Target option to require explicit cpu

Some targets have many different CPUs and no generic CPU that can be
used as a default. For these targets, the user needs to explicitly
specify a CPU through `-C target-cpu=`.

Add an option for targets and an error message if no CPU is set.

This affects the proposed amdgpu and avr targets.

compiler/rustc_codegen_ssa/messages.ftl

@@ -30,6 +30,8 @@ codegen_ssa_copy_path = could not copy {$from} to {$to}: {$error}
 
 codegen_ssa_copy_path_buf = unable to copy {$source_file} to {$output_path}: {$error}
 
+codegen_ssa_cpu_required = target requires explicitly specifying a cpu with `-C target-cpu`
+
 codegen_ssa_create_temp_dir = couldn't create a temp dir: {$error}
 
 codegen_ssa_dlltool_fail_import_library =

compiler/rustc_codegen_ssa/src/base.rs

@@ -616,6 +616,11 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
         return ongoing_codegen;
     }
 
+    if tcx.sess.target.need_explicit_cpu && tcx.sess.opts.cg.target_cpu.is_none() {
+        // The target has no default cpu, but none is set explicitly
+        tcx.dcx().emit_fatal(errors::CpuRequired);
+    }
+
     let cgu_name_builder = &mut CodegenUnitNameBuilder::new(tcx);
 
     // Run the monomorphization collector and partition the collected items into

compiler/rustc_codegen_ssa/src/errors.rs

@@ -491,6 +491,10 @@ pub(crate) struct CheckInstalledVisualStudio;
 #[diag(codegen_ssa_insufficient_vs_code_product)]
 pub(crate) struct InsufficientVSCodeProduct;
 
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_cpu_required)]
+pub(crate) struct CpuRequired;
+
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_processing_dymutil_failed)]
 #[note]

compiler/rustc_target/src/spec/json.rs

@@ -546,6 +546,7 @@ impl Target {
         key!(link_env_remove, list);
         key!(asm_args, list);
         key!(cpu);
+        key!(need_explicit_cpu, bool);
         key!(features);
         key!(dynamic_linking, bool);
         key!(direct_access_external_data, Option<bool>);
@@ -720,6 +721,7 @@ impl ToJson for Target {
         target_option_val!(link_env_remove);
         target_option_val!(asm_args);
         target_option_val!(cpu);
+        target_option_val!(need_explicit_cpu);
         target_option_val!(features);
         target_option_val!(dynamic_linking);
         target_option_val!(direct_access_external_data);

compiler/rustc_target/src/spec/mod.rs

@@ -2240,6 +2240,9 @@ pub struct TargetOptions {
     /// Default CPU to pass to LLVM. Corresponds to `llc -mcpu=$cpu`. Defaults
     /// to "generic".
     pub cpu: StaticCow<str>,
+    /// Whether a cpu needs to be explicitly set.
+    /// Set to true if there is no default cpu. Defaults to false.
+    pub need_explicit_cpu: bool,
     /// Default target features to pass to LLVM. These features overwrite
     /// `-Ctarget-cpu` but can be overwritten with `-Ctarget-features`.
     /// Corresponds to `llc -mattr=$features`.
@@ -2676,6 +2679,7 @@ impl Default for TargetOptions {
             link_script: None,
             asm_args: cvs![],
             cpu: "generic".into(),
+            need_explicit_cpu: false,
             features: "".into(),
             direct_access_external_data: None,
             dynamic_linking: false,

tests/run-make/target-specs/require-explicit-cpu.json

@@ -0,0 +1,11 @@
+{
+    "data-layout": "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",
+    "linker-flavor": "gcc",
+    "llvm-target": "i686-unknown-linux-gnu",
+    "target-endian": "little",
+    "target-pointer-width": "32",
+    "target-c-int-width": "32",
+    "arch": "x86",
+    "os": "linux",
+    "need-explicit-cpu": true
+}

tests/run-make/target-specs/rmake.rs

@@ -63,4 +63,17 @@ fn main() {
         .crate_type("lib")
         .run_fail()
         .assert_stderr_contains("data-layout for target");
+    rustc()
+        .input("foo.rs")
+        .target("require-explicit-cpu")
+        .crate_type("lib")
+        .run_fail()
+        .assert_stderr_contains("target requires explicitly specifying a cpu");
+    rustc()
+        .input("foo.rs")
+        .target("require-explicit-cpu")
+        .crate_type("lib")
+        .arg("-Ctarget-cpu=generic")
+        .run();
+    rustc().target("require-explicit-cpu").arg("--print=target-cpus").run();
 }