Remove code for ThinLTO from cg_gcc

It was just a dummy implementation to workarround the fact that thin
local lto is the default in rustc. By adding a thin_lto_supported thin
local lto can be automatically disabled for cg_gcc, removing the need
for this dummy implementation. This makes improvements to the LTO
handling on the cg_ssa side a lot easier.

Author: bjorn3

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -196,6 +196,10 @@ impl CodegenBackend for CraneliftCodegenBackend {
         println!("Cranelift version: {}", cranelift_codegen::VERSION);
     }
 
+    fn thin_lto_supported(&self) -> bool {
+        false
+    }
+
     fn codegen_crate(&self, tcx: TyCtxt<'_>) -> Box<dyn Any> {
         info!("codegen crate {}", tcx.crate_name(LOCAL_CRATE));
         let config = self.config.get().unwrap();

compiler/rustc_codegen_gcc/src/back/lto.rs

@@ -76,7 +76,6 @@ use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 
-use back::lto::{ThinBuffer, ThinData};
 use gccjit::{CType, Context, OptimizationLevel};
 #[cfg(feature = "master")]
 use gccjit::{TargetInfo, Version};
@@ -87,7 +86,9 @@ use rustc_codegen_ssa::back::write::{
 };
 use rustc_codegen_ssa::base::codegen_crate;
 use rustc_codegen_ssa::target_features::cfg_target_feature;
-use rustc_codegen_ssa::traits::{CodegenBackend, ExtraBackendMethods, WriteBackendMethods};
+use rustc_codegen_ssa::traits::{
+    CodegenBackend, ExtraBackendMethods, ThinBufferMethods, WriteBackendMethods,
+};
 use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleCodegen, TargetConfig};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::IntoDynSyncSend;
@@ -278,6 +279,10 @@ impl CodegenBackend for GccCodegenBackend {
         }
     }
 
+    fn thin_lto_supported(&self) -> bool {
+        false
+    }
+
     fn provide(&self, providers: &mut Providers) {
         providers.queries.global_backend_features =
             |tcx, ()| gcc_util::global_gcc_features(tcx.sess)
@@ -418,11 +423,19 @@ unsafe impl Send for SyncContext {}
 // FIXME(antoyo): that shouldn't be Sync. Parallel compilation is currently disabled with "CodegenBackend::supports_parallel()".
 unsafe impl Sync for SyncContext {}
 
+pub struct ThinBuffer;
+
+impl ThinBufferMethods for ThinBuffer {
+    fn data(&self) -> &[u8] {
+        &[]
+    }
+}
+
 impl WriteBackendMethods for GccCodegenBackend {
     type Module = GccContext;
     type TargetMachine = ();
     type ModuleBuffer = ModuleBuffer;
-    type ThinData = ThinData;
+    type ThinData = ();
     type ThinBuffer = ThinBuffer;
 
     fn run_and_optimize_fat_lto(
@@ -438,15 +451,15 @@ impl WriteBackendMethods for GccCodegenBackend {
     }
 
     fn run_thin_lto(
-        cgcx: &CodegenContext,
-        dcx: DiagCtxtHandle<'_>,
+        _cgcx: &CodegenContext,
+        _dcx: DiagCtxtHandle<'_>,
         // FIXME(bjorn3): Limit LTO exports to these symbols
         _exported_symbols_for_lto: &[String],
-        each_linked_rlib_for_lto: &[PathBuf],
-        modules: Vec<(String, Self::ThinBuffer)>,
-        cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
+        _each_linked_rlib_for_lto: &[PathBuf],
+        _modules: Vec<(String, Self::ThinBuffer)>,
+        _cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
     ) -> (Vec<ThinModule<Self>>, Vec<WorkProduct>) {
-        back::lto::run_thin(cgcx, dcx, each_linked_rlib_for_lto, modules, cached_modules)
+        unreachable!()
     }
 
     fn print_pass_timings(&self) {
@@ -467,12 +480,12 @@ impl WriteBackendMethods for GccCodegenBackend {
     }
 
     fn optimize_thin(
-        cgcx: &CodegenContext,
+        _cgcx: &CodegenContext,
         _shared_emitter: &SharedEmitter,
         _tm_factory: TargetMachineFactoryFn<Self>,
-        thin: ThinModule<Self>,
+        _thin: ThinModule<Self>,
     ) -> ModuleCodegen<Self::Module> {
-        back::lto::optimize_thin_module(thin, cgcx)
+        unreachable!()
     }
 
     fn codegen(
@@ -484,8 +497,8 @@ impl WriteBackendMethods for GccCodegenBackend {
         back::write::codegen(cgcx, shared_emitter, module, config)
     }
 
