvm@${LLVM_VERSION}/bin/clang cargo build
+```
+
 ### API Stability
 
 The instinsics are exported under `ethnum::intrinsics`. That being said, be

intrinsics/build/main.rs

@@ -15,6 +15,11 @@ use cc::Build;
 use std::{env, error::Error, fs, path::PathBuf, process::Command};
 
 fn main() -> Result<(), Box<dyn Error>> {
+    println!("cargo:rerun-if-env-changed=CLANG");
+    let clang = env::var("CLANG")
+        .map(PathBuf::from)
+        .unwrap_or_else(|_| "clang".into());
+
     let out_dir = PathBuf::from(env::var("OUT_DIR")?);
 
     let template = {
@@ -35,24 +40,15 @@ fn main() -> Result<(), Box<dyn Error>> {
         let source = template
             .replace("i128", "i256")
             .replace(" 127", " 255")
-            .replace("dereferenceable(16)", "dereferenceable(32)")
-            // TODO(nlordell): Figure out why Clang doesn't like these
-            .replace("mustprogress ", "")
-            .replace("noundef ", "")
-            .replace("memory(argmem: readwrite) ", "")
-            .replace("memory(argmem: read) ", "")
-            .replace("memory(none) ", "")
-            .replace(", !!1", "");
+            .replace("dereferenceable(16)", "dereferenceable(32)");
+
         let path = out_dir.join("intrinsics.ll");
         fs::write(&path, source)?;
         path
     };
 
     let mut build = Build::new();
-    build
-        .compiler("clang")
-        .file(intrinsics_ir_path)
-        .opt_level(3);
+    build.compiler(&clang).file(intrinsics_ir_path).opt_level(3);
 
     let linker_plugin_lto =
         matches!(env::var("RUSTFLAGS"), Ok(flags) if flags.contains("-Clinker-plugin-lto"));