unify LLVM version finding logic

src/bootstrap/src/core/build_steps/compile.rs

@@ -1092,9 +1092,10 @@ pub fn rustc_cargo(
 
     // We want to link against registerEnzyme and in the future we want to use additional
     // functionality from Enzyme core. For that we need to link against Enzyme.
-    // FIXME(ZuseZ4): Get the LLVM version number automatically instead of hardcoding it.
     if builder.config.llvm_enzyme {
-        cargo.rustflag("-l").rustflag("Enzyme-19");
+        let llvm_config = builder.llvm_config(builder.config.build).unwrap();
+        let llvm_version_major = llvm::get_llvm_version_major(builder, &llvm_config);
+        cargo.rustflag("-l").rustflag(&format!("Enzyme-{llvm_version_major}"));
     }
 
     // Building with protected visibility reduces the number of dynamic relocations needed, giving

src/bootstrap/src/core/build_steps/llvm.rs

@@ -571,10 +571,7 @@ impl Step for Llvm {
 
         // Helper to find the name of LLVM's shared library on darwin and linux.
         let find_llvm_lib_name = |extension| {
-            let version =
-                command(&res.llvm_config).arg("--version").run_capture_stdout(builder).stdout();
-            let major = version.split('.').next().unwrap();
-
+            let major = get_llvm_version_major(builder, &res.llvm_config);
             match &llvm_version_suffix {
                 Some(version_suffix) => format!("libLLVM-{major}{version_suffix}.{extension}"),
                 None => format!("libLLVM-{major}.{extension}"),
@@ -624,12 +621,22 @@ impl Step for Llvm {
     }
 }
 
+pub fn get_llvm_version(builder: &Builder<'_>, llvm_config: &Path) -> String {
+    command(llvm_config).arg("--version").run_capture_stdout(builder).stdout().trim().to_owned()
+}
+
+pub fn get_llvm_version_major(builder: &Builder<'_>, llvm_config: &Path) -> u8 {
+    let version = get_llvm_version(builder, llvm_config);
+    let major_str = version.split_once('.').expect("Failed to parse LLVM version").0;
+    major_str.parse().unwrap()
+}
+
 fn check_llvm_version(builder: &Builder<'_>, llvm_config: &Path) {
     if builder.config.dry_run() {
         return;
     }
 
-    let version = command(llvm_config).arg("--version").run_capture_stdout(builder).stdout();
+    let version = get_llvm_version(builder, llvm_config);
     let mut parts = version.split('.').take(2).filter_map(|s| s.parse::<u32>().ok());
     if let (Some(major), Some(_minor)) = (parts.next(), parts.next()) {
         if major >= 18 {

src/bootstrap/src/core/build_steps/test.rs

@@ -12,6 +12,7 @@ use clap_complete::shells;
 
 use crate::core::build_steps::compile::run_cargo;
 use crate::core::build_steps::doc::DocumentationFormat;
+use crate::core::build_steps::llvm::get_llvm_version;
 use crate::core::build_steps::synthetic_targets::MirOptPanicAbortSyntheticTarget;
 use crate::core::build_steps::tool::{self, SourceType, Tool};
 use crate::core::build_steps::toolstate::ToolState;
@@ -1945,8 +1946,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             let llvm::LlvmResult { llvm_config, .. } =
                 builder.ensure(llvm::Llvm { target: builder.config.build });
             if !builder.config.dry_run() {
-                let llvm_version =
-                    command(&llvm_config).arg("--version").run_capture_stdout(builder).stdout();
+                let llvm_version = get_llvm_version(builder, &llvm_config);
                 let llvm_components =
                     command(&llvm_config).arg("--components").run_capture_stdout(builder).stdout();
                 // Remove trailing newline from llvm-config output.

src/bootstrap/src/core/builder/mod.rs

@@ -1431,7 +1431,7 @@ impl<'a> Builder<'a> {
     ///
     /// Note that this returns `None` if LLVM is disabled, or if we're in a
     /// check build or dry-run, where there's no need to build all of LLVM.
-    fn llvm_config(&self, target: TargetSelection) -> Option<PathBuf> {
+    pub fn llvm_config(&self, target: TargetSelection) -> Option<PathBuf> {
         if self.config.llvm_enabled(target) && self.kind != Kind::Check && !self.config.dry_run() {
             let llvm::LlvmResult { llvm_config, .. } = self.ensure(llvm::Llvm { target });
             if llvm_config.is_file() {