Offload: Build offload as a single Step

bootstrap.example.toml

@@ -106,6 +106,9 @@
 # Whether to build LLVM with support for it's gpu offload runtime.
 #llvm.offload = false
 
+# Absolute path to the directory containing ClangConfig.cmake
+#llvm.offload-clang-dir = ""
+
 # When true, link libstdc++ statically into the rustc_llvm.
 # This is useful if you don't want to use the dynamic version of that
 # library provided by LLVM.

src/bootstrap/configure.py

@@ -120,6 +120,11 @@ def v(*args):
 o("llvm-assertions", "llvm.assertions", "build LLVM with assertions")
 o("llvm-enzyme", "llvm.enzyme", "build LLVM with enzyme")
 o("llvm-offload", "llvm.offload", "build LLVM with gpu offload support")
+o(
+    "llvm-offload-clang-dir",
+    "llvm.offload-clang-dir",
+    "pass the absolute directory of ClangConfig.cmake",
+)
 o("llvm-plugins", "llvm.plugins", "build LLVM with plugin interface")
 o("debug-assertions", "rust.debug-assertions", "build with debugging assertions")
 o(

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

@@ -1427,10 +1427,12 @@ fn rustc_llvm_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetSelect
     if builder.config.llvm_enzyme {
         cargo.env("LLVM_ENZYME", "1");
     }
+    let llvm::LlvmResult { host_llvm_config, .. } = builder.ensure(llvm::Llvm { target });
     if builder.config.llvm_offload {
+        builder.ensure(llvm::OmpOffload { target });
         cargo.env("LLVM_OFFLOAD", "1");
     }
-    let llvm::LlvmResult { host_llvm_config, .. } = builder.ensure(llvm::Llvm { target });
+
     cargo.env("LLVM_CONFIG", &host_llvm_config);
 
     // Some LLVM linker flags (-L and -l) may be needed to link `rustc_llvm`. Its build script
@@ -2293,6 +2295,45 @@ impl Step for Assemble {
             }
         }
 
+        if builder.config.llvm_offload && !builder.config.dry_run() {
+            debug!("`llvm_offload` requested");
+            let offload_install = builder.ensure(llvm::OmpOffload { target: build_compiler.host });
+            if let Some(_llvm_config) = builder.llvm_config(builder.config.host_target) {
+                let src_dir = offload_install.join("lib");
+                let libdir = builder.sysroot_target_libdir(build_compiler, build_compiler.host);
+                let target_libdir =
+                    builder.sysroot_target_libdir(target_compiler, target_compiler.host);
+                let lib_ext = std::env::consts::DLL_EXTENSION;
+
+                let liboffload = "libLLVMOffload";
+                let src_lib = src_dir.join(liboffload).with_extension(lib_ext);
+                let dst_lib = libdir.join(liboffload).with_extension(lib_ext);
+                let target_dst_lib = target_libdir.join(liboffload).with_extension(lib_ext);
+                builder.resolve_symlink_and_copy(&src_lib, &dst_lib);
+                builder.resolve_symlink_and_copy(&src_lib, &target_dst_lib);
+
+                // FIXME(offload): With LLVM-22, we should be able to drop everything below here.
+                let omp = "libomp";
+                let src_omp = src_dir.join(omp).with_extension(lib_ext);
+                let dst_omp_lib = libdir.join(omp).with_extension(lib_ext);
+                let target_omp_dst_lib = target_libdir.join(omp).with_extension(lib_ext);
+                builder.resolve_symlink_and_copy(&src_omp, &dst_omp_lib);
+                builder.resolve_symlink_and_copy(&src_omp, &target_omp_dst_lib);
+
+                let tgt = "libomptarget";
+                let src_tgt = src_dir.join(tgt).with_extension(lib_ext);
+                let dst_tgt_lib = libdir.join(tgt).with_extension(lib_ext);
+                let target_tgt_dst_lib = target_libdir.join(tgt).with_extension(lib_ext);
+                builder.resolve_symlink_and_copy(&src_tgt, &dst_tgt_lib);
+                builder.resolve_symlink_and_copy(&src_tgt, &target_tgt_dst_lib);
+
+                // FIXME(offload): Add amdgcn-amd-amdhsa and nvptx64-nvidia-cuda folder
+                // This one is slightly more tricky, since we have the same file twice, in two
+                // subfolders for amdgcn and nvptx64. We'll likely find two more in the future, once
+                // Intel and Spir-V support lands in offload.
+            }
+        }
+
         // Build the libraries for this compiler to link to (i.e., the libraries
         // it uses at runtime).
         debug!(

