Allow targets to override default codegen backend

compiler/rustc_driver_impl/src/lib.rs

@@ -35,7 +35,7 @@ use rustc_session::config::{nightly_options, CG_OPTIONS, Z_OPTIONS};
 use rustc_session::config::{ErrorOutputType, Input, OutFileName, OutputType};
 use rustc_session::getopts::{self, Matches};
 use rustc_session::lint::{Lint, LintId};
-use rustc_session::{config, EarlyDiagCtxt, Session};
+use rustc_session::{config, filesearch, EarlyDiagCtxt, Session};
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::source_map::FileLoader;
 use rustc_span::symbol::sym;
@@ -887,7 +887,13 @@ pub fn version_at_macro_invocation(
 
         let debug_flags = matches.opt_strs("Z");
         let backend_name = debug_flags.iter().find_map(|x| x.strip_prefix("codegen-backend="));
-        get_codegen_backend(early_dcx, &None, backend_name).print_version();
+        let opts = config::Options::default();
+        let sysroot = opts.maybe_sysroot.clone().unwrap_or_else(|| {
+            filesearch::get_or_default_sysroot().expect("Failed finding sysroot")
+        });
+        let target = config::build_target_config(early_dcx, &opts, None, &sysroot);
+
+        get_codegen_backend(early_dcx, &sysroot, backend_name, &target).print_version();
     }
 }
 
@@ -1092,7 +1098,14 @@ pub fn describe_flag_categories(early_dcx: &EarlyDiagCtxt, matches: &Matches) ->
 
     if cg_flags.iter().any(|x| *x == "passes=list") {
         let backend_name = debug_flags.iter().find_map(|x| x.strip_prefix("codegen-backend="));
-        get_codegen_backend(early_dcx, &None, backend_name).print_passes();
+
+        let opts = config::Options::default();
+        let sysroot = opts.maybe_sysroot.clone().unwrap_or_else(|| {
+            filesearch::get_or_default_sysroot().expect("Failed finding sysroot")
+        });
+        let target = config::build_target_config(early_dcx, &opts, None, &sysroot);
+
+        get_codegen_backend(early_dcx, &sysroot, backend_name, &target).print_passes();
         return true;
     }
 

compiler/rustc_interface/src/interface.rs

@@ -16,7 +16,7 @@ use rustc_parse::maybe_new_parser_from_source_str;
 use rustc_query_impl::QueryCtxt;
 use rustc_query_system::query::print_query_stack;
 use rustc_session::config::{self, Cfg, CheckCfg, ExpectedValues, Input, OutFileName};
-use rustc_session::filesearch::sysroot_candidates;
+use rustc_session::filesearch::{self, sysroot_candidates};
 use rustc_session::parse::ParseSess;
 use rustc_session::{lint, CompilerIO, EarlyDiagCtxt, Session};
 use rustc_span::source_map::FileLoader;
@@ -336,14 +336,66 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
 
             let early_dcx = EarlyDiagCtxt::new(config.opts.error_format);
 
-            let codegen_backend = if let Some(make_codegen_backend) = config.make_codegen_backend {
-                make_codegen_backend(&config.opts)
-            } else {
-                util::get_codegen_backend(
-                    &early_dcx,
-                    &config.opts.maybe_sysroot,
-                    config.opts.unstable_opts.codegen_backend.as_deref(),
-                )
+            let sysroot = match &config.opts.maybe_sysroot {
+                Some(sysroot) => sysroot.clone(),
+                None => filesearch::get_or_default_sysroot().expect("Failed finding sysroot"),
+            };
+
+            let (codegen_backend, target_cfg) = match config.make_codegen_backend {
+                None => {
+                    // Build a target without override, so that it can override the backend if needed
+                    let target =
+                        config::build_target_config(&early_dcx, &config.opts, None, &sysroot);
+
+                    let backend = util::get_codegen_backend(
+                        &early_dcx,
+                        &sysroot,
+                        config.opts.unstable_opts.codegen_backend.as_deref(),
+                        &target,
+                    );
+
+                    // target_override is documented to be called before init(), so this is okay
+                    let target_override = backend.target_override(&config.opts);
+
+                    // Assert that we don't use target's override of the backend and
+                    // backend's override of the target at the same time
+                    if config.opts.unstable_opts.codegen_backend.is_none()
+                        && target.default_codegen_backend.is_some()
+                        && target_override.is_some()
+                    {
+                        rustc_middle::bug!(
+                            "Codegen backend requested target override even though the target requested the backend"
+                        );
+                    }
+
+                    // Re-build target with the (potential) override
+                    let target = config::build_target_config(
+                        &early_dcx,
+                        &config.opts,
+                        target_override,
+                        &sysroot,
+                    );
+
+                    (backend, target)
+                }
+                Some(make_codegen_backend) => {
+                    // N.B. `make_codegen_backend` takes precedence over `target.default_codegen_backend`,
+                    //      which is ignored in this case.
+
+                    let backend = make_codegen_backend(&config.opts);
+
+                    // target_override is documented to be called before init(), so this is okay
+                    let target_override = backend.target_override(&config.opts);
+
+                    let target = config::build_target_config(
+                        &early_dcx,
+                        &config.opts,
+                        target_override,
+                        &sysroot,
+                    );
+
+                    (backend, target)
+                }
             };
 