-    fn prepare_thin(module: ModuleCodegen<Self::Module>) -> (String, Self::ThinBuffer) {
-        back::lto::prepare_thin(module)
+    fn prepare_thin(_module: ModuleCodegen<Self::Module>) -> (String, Self::ThinBuffer) {
+        unreachable!()
     }
 
     fn serialize_module(_module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer) {

compiler/rustc_codegen_gcc/src/lib.rs

@@ -349,6 +349,10 @@ impl CodegenBackend for LlvmCodegenBackend {
         vec![sym::unchecked_funnel_shl, sym::unchecked_funnel_shr, sym::carrying_mul_add]
     }
 
+    fn thin_lto_supported(&self) -> bool {
+        true
+    }
+
     fn codegen_crate<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Box<dyn Any> {
         Box::new(rustc_codegen_ssa::base::codegen_crate(
             LlvmCodegenBackend(()),

compiler/rustc_codegen_llvm/src/lib.rs

@@ -80,6 +80,9 @@ pub trait CodegenBackend {
         vec![]
     }
 
+    /// Is ThinLTO supported by this backend?
+    fn thin_lto_supported(&self) -> bool;
+
     /// Value printed by `--print=backend-has-zstd`.
     ///
     /// Used by compiletest to determine whether tests involving zstd compression

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -463,6 +463,7 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
 
             codegen_backend.init(&sess);
             sess.replaced_intrinsics = FxHashSet::from_iter(codegen_backend.replaced_intrinsics());
+            sess.thin_lto_supported = codegen_backend.thin_lto_supported();
 
             let cfg = parse_cfg(sess.dcx(), config.crate_cfg);
             let mut cfg = config::build_configuration(&sess, cfg);

compiler/rustc_interface/src/interface.rs

@@ -400,6 +400,10 @@ impl CodegenBackend for DummyCodegenBackend {
         vec![CrateType::Rlib, CrateType::Executable]
     }
 
+    fn thin_lto_supported(&self) -> bool {
+        false
+    }
+
     fn codegen_crate<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Box<dyn Any> {
         Box::new(CodegenResults {
             modules: vec![],

compiler/rustc_interface/src/util.rs

@@ -101,6 +101,7 @@ const ALLOWED_CAPITALIZED_WORDS: &[&str] = &[
     "NaNs",
     "OK",
     "Rust",
+    "ThinLTO",
     "Unicode",
     "VS",
     // tidy-alphabetical-end

compiler/rustc_macros/src/diagnostics/message.rs

@@ -537,3 +537,7 @@ pub(crate) struct UnexpectedBuiltinCfg {
     pub(crate) cfg_name: Symbol,
     pub(crate) controlled_by: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag("ThinLTO is not supported by the codegen backend")]
+pub(crate) struct ThinLtoNotSupportedByBackend;

compiler/rustc_session/src/errors.rs

@@ -158,6 +158,9 @@ pub struct Session {
     /// The names of intrinsics that the current codegen backend replaces
     /// with its own implementations.
     pub replaced_intrinsics: FxHashSet<Symbol>,
+
+    /// Does the codegen backend support ThinLTO?
+    pub thin_lto_supported: bool,
 }
 
 #[derive(Clone, Copy)]
@@ -606,6 +609,9 @@ impl Session {
             }
             config::LtoCli::Thin => {
                 // The user explicitly asked for ThinLTO
+                if !self.thin_lto_supported {
+                    self.dcx().emit_fatal(errors::ThinLtoNotSupportedByBackend);
+                }
                 return config::Lto::Thin;
             }
         }
@@ -626,6 +632,9 @@ impl Session {
         // a deprecated option now that `-C lto=thin` exists.
         if let Some(enabled) = self.opts.unstable_opts.thinlto {
             if enabled {
+                if !self.thin_lto_supported {
+                    self.dcx().emit_fatal(errors::ThinLtoNotSupportedByBackend);
+                }
                 return config::Lto::ThinLocal;
             } else {
                 return config::Lto::No;
@@ -642,7 +651,8 @@ impl Session {
         // optimized compiles (anything greater than O0).
         match self.opts.optimize {
             config::OptLevel::No => config::Lto::No,
-            _ => config::Lto::ThinLocal,
+            _ if self.thin_lto_supported => config::Lto::ThinLocal,
+            _ => config::Lto::No,
         }
     }
 
@@ -1088,6 +1098,7 @@ pub fn build_session(
         host_filesearch,
         invocation_temp,
         replaced_intrinsics: FxHashSet::default(), // filled by `run_compiler`
+        thin_lto_supported: true,                  // filled by `run_compiler`
     };
 
     validate_commandline_args_with_session_available(&sess);