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

@@ -454,16 +454,6 @@ impl Step for Llvm {
             enabled_llvm_runtimes.push("compiler-rt");
         }
 
-        // This is an experimental flag, which likely builds more than necessary.
-        // We will optimize it when we get closer to releasing it on nightly.
-        if builder.config.llvm_offload {
-            enabled_llvm_runtimes.push("offload");
-            //FIXME(ZuseZ4): LLVM intends to drop the offload dependency on openmp.
-            //Remove this line once they achieved it.
-            enabled_llvm_runtimes.push("openmp");
-            enabled_llvm_projects.push("compiler-rt");
-        }
-
         if !enabled_llvm_projects.is_empty() {
             enabled_llvm_projects.sort();
             enabled_llvm_projects.dedup();
@@ -778,6 +768,17 @@ fn configure_cmake(
         .collect::<Vec<String>>()
         .join(" ")
         .into();
+
+    // If we use an external clang as opposed to building our own llvm_clang, than that clang will
+    // come with it's own set of default include directories, which are based on a potentially older
+    // LLVM. This can cause issues, so we overwrite it to include headers based on our
+    // `src/llvm-project` submodule instead.
+    // FIXME(offload): With LLVM-22 we hopefully won't need an external clang anymore.
+    let base = builder.llvm_out(target).join("include");
+    let inc_dir = base.display();
+    if builder.config.llvm_offload && !builder.config.llvm_clang {
+        cflags.push(format!(" -I {inc_dir}"));
+    }
     if let Some(ref s) = builder.config.llvm_cflags {
         cflags.push(" ");
         cflags.push(s);
@@ -789,6 +790,7 @@ fn configure_cmake(
         cflags.push(format!(" --target={target}"));
     }
     cfg.define("CMAKE_C_FLAGS", cflags);
+
     let mut cxxflags: OsString = builder
         .cc_handled_clags(target, CLang::Cxx)
         .into_iter()
@@ -801,6 +803,9 @@ fn configure_cmake(
         .collect::<Vec<String>>()
         .join(" ")
         .into();
+    if builder.config.llvm_offload && !builder.config.llvm_clang {
+        cxxflags.push(format!(" -I {inc_dir}"));
+    }
     if let Some(ref s) = builder.config.llvm_cxxflags {
         cxxflags.push(" ");
         cxxflags.push(s);
@@ -896,6 +901,133 @@ fn get_var(var_base: &str, host: &str, target: &str) -> Option<OsString> {
         .or_else(|| env::var_os(var_base))
 }
 
