bootstrap: Memoize the LLVM rebuild hash to avoid very slow x check

Author: Zalathar

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

@@ -105,13 +105,15 @@ pub fn prebuilt_llvm_config(
     let llvm_cmake_dir = out_dir.join("lib/cmake/llvm");
     let res = LlvmResult { llvm_config: build_llvm_config, llvm_cmake_dir };
 
-    let smart_stamp_hash = generate_smart_stamp_hash(
-        &builder.config.src.join("src/llvm-project"),
-        &builder.in_tree_llvm_info.sha().unwrap_or_default(),
-    );
+    let smart_stamp_hash = builder.llvm_stamp_hash.get_or_init(|| {
+        generate_smart_stamp_hash(
+            &builder.config.src.join("src/llvm-project"),
+            &builder.in_tree_llvm_info.sha().unwrap_or_default(),
+        )
+    });
 
     let stamp = out_dir.join("llvm-finished-building");
-    let stamp = HashStamp::new(stamp, Some(&smart_stamp_hash));
+    let stamp = HashStamp::new(stamp, Some(smart_stamp_hash));
 
     if stamp.is_done() {
         if stamp.hash.is_none() {

src/bootstrap/src/core/builder.rs

@@ -10,6 +10,7 @@ use std::io::{BufRead, BufReader};
 use std::ops::Deref;
 use std::path::{Path, PathBuf};
 use std::process::Command;
+use std::sync::OnceLock;
 use std::time::{Duration, Instant};
 
 use crate::core::build_steps::llvm;
@@ -44,6 +45,7 @@ pub struct Builder<'a> {
     stack: RefCell<Vec<Box<dyn Any>>>,
     time_spent_on_dependencies: Cell<Duration>,
     pub paths: Vec<PathBuf>,
+    pub(crate) llvm_stamp_hash: OnceLock<String>,
 }
 
 impl<'a> Deref for Builder<'a> {
@@ -917,6 +919,7 @@ impl<'a> Builder<'a> {
             stack: RefCell::new(Vec::new()),
             time_spent_on_dependencies: Cell::new(Duration::new(0, 0)),
             paths,
+            llvm_stamp_hash: OnceLock::new(),
         }
     }