             let temps_dir = config.opts.unstable_opts.temps_dir.as_deref().map(PathBuf::from);
@@ -364,9 +416,6 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
             let mut locale_resources = Vec::from(config.locale_resources);
             locale_resources.push(codegen_backend.locale_resource());
 
-            // target_override is documented to be called before init(), so this is okay
-            let target_override = codegen_backend.target_override(&config.opts);
-
             let mut sess = rustc_session::build_session(
                 early_dcx,
                 config.opts,
@@ -381,7 +430,8 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
                 locale_resources,
                 config.lint_caps,
                 config.file_loader,
-                target_override,
+                target_cfg,
+                sysroot,
                 util::rustc_version_str().unwrap_or("unknown"),
                 config.ice_file,
                 config.using_internal_features,

compiler/rustc_interface/src/tests.rs

@@ -37,6 +37,17 @@ fn mk_session(matches: getopts::Matches) -> (Session, Cfg) {
         output_file: None,
         temps_dir,
     };
+
+    let sysroot = match &sessopts.maybe_sysroot {
+        Some(sysroot) => sysroot.clone(),
+        None => {
+            rustc_session::filesearch::get_or_default_sysroot().expect("Failed finding sysroot")
+        }
+    };
+
+    let target_cfg =
+        rustc_session::config::build_target_config(&early_dcx, &sessopts, None, &sysroot);
+
     let sess = build_session(
         early_dcx,
         sessopts,
@@ -46,7 +57,8 @@ fn mk_session(matches: getopts::Matches) -> (Session, Cfg) {
         vec![],
         Default::default(),
         None,
-        None,
+        target_cfg,
+        sysroot,
         "",
         None,
         Arc::default(),

compiler/rustc_interface/src/util.rs

@@ -14,14 +14,15 @@ use rustc_session::{filesearch, output, Session};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
 use rustc_span::symbol::{sym, Symbol};
+use rustc_target::spec::Target;
 use session::EarlyDiagCtxt;
-use std::env;
 use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
 use std::mem;
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::OnceLock;
 use std::thread;
+use std::{env, iter};
 
 /// Function pointer type that constructs a new CodegenBackend.
 pub type MakeBackendFn = fn() -> Box<dyn CodegenBackend>;
@@ -192,21 +193,25 @@ fn load_backend_from_dylib(early_dcx: &EarlyDiagCtxt, path: &Path) -> MakeBacken
 /// A name of `None` indicates that the default backend should be used.
 pub fn get_codegen_backend(
     early_dcx: &EarlyDiagCtxt,
-    maybe_sysroot: &Option<PathBuf>,
+    sysroot: &Path,
     backend_name: Option<&str>,
+    target: &Target,
 ) -> Box<dyn CodegenBackend> {
     static LOAD: OnceLock<unsafe fn() -> Box<dyn CodegenBackend>> = OnceLock::new();
 
     let load = LOAD.get_or_init(|| {
-        let default_codegen_backend = option_env!("CFG_DEFAULT_CODEGEN_BACKEND").unwrap_or("llvm");
+        let backend = backend_name
+            .or(target.default_codegen_backend.as_deref())
+            .or(option_env!("CFG_DEFAULT_CODEGEN_BACKEND"))
+            .unwrap_or("llvm");
 
-        match backend_name.unwrap_or(default_codegen_backend) {
+        match backend {
             filename if filename.contains('.') => {
                 load_backend_from_dylib(early_dcx, filename.as_ref())
             }
             #[cfg(feature = "llvm")]
             "llvm" => rustc_codegen_llvm::LlvmCodegenBackend::new,
-            backend_name => get_codegen_sysroot(early_dcx, maybe_sysroot, backend_name),
+            backend_name => get_codegen_sysroot(early_dcx, sysroot, backend_name),
         }
     });
 