+// FIXME(offload): In an ideal world, we would just enable the offload runtime in our previous LLVM
+// build step. For now, we still depend on the openmp runtime since we use some of it's API, so we
+// build both. However, when building those runtimes as part of the LLVM step, then LLVM's cmake
+// implicitly assumes that Clang has also been build and will try to use it. In the Rust CI, we
+// don't always build clang (due to compile times), but instead use a slightly older external clang.
+// LLVM tries to remove this build dependency of offload/openmp on Clang for LLVM-22, so in the
+// future we might be able to integrate this step into the LLVM step. For now, we instead introduce
+// a Clang_DIR bootstrap option, which allows us tell CMake to use an external clang for these two
+// runtimes. This external clang will try to use it's own (older) include dirs when building our
+// in-tree LLVM submodule, which will cause build failures. To prevent those, we now also
+// explicitly set our include dirs.
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+pub struct OmpOffload {
+    pub target: TargetSelection,
+}
+
+impl Step for OmpOffload {
+    type Output = PathBuf;
+    const IS_HOST: bool = true;
+
+    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        run.path("src/llvm-project/offload")
+    }
+
+    fn make_run(run: RunConfig<'_>) {
+        run.builder.ensure(OmpOffload { target: run.target });
+    }
+
+    /// Compile OpenMP offload runtimes for `target`.
+    #[allow(unused)]
+    fn run(self, builder: &Builder<'_>) -> PathBuf {
+        if builder.config.dry_run() {
+            return builder.config.tempdir().join("llvm-offload-dry-run");
+        }
+        let target = self.target;
+
+        let LlvmResult { host_llvm_config, .. } = builder.ensure(Llvm { target: self.target });
+
+        // Running cmake twice in the same folder is known to cause issues, like deleting existing
+        // binaries. We therefore write our offload artifacts into it's own subfolder. We use a
+        // subfolder, so that all the logic that processes our build artifacts (hopefully) also
+        // automatically manages our artifacts in the subfolder.
+        let out_dir = builder.llvm_out(target).join("offload-outdir");
+        if std::fs::exists(&out_dir).is_ok_and(|x| !x) {
+            std::fs::create_dir_all(&out_dir).unwrap();
+        }
+
+        // Offload/OpenMP are just subfolders of LLVM, so we can use the LLVM sha.
+        static STAMP_HASH_MEMO: OnceLock<String> = OnceLock::new();
+        let smart_stamp_hash = STAMP_HASH_MEMO.get_or_init(|| {
+            generate_smart_stamp_hash(
+                builder,
+                &builder.config.src.join("src/llvm-project/offload"),
+                builder.in_tree_llvm_info.sha().unwrap_or_default(),
+            )
+        });
+        let stamp = BuildStamp::new(&out_dir).with_prefix("offload").add_stamp(smart_stamp_hash);
+
+        trace!("checking build stamp to see if we need to rebuild offload/openmp artifacts");
+        if stamp.is_up_to_date() {
+            trace!(?out_dir, "offload/openmp build artifacts are up to date");
+            if stamp.stamp().is_empty() {
+                builder.info(
+                    "Could not determine the Offload submodule commit hash. \
+                     Assuming that an Offload rebuild is not necessary.",
+                );
+                builder.info(&format!(
+                    "To force Offload/OpenMP to rebuild, remove the file `{}`",
+                    stamp.path().display()
+                ));
+            }
+            return out_dir;
+        }
+
+        trace!(?target, "(re)building offload/openmp artifacts");
+        builder.info(&format!("Building OpenMP/Offload for {target}"));
+        t!(stamp.remove());
+        let _time = helpers::timeit(builder);
+        t!(fs::create_dir_all(&out_dir));
+
+        builder.config.update_submodule("src/llvm-project");
+        let mut cfg = cmake::Config::new(builder.src.join("src/llvm-project/runtimes/"));
+        configure_cmake(builder, target, &mut cfg, true, LdFlags::default(), &[]);
+
+        // Re-use the same flags as llvm to control the level of debug information
+        // generated for offload.
+        let profile = match (builder.config.llvm_optimize, builder.config.llvm_release_debuginfo) {
+            (false, _) => "Debug",
+            (true, false) => "Release",
+            (true, true) => "RelWithDebInfo",
+        };
+        trace!(?profile);
+
+        // OpenMP/Offload builds currently (LLVM-21) still depend on Clang, although there are
+        // intentions to loosen this requirement for LLVM-22. If we were to
+        let clang_dir = if !builder.config.llvm_clang {
+            // We must have an external clang to use.
+            assert!(&builder.build.config.llvm_clang_dir.is_some());
+            builder.build.config.llvm_clang_dir.clone()
+        } else {
+            // No need to specify it, since we use the in-tree clang
+            None
+        };
+
+        // FIXME(offload): Once we move from OMP to Offload (Ol) APIs, we should drop the openmp
+        // runtime to simplify our build. We should also re-evaluate the LLVM_Root and try to get
+        // rid of the Clang_DIR, once we upgrade to LLVM-22.
+        cfg.out_dir(&out_dir)
+            .profile(profile)
+            .env("LLVM_CONFIG_REAL", &host_llvm_config)
+            .define("LLVM_ENABLE_ASSERTIONS", "ON")
+            .define("LLVM_ENABLE_RUNTIMES", "openmp;offload")
+            .define("LLVM_INCLUDE_TESTS", "OFF")
+            .define("OFFLOAD_INCLUDE_TESTS", "OFF")
+            .define("OPENMP_STANDALONE_BUILD", "ON")
+            .define("LLVM_ROOT", builder.llvm_out(target).join("build"))
+            .define("LLVM_DIR", builder.llvm_out(target).join("lib").join("cmake").join("llvm"));
+        if let Some(p) = clang_dir {
+            cfg.define("Clang_DIR", p);
+        }
+        cfg.build();
+
+        t!(stamp.write());
+        out_dir
+    }
+}
+
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct Enzyme {
     pub target: TargetSelection,

