Add a new if-identical mode to llvm.from_ci

It downloads llvm-opts from CI and checks if it is equal to the LLVM compiled on CI.

src/bootstrap/builder.rs

@@ -1105,7 +1105,7 @@ impl<'a> Builder<'a> {
         let mut dylib_dirs = vec![self.rustc_libdir(compiler)];
 
         // Ensure that the downloaded LLVM libraries can be found.
-        if self.config.llvm.from_ci {
+        if self.config.llvm_from_ci {
             let ci_llvm_lib = self.out.join(&*compiler.host.triple).join("ci-llvm").join("lib");
             dylib_dirs.push(ci_llvm_lib);
         }

src/bootstrap/config.rs

@@ -101,7 +101,7 @@ impl Display for DebuginfoLevel {
     }
 }
 
-#[derive(Default, Clone)]
+#[derive(Default, Clone, PartialEq, serde_derive::Serialize, serde_derive::Deserialize)]
 pub struct LLVMConfig {
     pub assertions: bool,
     pub tests: bool,
@@ -111,9 +111,6 @@ pub struct LLVMConfig {
     pub release_debuginfo: bool,
     pub static_stdcpp: bool,
     /// `None` if `llvm_from_ci` is true and we haven't yet downloaded llvm.
-    #[cfg(not(test))]
-    link_shared: Cell<Option<bool>>,
-    #[cfg(test)]
     pub link_shared: Cell<Option<bool>>,
     pub clang_cl: Option<String>,
     pub targets: Option<String>,
@@ -125,7 +122,6 @@ pub struct LLVMConfig {
     pub polly: bool,
     pub clang: bool,
     pub enable_warnings: bool,
-    pub from_ci: bool,
     pub build_config: HashMap<String, String>,
 
     pub use_lld: bool,
@@ -204,6 +200,7 @@ pub struct Config {
     pub backtrace_on_ice: bool,
 
     // llvm codegen options
+    pub llvm_from_ci: bool,
     pub llvm: LLVMConfig,
 
     // rust codegen options
@@ -1519,18 +1516,26 @@ impl Config {
             config.llvm.build_config = llvm.build_config.clone().unwrap_or(Default::default());
 
             let asserts = llvm_assertions.unwrap_or(false);
-            config.llvm.from_ci = match llvm.download_ci_llvm {
-                Some(StringOrBool::String(s)) => {
-                    assert!(s == "if-available", "unknown option `{}` for download-ci-llvm", s);
-                    crate::llvm::is_ci_llvm_available(&config, asserts)
-                }
+            let mut downloaded_config = false;
+            config.llvm_from_ci = match llvm.download_ci_llvm {
+                Some(StringOrBool::String(s)) => match s.as_str() {
+                    "if-available" => crate::llvm::is_ci_llvm_available(&config, asserts),
+                    "if-identical" => match crate::llvm::get_llvm_opts_from_ci(&config) {
+                        Some(config_ci) => {
+                            downloaded_config = true;
+                            is_llvm_config_identical(&config_ci, &config.llvm)
+                        }
+                        None => false,
+                    },
+                    _ => panic!("unknown option `{s}` for download-ci-llvm"),
+                },
                 Some(StringOrBool::Bool(b)) => b,
                 None => {
                     config.channel == "dev" && crate::llvm::is_ci_llvm_available(&config, asserts)
                 }
             };
 
-            if config.llvm.from_ci {
+            if config.llvm_from_ci && !downloaded_config {
                 // None of the LLVM options, except assertions, are supported
                 // when using downloaded LLVM. We could just ignore these but
                 // that's potentially confusing, so force them to not be
@@ -1561,14 +1566,14 @@ impl Config {
             }
 
             // NOTE: can never be hit when downloading from CI, since we call `check_ci_llvm!(thin_lto)` above.
-            if config.llvm.thin_lto && llvm.link_shared.is_none() {
+            if !downloaded_config && config.llvm.thin_lto && llvm.link_shared.is_none() {
                 // If we're building with ThinLTO on, by default we want to link
                 // to LLVM shared, to avoid re-doing ThinLTO (which happens in
                 // the link step) with each stage.
                 config.llvm.link_shared.set(Some(true));
             }
         } else {
-            config.llvm.from_ci =
+            config.llvm_from_ci =
                 config.channel == "dev" && crate::llvm::is_ci_llvm_available(&config, false);
         }
 
@@ -1620,7 +1625,7 @@ impl Config {
             }
         }
 
-        if config.llvm.from_ci {
+        if config.llvm_from_ci {
             let triple = &config.build.triple;
             let ci_llvm_bin = config.ci_llvm_root().join("bin");
             let build_target = config
@@ -1860,10 +1865,14 @@ impl Config {
 
     /// The absolute path to the downloaded LLVM artifacts.
     pub(crate) fn ci_llvm_root(&self) -> PathBuf {
-        assert!(self.llvm.from_ci);
+        assert!(self.llvm_from_ci);
         self.out.join(&*self.build.triple).join("ci-llvm")
     }
 
+    pub(crate) fn ci_llvm_root_opts(&self) -> PathBuf {
+        self.out.join(&*self.build.triple).join("ci-llvm-opts")
+    }
+
     /// Directory where the extracted `rustc-dev` component is stored.
     pub(crate) fn ci_rustc_dir(&self) -> PathBuf {
         assert!(self.download_rustc());
@@ -1882,7 +1891,7 @@ impl Config {
         }
 
         let llvm_link_shared = *opt.get_or_insert_with(|| {
-            if self.llvm.from_ci {
+            if self.llvm_from_ci {
                 self.maybe_download_ci_llvm();
                 let ci_llvm = self.ci_llvm_root();
                 let link_type = t!(
@@ -2089,6 +2098,10 @@ impl Config {
     }
 }
 
