Auto merge of #99967 - Mark-Simulacrum:download-llvm-ci, r=jyn514

Download, rather than sccache-cache, LLVM in CI

My hope/expectation is that we can do better than sccache in CI for cached builds -- currently it looks like on macOS those still take upwards of 10-11 minutes, which is a significant amount of time that we could potentially cut.

This enables this mode for all non-dist builders; this should avoid any problems with the artifacts we distribute, while also providing for faster test builders (since they'll make use of PGO'd LLVM on the platforms we do that on, which is hopefully a nice win). It slightly increases the chance of test builders starting to fail only after a PR is merged (if PGO changes runtime behavior), but that should hopefully never happen, so I think this is worthwhile.

Measurements on the PR for apple-1 don't show any noticeable improvement in CI times, but those can be pretty noisy -- I'm inclined to land this since it *should* pretty much always be better and we can reconsider if that ever turns out not to be the case.

1 files changed, 80 insertions, 9 deletions

diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index 1bba5e68be4..7ecf74d3068 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -114,23 +114,20 @@ pub fn prebuilt_llvm_config(
     Err(Meta { stamp, build_llvm_config, out_dir, root: root.into() })
 }
 
-pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
-    let config = &builder.config;
-    if !config.llvm_from_ci {
-        return;
-    }
+/// This retrieves the LLVM sha we *want* to use, according to git history.
+pub(crate) fn detect_llvm_sha(config: &crate::config::Config) -> String {
     let mut rev_list = config.git();
     rev_list.args(&[
         PathBuf::from("rev-list"),
-        format!("--author={}", builder.config.stage0_metadata.config.git_merge_commit_email).into(),
+        format!("--author={}", config.stage0_metadata.config.git_merge_commit_email).into(),
         "-n1".into(),
         "--first-parent".into(),
         "HEAD".into(),
         "--".into(),
-        builder.src.join("src/llvm-project"),
-        builder.src.join("src/bootstrap/download-ci-llvm-stamp"),
+        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`
-        builder.src.join("src/version"),
+        config.src.join("src/version"),
     ]);
     let llvm_sha = output(&mut rev_list);
     let llvm_sha = llvm_sha.trim();
@@ -143,8 +140,82 @@ pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
         panic!();
     }
 
+    llvm_sha.to_owned()
+}
+
+/// Returns whether the CI-found LLVM is currently usable.
+///
+/// This checks both the build triple platform to confirm we're usable at all,
+/// and then verifies if the current HEAD matches the detected LLVM SHA head,
+/// in which case LLVM is indicated as not available.
+pub(crate) fn is_ci_llvm_available(config: &crate::config::Config, asserts: bool) -> bool {
+    // This is currently all tier 1 targets and tier 2 targets with host tools
+    // (since others may not have CI artifacts)
+    // https://doc.rust-lang.org/rustc/platform-support.html#tier-1
+    let supported_platforms = [
+        // tier 1
+        "aarch64-unknown-linux-gnu",
+        "i686-pc-windows-gnu",
+        "i686-pc-windows-msvc",
+        "i686-unknown-linux-gnu",
+        "x86_64-unknown-linux-gnu",
+        "x86_64-apple-darwin",
+        "x86_64-pc-windows-gnu",
+        "x86_64-pc-windows-msvc",
+        // tier 2 with host tools
+        "aarch64-apple-darwin",
+        "aarch64-pc-windows-msvc",
+        "aarch64-unknown-linux-musl",
+        "arm-unknown-linux-gnueabi",
+        "arm-unknown-linux-gnueabihf",
+        "armv7-unknown-linux-gnueabihf",
+        "mips-unknown-linux-gnu",
+        "mips64-unknown-linux-gnuabi64",
+        "mips64el-unknown-linux-gnuabi64",
+        "mipsel-unknown-linux-gnu",
+        "powerpc-unknown-linux-gnu",
+        "powerpc64-unknown-linux-gnu",
+        "powerpc64le-unknown-linux-gnu",
+        "riscv64gc-unknown-linux-gnu",
+        "s390x-unknown-linux-gnu",
+        "x86_64-unknown-freebsd",
+        "x86_64-unknown-illumos",
+        "x86_64-unknown-linux-musl",
+        "x86_64-unknown-netbsd",
+    ];
+    if !supported_platforms.contains(&&*config.build.triple) {
+        return false;
+    }
+
+    let triple = &*config.build.triple;
+    if (triple == "aarch64-unknown-linux-gnu" || triple.contains("i686")) && asserts {
+        // No alt builder for aarch64-unknown-linux-gnu today.
+        return false;
+    }
+
+    if crate::util::CiEnv::is_ci() {
+        let llvm_sha = detect_llvm_sha(config);
+        let head_sha = output(config.git().arg("rev-parse").arg("HEAD"));
+        let head_sha = head_sha.trim();
+        if llvm_sha == head_sha {
+            eprintln!(
+                "Detected LLVM as non-available: running in CI and modified LLVM in this change"
+            );
+            return false;
+        }
+    }
+
+    true
+}
+
+pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
+    let config = &builder.config;
+    if !config.llvm_from_ci {
+        return;
+    }
     let llvm_root = config.ci_llvm_root();
     let llvm_stamp = llvm_root.join(".llvm-stamp");
+    let llvm_sha = detect_llvm_sha(&config);
     let key = format!("{}{}", llvm_sha, config.llvm_assertions);
     if program_out_of_date(&llvm_stamp, &key) && !config.dry_run {
         download_ci_llvm(builder, &llvm_sha);