Allow building rustc's LLVM with Offload support

Author: ZuseZ4

config.example.toml

@@ -84,6 +84,9 @@
 # Wheter to build Enzyme as AutoDiff backend.
 #enzyme = false
 
+# Whether to build LLVM with support for it's gpu offload runtime.
+#offload = false
+
 # Indicates whether ccache is used when building LLVM. Set to `true` to use the first `ccache` in
 # PATH, or set an absolute path to use a specific version.
 #ccache = false

src/bootstrap/configure.py

@@ -72,6 +72,7 @@ def v(*args):
 o("optimize-llvm", "llvm.optimize", "build optimized LLVM")
 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-plugins", "llvm.plugins", "build LLVM with plugin interface")
 o("debug-assertions", "rust.debug-assertions", "build with debugging assertions")
 o("debug-assertions-std", "rust.debug-assertions-std", "build the standard library with debugging assertions")

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

@@ -459,6 +459,21 @@ impl Step for Llvm {
             cfg.define("LLVM_ENABLE_PROJECTS", enabled_llvm_projects.join(";"));
         }
 
+        let mut enabled_llvm_runtimes = Vec::new();
+
+        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");
+        }
+
+        if !enabled_llvm_runtimes.is_empty() {
+            enabled_llvm_runtimes.sort();
+            enabled_llvm_runtimes.dedup();
+            cfg.define("LLVM_ENABLE_RUNTIMES", enabled_llvm_runtimes.join(";"));
+        }
+
         if let Some(num_linkers) = builder.config.llvm_link_jobs {
             if num_linkers > 0 {
                 cfg.define("LLVM_PARALLEL_LINK_JOBS", num_linkers.to_string());

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

@@ -224,6 +224,7 @@ pub struct Config {
     pub llvm_assertions: bool,
     pub llvm_tests: bool,
     pub llvm_enzyme: bool,
+    pub llvm_offload: bool,
     pub llvm_plugins: bool,
     pub llvm_optimize: bool,
     pub llvm_thin_lto: bool,
@@ -930,6 +931,7 @@ define_config! {
         use_libcxx: Option<bool> = "use-libcxx",
         use_linker: Option<String> = "use-linker",
         allow_old_toolchain: Option<bool> = "allow-old-toolchain",
+        offload: Option<bool> = "offload",
         polly: Option<bool> = "polly",
         clang: Option<bool> = "clang",
         enable_warnings: Option<bool> = "enable-warnings",
@@ -1636,6 +1638,7 @@ impl Config {
         // we'll infer default values for them later
         let mut llvm_tests = None;
         let mut llvm_enzyme = None;
+        let mut llvm_offload = None;
         let mut llvm_plugins = None;
         let mut debug = None;
         let mut debug_assertions = None;
@@ -1873,6 +1876,7 @@ impl Config {
                 use_libcxx,
                 use_linker,
                 allow_old_toolchain,
+                offload,
                 polly,
                 clang,
                 enable_warnings,
@@ -1889,6 +1893,7 @@ impl Config {
             set(&mut config.ninja_in_file, ninja);
             llvm_tests = tests;
             llvm_enzyme = enzyme;
+            llvm_offload = offload;
             llvm_plugins = plugins;
             set(&mut config.llvm_optimize, optimize_toml);
             set(&mut config.llvm_thin_lto, thin_lto);
@@ -1910,6 +1915,7 @@ impl Config {
             set(&mut config.llvm_use_libcxx, use_libcxx);
             config.llvm_use_linker.clone_from(&use_linker);
             config.llvm_allow_old_toolchain = allow_old_toolchain.unwrap_or(false);
+            config.llvm_offload = offload.unwrap_or(false);
             config.llvm_polly = polly.unwrap_or(false);
             config.llvm_clang = clang.unwrap_or(false);
             config.llvm_enable_warnings = enable_warnings.unwrap_or(false);
@@ -2086,6 +2092,7 @@ impl Config {
 
         config.llvm_tests = llvm_tests.unwrap_or(false);
         config.llvm_enzyme = llvm_enzyme.unwrap_or(false);
+        config.llvm_offload = llvm_offload.unwrap_or(false);
         config.llvm_plugins = llvm_plugins.unwrap_or(false);
         config.rust_optimize = optimize.unwrap_or(RustOptimize::Bool(true));
 
@@ -2970,6 +2977,7 @@ pub(crate) fn check_incompatible_options_for_ci_llvm(
         use_libcxx,
         use_linker,
         allow_old_toolchain,
+        offload,
         polly,
         clang,
         enable_warnings,
@@ -2992,6 +3000,7 @@ pub(crate) fn check_incompatible_options_for_ci_llvm(
     err!(current_llvm_config.use_libcxx, use_libcxx);
     err!(current_llvm_config.use_linker, use_linker);
     err!(current_llvm_config.allow_old_toolchain, allow_old_toolchain);
+    err!(current_llvm_config.offload, offload);
     err!(current_llvm_config.polly, polly);
     err!(current_llvm_config.clang, clang);
     err!(current_llvm_config.build_config, build_config);

src/bootstrap/src/utils/change_tracker.rs

@@ -275,4 +275,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "New option `./x setup editor` added, replacing `./x setup vscode` and adding support for vim, emacs and helix.",
     },
+    ChangeInfo {
+        change_id: 131513,
+        severity: ChangeSeverity::Info,
+        summary: "New option `llvm.offload` to control whether the llvm offload runtime for GPU support is built. Implicitly enables the openmp runtime as dependency.",
+    },
 ];