diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index 888290a0479ba..b6b8906746206 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -26,7 +26,6 @@ use crate::{generate_smart_stamp_hash, CLang, GitRepo, Kind};
 
 use crate::utils::exec::command;
 use build_helper::ci::CiEnv;
-use build_helper::git::get_git_merge_base;
 
 #[derive(Clone)]
 pub struct LlvmResult {
@@ -154,26 +153,18 @@ pub fn prebuilt_llvm_config(builder: &Builder<'_>, target: TargetSelection) -> L
 /// This retrieves the LLVM sha we *want* to use, according to git history.
 pub(crate) fn detect_llvm_sha(config: &Config, is_git: bool) -> String {
     let llvm_sha = if is_git {
-        // We proceed in 2 steps. First we get the closest commit that is actually upstream. Then we
-        // walk back further to the last bors merge commit that actually changed LLVM. The first
-        // step will fail on CI because only the `auto` branch exists; we just fall back to `HEAD`
-        // in that case.
-        let closest_upstream = get_git_merge_base(&config.git_config(), Some(&config.src))
-            .unwrap_or_else(|_| "HEAD".into());
-        let mut rev_list = helpers::git(Some(&config.src));
-        rev_list.args(&[
-            PathBuf::from("rev-list"),
-            format!("--author={}", config.stage0_metadata.config.git_merge_commit_email).into(),
-            "-n1".into(),
-            "--first-parent".into(),
-            closest_upstream.into(),
-            "--".into(),
-            config.src.join("src/llvm-project"),
-            config.src.join("src/bootstrap/download-ci-llvm-stamp"),
-            // the LLVM shared object file is named `LLVM-12-rust-{version}-nightly`
-            config.src.join("src/version"),
-        ]);
-        output(rev_list.as_command_mut()).trim().to_owned()
+        helpers::get_closest_merge_base_commit(
+            Some(&config.src),
+            &config.git_config(),
+            &config.stage0_metadata.config.git_merge_commit_email,
+            &[
+                config.src.join("src/llvm-project"),
+                config.src.join("src/bootstrap/download-ci-llvm-stamp"),
+                // the LLVM shared object file is named `LLVM-12-rust-{version}-nightly`
+                config.src.join("src/version"),
+            ],
+        )
+        .unwrap()
     } else if let Some(info) = channel::read_commit_info_file(&config.src) {
         info.sha.trim().to_owned()
     } else {
diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index 7bc410b9e8878..459e59e1b3446 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/src/bootstrap/src/core/build_steps/tool.rs
@@ -9,7 +9,7 @@ use crate::core::builder::{Builder, Cargo as CargoCommand, RunConfig, ShouldRun,
 use crate::core::config::TargetSelection;
 use crate::utils::channel::GitInfo;
 use crate::utils::exec::{command, BootstrapCommand};
-use crate::utils::helpers::{add_dylib_path, exe, t};
+use crate::utils::helpers::{add_dylib_path, exe, get_closest_merge_base_commit, git, t};
 use crate::Compiler;
 use crate::Mode;
 use crate::{gha, Kind};
@@ -554,6 +554,57 @@ impl Step for Rustdoc {
         }
         let target = target_compiler.host;
 
+        let bin_rustdoc = || {
+            let sysroot = builder.sysroot(target_compiler);
+            let bindir = sysroot.join("bin");
+            t!(fs::create_dir_all(&bindir));
+            let bin_rustdoc = bindir.join(exe("rustdoc", target_compiler.host));
+            let _ = fs::remove_file(&bin_rustdoc);
+            bin_rustdoc
+        };
+
+        // If CI rustc is enabled and we haven't modified the rustdoc sources,
+        // use the precompiled rustdoc from CI rustc's sysroot to speed up bootstrapping.
+        if builder.download_rustc()
+            && target_compiler.stage > 0
+            && builder.rust_info().is_managed_git_subrepository()
+        {
+            let commit = get_closest_merge_base_commit(
+                Some(&builder.config.src),
+                &builder.config.git_config(),
+                &builder.config.stage0_metadata.config.git_merge_commit_email,
+                &[],
+            )
+            .unwrap();
+
+            let librustdoc_src = builder.config.src.join("src/librustdoc");
+            let rustdoc_src = builder.config.src.join("src/tools/rustdoc");
+
+            // FIXME: The change detection logic here is quite similar to `Config::download_ci_rustc_commit`.
+            // It would be better to unify them.
+            let has_changes = !git(Some(&builder.config.src))
+                .allow_failure()
+                .run_always()
+                .args(["diff-index", "--quiet", &commit])
+                .arg("--")
+                .arg(librustdoc_src)
+                .arg(rustdoc_src)
+                .run(builder)
+                .is_success();
+
+            if !has_changes {
+                let precompiled_rustdoc = builder
+                    .config
+                    .ci_rustc_dir()
+                    .join("bin")
+                    .join(exe("rustdoc", target_compiler.host));
+
+                let bin_rustdoc = bin_rustdoc();
+                builder.copy_link(&precompiled_rustdoc, &bin_rustdoc);
+                return bin_rustdoc;
+            }
+        }
+
         let build_compiler = if builder.download_rustc() && target_compiler.stage == 1 {
             // We already have the stage 1 compiler, we don't need to cut the stage.
             builder.compiler(target_compiler.stage, builder.config.build)
@@ -614,11 +665,7 @@ impl Step for Rustdoc {
 