@@ -240,7 +245,7 @@ fn get_rustc_path_inner(bin_path: &str) -> Option<PathBuf> {
 
 fn get_codegen_sysroot(
     early_dcx: &EarlyDiagCtxt,
-    maybe_sysroot: &Option<PathBuf>,
+    sysroot: &Path,
     backend_name: &str,
 ) -> MakeBackendFn {
     // For now we only allow this function to be called once as it'll dlopen a
@@ -257,28 +262,28 @@ fn get_codegen_sysroot(
     let target = session::config::host_triple();
     let sysroot_candidates = sysroot_candidates();
 
-    let sysroot = maybe_sysroot
-        .iter()
-        .chain(sysroot_candidates.iter())
+    let sysroot = iter::once(sysroot)
+        .chain(sysroot_candidates.iter().map(<_>::as_ref))
         .map(|sysroot| {
             filesearch::make_target_lib_path(sysroot, target).with_file_name("codegen-backends")
         })
         .find(|f| {
             info!("codegen backend candidate: {}", f.display());
             f.exists()
-        });
-    let sysroot = sysroot.unwrap_or_else(|| {
-        let candidates = sysroot_candidates
-            .iter()
-            .map(|p| p.display().to_string())
-            .collect::<Vec<_>>()
-            .join("\n* ");
-        let err = format!(
-            "failed to find a `codegen-backends` folder \
+        })
+        .unwrap_or_else(|| {
+            let candidates = sysroot_candidates
+                .iter()
+                .map(|p| p.display().to_string())
+                .collect::<Vec<_>>()
+                .join("\n* ");
+            let err = format!(
+                "failed to find a `codegen-backends` folder \
                            in the sysroot candidates:\n* {candidates}"
-        );
-        early_dcx.early_fatal(err);
-    });
+            );
+            early_dcx.early_fatal(err);
+        });
+
     info!("probing {} for a codegen backend", sysroot.display());
 
     let d = sysroot.read_dir().unwrap_or_else(|e| {

compiler/rustc_session/src/config.rs

@@ -1558,7 +1558,7 @@ pub fn build_configuration(sess: &Session, mut user_cfg: Cfg) -> Cfg {
     user_cfg
 }
 
-pub(super) fn build_target_config(
+pub fn build_target_config(
     early_dcx: &EarlyDiagCtxt,
     opts: &Options,
     target_override: Option<Target>,

compiler/rustc_session/src/session.rs

@@ -1034,7 +1034,8 @@ pub fn build_session(
     fluent_resources: Vec<&'static str>,
     driver_lint_caps: FxHashMap<lint::LintId, lint::Level>,
     file_loader: Option<Box<dyn FileLoader + Send + Sync + 'static>>,
-    target_override: Option<Target>,
+    target_cfg: Target,
+    sysroot: PathBuf,
     cfg_version: &'static str,
     ice_file: Option<PathBuf>,
     using_internal_features: Arc<AtomicBool>,
@@ -1051,12 +1052,6 @@ pub fn build_session(
     let cap_lints_allow = sopts.lint_cap.is_some_and(|cap| cap == lint::Allow);
     let can_emit_warnings = !(warnings_allow || cap_lints_allow);
 
-    let sysroot = match &sopts.maybe_sysroot {
-        Some(sysroot) => sysroot.clone(),
-        None => filesearch::get_or_default_sysroot().expect("Failed finding sysroot"),
-    };
-
-    let target_cfg = config::build_target_config(&early_dcx, &sopts, target_override, &sysroot);
     let host_triple = TargetTriple::from_triple(config::host_triple());
     let (host, target_warnings) = Target::search(&host_triple, &sysroot).unwrap_or_else(|e| {
         early_dcx.early_fatal(format!("Error loading host specification: {e}"))

compiler/rustc_target/src/spec/mod.rs

@@ -2044,6 +2044,14 @@ pub struct TargetOptions {
     /// Default number of codegen units to use in debug mode
     pub default_codegen_units: Option<u64>,
 
+    /// Default codegen backend used for this target. Defaults to `None`.
+    ///
+    /// If `None`, then `CFG_DEFAULT_CODEGEN_BACKEND` environmental variable captured when
+    /// compiling `rustc` will be used instead (or llvm if it is not set).
+    ///
+    /// N.B. when *using* the compiler, backend can always be overriden with `-Zcodegen-backend`.
+    pub default_codegen_backend: Option<StaticCow<str>>,
+
     /// Whether to generate trap instructions in places where optimization would
     /// otherwise produce control flow that falls through into unrelated memory.
     pub trap_unreachable: bool,
@@ -2350,6 +2358,7 @@ impl Default for TargetOptions {
             stack_probes: StackProbeType::None,
             min_global_align: None,
             default_codegen_units: None,
+            default_codegen_backend: None,
             trap_unreachable: true,
             requires_lto: false,
             singlethread: false,