src/bootstrap/src/core/config/config.rs

@@ -169,6 +169,7 @@ pub struct Config {
     pub llvm_link_jobs: Option<u32>,
     pub llvm_version_suffix: Option<String>,
     pub llvm_use_linker: Option<String>,
+    pub llvm_clang_dir: Option<PathBuf>,
     pub llvm_allow_old_toolchain: bool,
     pub llvm_polly: bool,
     pub llvm_clang: bool,
@@ -604,6 +605,7 @@ impl Config {
             use_linker: llvm_use_linker,
             allow_old_toolchain: llvm_allow_old_toolchain,
             offload: llvm_offload,
+            offload_clang_dir: llvm_clang_dir,
             polly: llvm_polly,
             clang: llvm_clang,
             enable_warnings: llvm_enable_warnings,
@@ -1361,6 +1363,7 @@ impl Config {
             llvm_cflags,
             llvm_clang: llvm_clang.unwrap_or(false),
             llvm_clang_cl,
+            llvm_clang_dir: llvm_clang_dir.map(PathBuf::from),
             llvm_cxxflags,
             llvm_enable_warnings: llvm_enable_warnings.unwrap_or(false),
             llvm_enzyme: llvm_enzyme.unwrap_or(false),

src/bootstrap/src/core/config/toml/llvm.rs

@@ -35,6 +35,7 @@ define_config! {
         allow_old_toolchain: Option<bool> = "allow-old-toolchain",
         offload: Option<bool> = "offload",
         polly: Option<bool> = "polly",
+        offload_clang_dir: Option<String> = "offload-clang-dir",
         clang: Option<bool> = "clang",
         enable_warnings: Option<bool> = "enable-warnings",
         download_ci_llvm: Option<StringOrBool> = "download-ci-llvm",
@@ -112,6 +113,7 @@ pub fn check_incompatible_options_for_ci_llvm(
         use_linker,
         allow_old_toolchain,
         offload,
+        offload_clang_dir: _,
         polly,
         clang,
         enable_warnings,

src/bootstrap/src/utils/change_tracker.rs

@@ -606,4 +606,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "New option `gcc.libgccjit-libs-dir` to specify which libgccjit.so to use per target.",
     },
+    ChangeInfo {
+        change_id: 148671,
+        severity: ChangeSeverity::Info,
+        summary: "New option `llvm.offload-clang-dir` to allow building an in-tree llvm offload and openmp runtime with an external clang.",
+    },
 ];