Allow targets to override default codegen backend

compiler/rustc_driver_impl/src/lib.rs

@@ -42,7 +42,7 @@ use rustc_session::config::{ErrorOutputType, Input, OutFileName, OutputType, Tri
 use rustc_session::cstore::MetadataLoader;
 use rustc_session::getopts::{self, Matches};
 use rustc_session::lint::{Lint, LintId};
-use rustc_session::{config, EarlyErrorHandler, Session};
+use rustc_session::{config, filesearch, EarlyErrorHandler, Session};
 use rustc_span::source_map::{FileLoader, FileName};
 use rustc_span::symbol::sym;
 use rustc_target::json::ToJson;
@@ -923,7 +923,13 @@ 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(handler, &None, backend_name).print_version();
+        let sopts = config::Options::default();
+        let sysroot = sopts.maybe_sysroot.clone().unwrap_or_else(|| {
+            filesearch::get_or_default_sysroot().expect("Failed finding sysroot")
+        });
+        let target = config::build_target_config(handler, &sopts, None, &sysroot);
+
+        get_codegen_backend(handler, &sysroot, backend_name, &target).print_version();
     }
 }
 
@@ -1127,7 +1133,13 @@ pub fn describe_flag_categories(handler: &EarlyErrorHandler, 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(handler, &None, backend_name).print_passes();
+        let sopts = config::Options::default();
+        let sysroot = sopts.maybe_sysroot.clone().unwrap_or_else(|| {
+            filesearch::get_or_default_sysroot().expect("Failed finding sysroot")
+        });
+        let target = config::build_target_config(handler, &sopts, None, &sysroot);
+
+        get_codegen_backend(handler, &sysroot, backend_name, &target).print_passes();
         return true;
     }
 

compiler/rustc_interface/src/util.rs

@@ -20,14 +20,16 @@ use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::FileLoader;
 use rustc_span::symbol::{sym, Symbol};
+use rustc_target::spec::Target;
 use session::{CompilerIO, EarlyErrorHandler};
-use std::env;
 use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
 use std::mem;
+use std::ops::Deref;
 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>;
@@ -79,24 +81,41 @@ pub fn create_session(
     };
 
     let (codegen_backend, target_cfg) = match make_codegen_backend {
-        Some(make_codegen_backend) => {
-            let backend = make_codegen_backend(&sopts);
+        None => {
+            // Build a target without override, so that it can override the backend if needed
+            let target = config::build_target_config(handler, &sopts, None, &sysroot);
+
+            let backend = get_codegen_backend(
+                handler,
+                &sysroot,
+                sopts.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(&sopts);
 
+            // Assert that we don't use target's override of the backend and
+            // backend's override of the target at the same time
+            if sopts.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(handler, &sopts, target_override, &sysroot);
 
             (backend, target)
         }
-        None => {
-            let _target = config::build_target_config(handler, &sopts, None, &sysroot);
+        Some(make_codegen_backend) => {
+            // N.B. `make_codegen_backend` takes precedence over `target.default_codegen_backend`,
+            //      which is ignored in this case.
 
-            let backend = get_codegen_backend(
-                handler,
-                &sopts.maybe_sysroot,
-                sopts.unstable_opts.codegen_backend.as_deref(),
-            );
+            let backend = make_codegen_backend(&sopts);
 
             // target_override is documented to be called before init(), so this is okay
             let target_override = backend.target_override(&sopts);
@@ -295,21 +314,25 @@ fn load_backend_from_dylib(handler: &EarlyErrorHandler, path: &Path) -> MakeBack
 /// A name of `None` indicates that the default backend should be used.
 pub fn get_codegen_backend(
     handler: &EarlyErrorHandler,
-    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(handler, filename.as_ref())
             }
             #[cfg(feature = "llvm")]
             "llvm" => rustc_codegen_llvm::LlvmCodegenBackend::new,
-            backend_name => get_codegen_sysroot(handler, maybe_sysroot, backend_name),
+            backend_name => get_codegen_sysroot(handler, sysroot, backend_name),
         }
     });
 
@@ -343,7 +366,7 @@ fn get_rustc_path_inner(bin_path: &str) -> Option<PathBuf> {
 
 fn get_codegen_sysroot(
     handler: &EarlyErrorHandler,
-    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
@@ -360,9 +383,8 @@ 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(<_>::deref))
         .map(|sysroot| {
             filesearch::make_target_lib_path(sysroot, target).with_file_name("codegen-backends")
         })

compiler/rustc_target/src/spec/mod.rs

@@ -1966,6 +1966,14 @@ 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,
@@ -2272,6 +2280,7 @@ 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,