         // don't create a stage0-sysroot/bin directory.
         if target_compiler.stage > 0 {
-            let sysroot = builder.sysroot(target_compiler);
-            let bindir = sysroot.join("bin");
-            t!(fs::create_dir_all(&bindir));
-            let bin_rustdoc = bindir.join(exe("rustdoc", target_compiler.host));
-            let _ = fs::remove_file(&bin_rustdoc);
+            let bin_rustdoc = bin_rustdoc();
             builder.copy_link(&tool_rustdoc, &bin_rustdoc);
             bin_rustdoc
         } else {
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index f96633b059a17..9d5aa795c6c0d 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -20,7 +20,7 @@ use crate::core::build_steps::llvm;
 use crate::core::config::flags::{Color, Flags, Warnings};
 use crate::utils::cache::{Interned, INTERNER};
 use crate::utils::channel::{self, GitInfo};
-use crate::utils::helpers::{self, exe, output, t};
+use crate::utils::helpers::{self, exe, get_closest_merge_base_commit, output, t};
 use build_helper::exit;
 use serde::{Deserialize, Deserializer};
 use serde_derive::Deserialize;
@@ -2471,14 +2471,13 @@ impl Config {
 
         // Look for a version to compare to based on the current commit.
         // Only commits merged by bors will have CI artifacts.
-        let merge_base = output(
-            helpers::git(Some(&self.src))
-                .arg("rev-list")
-                .arg(format!("--author={}", self.stage0_metadata.config.git_merge_commit_email))
-                .args(["-n1", "--first-parent", "HEAD"])
-                .as_command_mut(),
-        );
-        let commit = merge_base.trim_end();
+        let commit = get_closest_merge_base_commit(
+            Some(&self.src),
+            &self.git_config(),
+            &self.stage0_metadata.config.git_merge_commit_email,
+            &[],
+        )
+        .unwrap();
         if commit.is_empty() {
             println!("ERROR: could not find commit hash for downloading rustc");
             println!("HELP: maybe your repository history is too shallow?");
@@ -2489,7 +2488,7 @@ impl Config {
 
         // Warn if there were changes to the compiler or standard library since the ancestor commit.
         let has_changes = !t!(helpers::git(Some(&self.src))
-            .args(["diff-index", "--quiet", commit])
+            .args(["diff-index", "--quiet", &commit])
             .arg("--")
             .args([self.src.join("compiler"), self.src.join("library")])
             .as_command_mut()
@@ -2565,14 +2564,13 @@ impl Config {
     ) -> Option<String> {
         // Look for a version to compare to based on the current commit.
         // Only commits merged by bors will have CI artifacts.
-        let merge_base = output(
-            helpers::git(Some(&self.src))
-                .arg("rev-list")
-                .arg(format!("--author={}", self.stage0_metadata.config.git_merge_commit_email))
-                .args(["-n1", "--first-parent", "HEAD"])
-                .as_command_mut(),
-        );
-        let commit = merge_base.trim_end();
+        let commit = get_closest_merge_base_commit(
+            Some(&self.src),
+            &self.git_config(),
+            &self.stage0_metadata.config.git_merge_commit_email,
+            &[],
+        )
+        .unwrap();
         if commit.is_empty() {
             println!("error: could not find commit hash for downloading components from CI");
             println!("help: maybe your repository history is too shallow?");
@@ -2583,7 +2581,7 @@ impl Config {
 
         // Warn if there were changes to the compiler or standard library since the ancestor commit.
         let mut git = helpers::git(Some(&self.src));
-        git.args(["diff-index", "--quiet", commit, "--"]);
+        git.args(["diff-index", "--quiet", &commit, "--"]);
 
         // Handle running from a directory other than the top level
         let top_level = &self.src;
diff --git a/src/bootstrap/src/utils/helpers.rs b/src/bootstrap/src/utils/helpers.rs
index 3c82fa189be36..773a873e47cc0 100644
--- a/src/bootstrap/src/utils/helpers.rs
+++ b/src/bootstrap/src/utils/helpers.rs
@@ -3,6 +3,7 @@
 //! Simple things like testing the various filesystem operations here and there,
 //! not a lot of interesting happenings here unfortunately.
 
+use build_helper::git::{get_git_merge_base, output_result, GitConfig};
 use build_helper::util::fail;
 use std::env;
 use std::ffi::OsStr;
@@ -521,3 +522,26 @@ pub fn git(source_dir: Option<&Path>) -> BootstrapCommand {
 
     git
 }
+
+/// Returns the closest commit available from upstream for the given `author` and `target_paths`.
+///
+/// If it fails to find the commit from upstream using `git merge-base`, fallbacks to HEAD.
+pub fn get_closest_merge_base_commit(
+    source_dir: Option<&Path>,
+    config: &GitConfig<'_>,
+    author: &str,
+    target_paths: &[PathBuf],
+) -> Result<String, String> {
+    let mut git = git(source_dir).capture_stdout();
+
+    let merge_base = get_git_merge_base(config, source_dir).unwrap_or_else(|_| "HEAD".into());
+
+    git.arg(Path::new("rev-list"));
+    git.args([&format!("--author={author}"), "-n1", "--first-parent", &merge_base]);
+
+    if !target_paths.is_empty() {
+        git.arg("--").args(target_paths);
+    }
+
+    Ok(output_result(git.as_command_mut())?.trim().to_owned())
+}
diff --git a/src/tools/build_helper/src/git.rs b/src/tools/build_helper/src/git.rs
index b4522de6897d4..8be38dc855f7a 100644
--- a/src/tools/build_helper/src/git.rs
+++ b/src/tools/build_helper/src/git.rs
@@ -7,7 +7,7 @@ pub struct GitConfig<'a> {
 }
 
 /// Runs a command and returns the output
-fn output_result(cmd: &mut Command) -> Result<String, String> {
+pub fn output_result(cmd: &mut Command) -> Result<String, String> {
     let output = match cmd.stderr(Stdio::inherit()).output() {
         Ok(status) => status,
         Err(e) => return Err(format!("failed to run command: {:?}: {}", cmd, e)),