+fn is_llvm_config_identical(from_ci: &LLVMConfig, current: &LLVMConfig) -> bool {
+    from_ci == current
+}
+
 fn set<T>(field: &mut T, val: Option<T>) {
     if let Some(v) = val {
         *field = v;

src/bootstrap/dist.rs

@@ -1968,7 +1968,7 @@ fn install_llvm_file(builder: &Builder<'_>, source: &Path, destination: &Path) {
 /// Returns whether the files were actually copied.
 fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir: &Path) -> bool {
     if let Some(config) = builder.config.target_config.get(&target) {
-        if config.llvm_config.is_some() && !builder.config.llvm.from_ci {
+        if config.llvm_config.is_some() && !builder.config.llvm_from_ci {
             // If the LLVM was externally provided, then we don't currently copy
             // artifacts into the sysroot. This is not necessarily the right
             // choice (in particular, it will require the LLVM dylib to be in
@@ -2090,7 +2090,7 @@ impl Step for BoltInstrument {
             return self.file.clone();
         }
 
-        if builder.build.config.llvm.from_ci {
+        if builder.build.config.llvm_from_ci {
             println!("warning: trying to use BOLT with LLVM from CI, this will probably not work");
         }
 
@@ -2142,7 +2142,7 @@ impl Step for BoltOptimize {
             return self.file.clone();
         }
 
-        if builder.build.config.llvm.from_ci {
+        if builder.build.config.llvm_from_ci {
             println!("warning: trying to use BOLT with LLVM from CI, this will probably not work");
         }
 

src/bootstrap/download.rs

@@ -602,7 +602,7 @@ download-rustc = false
     }
 
     pub(crate) fn maybe_download_ci_llvm(&self) {
-        if !self.llvm.from_ci {
+        if !self.llvm_from_ci {
             return;
         }
         let llvm_root = self.ci_llvm_root();
@@ -674,4 +674,33 @@ download-rustc = false
         let llvm_root = self.ci_llvm_root();
         self.unpack(&tarball, &llvm_root, "rust-dev");
     }
+
+    pub(crate) fn download_ci_llvm_opts(&self, llvm_sha: &str) {
+        let cache_prefix = format!("llvm-opts-{llvm_sha}");
+        let cache_dst = self.out.join("cache");
+        let rustc_cache = cache_dst.join(cache_prefix);
+        t!(fs::create_dir_all(&rustc_cache));
+        let base = if self.llvm.assertions {
+            &self.stage0_metadata.config.artifacts_with_llvm_assertions_server
+        } else {
+            &self.stage0_metadata.config.artifacts_server
+        };
+        let version = self.artifact_version_part(llvm_sha);
+        let filename = format!("rust-dev-config-{}-{}.tar.xz", version, self.build.triple);
+        let tarball = rustc_cache.join(&filename);
+        if !tarball.exists() {
+            let help_on_error = "error: failed to download llvm config from ci
+
+    help: old builds get deleted after a certain time
+    help: if trying to compile an old commit of rustc, disable `download-ci-llvm` in config.toml:
+
+    [llvm]
+    download-ci-llvm = false
+    ";
+            self.download_file(&format!("{base}/{llvm_sha}/{filename}"), &tarball, help_on_error);
+        }
+
+        let llvm_root = self.ci_llvm_root_opts();
+        self.unpack(&tarball, &llvm_root, "rust-dev-config");
+    }
 }

src/bootstrap/lib.rs

@@ -868,7 +868,7 @@ impl Build {
             Some(Target { llvm_config, .. }) => {
                 // If the user set llvm-config we assume Rust is not patched,
                 // but first check to see if it was configured by llvm-from-ci.
-                (self.config.llvm.from_ci && target == self.config.build) || llvm_config.is_none()
+                (self.config.llvm_from_ci && target == self.config.build) || llvm_config.is_none()
             }
             None => true,
         }

src/bootstrap/llvm.rs

@@ -18,7 +18,7 @@ use std::process::Command;
 
 use crate::builder::{Builder, RunConfig, ShouldRun, Step};
 use crate::channel;
-use crate::config::{Config, TargetSelection};
+use crate::config::{Config, LLVMConfig, TargetSelection};
 use crate::util::get_clang_cl_resource_dir;
 use crate::util::{self, exe, output, t, up_to_date};
 use crate::{CLang, GitRepo, Kind};
@@ -215,6 +215,23 @@ pub(crate) fn is_ci_llvm_available(config: &Config, asserts: bool) -> bool {
     true
 }
 
+pub(crate) fn get_llvm_opts_from_ci(config: &Config) -> Option<LLVMConfig> {
+    if config.dry_run() || is_ci_llvm_modified(config) {
+        return None;
+    }
+
+    let llvm_sha = detect_llvm_sha(config, config.rust_info.is_managed_git_subrepository());
+    config.download_ci_llvm_opts(&llvm_sha);
+
+    let config_path = config.ci_llvm_root_opts().join("llvm-opts.json");
+    let Ok(config) = serde_json::from_slice::<LLVMConfig>(&t!(std::fs::read(config_path))) else {
+        // The LLVM config has changed its format or is corrupted in some way
+        eprintln!("Cannot deserialize LLVM config from llvm-opts.json");
+        return None;
+    };
+    Some(config)
+}
+
 /// Returns true if we're running in CI with modified LLVM (and thus can't download it)
 pub(crate) fn is_ci_llvm_modified(config: &Config) -> bool {
     CiEnv::is_ci() && config.rust_info.is_managed_git_subrepository() && {