Merge run_fat_lto, optimize_fat and autodiff into run_and_optimize_fa…

…t_lto
rust-lang · bors · Jul 17, 2025 · Jul 3, 2025 · Jul 3, 2025 · Jul 3, 2025
commit 21026cae8d34241b65924198b72c7231ce4f5a3d
diff --git a/compiler/rustc_codegen_gcc/src/lib.rs b/compiler/rustc_codegen_gcc/src/lib.rs
@@ -357,11 +357,16 @@ impl WriteBackendMethods for GccCodegenBackend {
     type ThinData = ThinData;
     type ThinBuffer = ThinBuffer;
 
-    fn run_fat_lto(
+    fn run_and_optimize_fat_lto(
         cgcx: &CodegenContext<Self>,
         modules: Vec<FatLtoInput<Self>>,
         cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
+        diff_fncs: Vec<AutoDiffItem>,
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
+        if !diff_fncs.is_empty() {
+            unimplemented!();
+        }
+
         back::lto::run_fat(cgcx, modules, cached_modules)
     }
 
@@ -391,14 +396,6 @@ impl WriteBackendMethods for GccCodegenBackend {
         Ok(())
     }
 
-    fn optimize_fat(
-        _cgcx: &CodegenContext<Self>,
-        _module: &mut ModuleCodegen<Self::Module>,
-    ) -> Result<(), FatalError> {
-        // TODO(antoyo)
-        Ok(())
-    }
-
     fn optimize_thin(
         cgcx: &CodegenContext<Self>,
         thin: ThinModule<Self>,
@@ -432,14 +429,6 @@ impl WriteBackendMethods for GccCodegenBackend {
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
         back::write::link(cgcx, dcx, modules)
     }
-
-    fn autodiff(
-        _cgcx: &CodegenContext<Self>,
-        _module: &ModuleCodegen<Self::Module>,
-        _diff_functions: Vec<AutoDiffItem>,
-    ) -> Result<(), FatalError> {
-        unimplemented!()
-    }
 }
 
 /// This is the entrypoint for a hot plugged rustc_codegen_gccjit

diff --git a/compiler/rustc_codegen_llvm/messages.ftl b/compiler/rustc_codegen_llvm/messages.ftl
@@ -1,5 +1,4 @@
 codegen_llvm_autodiff_without_enable = using the autodiff feature requires -Z autodiff=Enable
-codegen_llvm_autodiff_without_lto = using the autodiff feature requires using fat-lto
 
 codegen_llvm_copy_bitcode = failed to copy bitcode to object file: {$err}
 

diff --git a/compiler/rustc_codegen_llvm/src/errors.rs b/compiler/rustc_codegen_llvm/src/errors.rs
@@ -37,10 +37,6 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for ParseTargetMachineConfig<'_> {
     }
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_llvm_autodiff_without_lto)]
-pub(crate) struct AutoDiffWithoutLTO;
-
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_autodiff_without_enable)]
 pub(crate) struct AutoDiffWithoutEnable;

diff --git a/compiler/rustc_codegen_llvm/src/lib.rs b/compiler/rustc_codegen_llvm/src/lib.rs
@@ -26,7 +26,7 @@ use std::mem::ManuallyDrop;
 use back::owned_target_machine::OwnedTargetMachine;
 use back::write::{create_informational_target_machine, create_target_machine};
 use context::SimpleCx;
-use errors::{AutoDiffWithoutLTO, ParseTargetMachineConfig};
+use errors::ParseTargetMachineConfig;
 use llvm_util::target_config;
 use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_ast::expand::autodiff_attrs::AutoDiffItem;
@@ -43,7 +43,7 @@ use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_middle::ty::TyCtxt;
 use rustc_middle::util::Providers;
 use rustc_session::Session;
-use rustc_session::config::{Lto, OptLevel, OutputFilenames, PrintKind, PrintRequest};
+use rustc_session::config::{OptLevel, OutputFilenames, PrintKind, PrintRequest};
 use rustc_span::Symbol;
 
 mod back {
@@ -174,12 +174,23 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
         back::write::link(cgcx, dcx, modules)
     }
-    fn run_fat_lto(
+    fn run_and_optimize_fat_lto(
         cgcx: &CodegenContext<Self>,
         modules: Vec<FatLtoInput<Self>>,
         cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
+        diff_fncs: Vec<AutoDiffItem>,
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
-        back::lto::run_fat(cgcx, modules, cached_modules)
+        let mut module = back::lto::run_fat(cgcx, modules, cached_modules)?;
+
+        if !diff_fncs.is_empty() {
+            builder::autodiff::differentiate(&module, cgcx, diff_fncs)?;
+        }
+
+        let dcx = cgcx.create_dcx();
+        let dcx = dcx.handle();
+        back::lto::run_pass_manager(cgcx, dcx, &mut module, false)?;
+
+        Ok(module)
     }
     fn run_thin_lto(
         cgcx: &CodegenContext<Self>,
@@ -196,14 +207,6 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     ) -> Result<(), FatalError> {
         back::write::optimize(cgcx, dcx, module, config)
     }
-    fn optimize_fat(
-        cgcx: &CodegenContext<Self>,
-        module: &mut ModuleCodegen<Self::Module>,
-    ) -> Result<(), FatalError> {
-        let dcx = cgcx.create_dcx();
-        let dcx = dcx.handle();
-        back::lto::run_pass_manager(cgcx, dcx, module, false)
-    }
     fn optimize_thin(
         cgcx: &CodegenContext<Self>,
         thin: ThinModule<Self>,
@@ -226,18 +229,6 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     fn serialize_module(module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer) {
         (module.name, back::lto::ModuleBuffer::new(module.module_llvm.llmod()))
     }
-    /// Generate autodiff rules
-    fn autodiff(
-        cgcx: &CodegenContext<Self>,
-        module: &ModuleCodegen<Self::Module>,
-        diff_fncs: Vec<AutoDiffItem>,
-    ) -> Result<(), FatalError> {
-        if cgcx.lto != Lto::Fat {
-            let dcx = cgcx.create_dcx();
-            return Err(dcx.handle().emit_almost_fatal(AutoDiffWithoutLTO));
-        }
-        builder::autodiff::differentiate(module, cgcx, diff_fncs)
-    }
 }
 
 impl LlvmCodegenBackend {

diff --git a/compiler/rustc_codegen_ssa/src/back/write.rs b/compiler/rustc_codegen_ssa/src/back/write.rs
@@ -995,17 +995,7 @@ fn execute_fat_lto_work_item<B: ExtraBackendMethods>(
     autodiff: Vec<AutoDiffItem>,
     module_config: &ModuleConfig,
 ) -> Result<WorkItemResult<B>, FatalError> {
-    let mut module =
-        B::run_fat_lto(cgcx, needs_fat_lto, import_only_modules).unwrap_or_else(|e| e.raise());
-
-    if !autodiff.is_empty() {
-        if let Err(err) = B::autodiff(cgcx, &module, autodiff) {
-            err.raise();
-        }
-    }
-
-    B::optimize_fat(cgcx, &mut module)?;
-
+    let module = B::run_and_optimize_fat_lto(cgcx, needs_fat_lto, import_only_modules, autodiff)?;
     let module = B::codegen(cgcx, module, module_config)?;
     Ok(WorkItemResult::Finished(module))
 }

diff --git a/compiler/rustc_codegen_ssa/src/traits/write.rs b/compiler/rustc_codegen_ssa/src/traits/write.rs
@@ -20,12 +20,13 @@ pub trait WriteBackendMethods: Clone + 'static {
         dcx: DiagCtxtHandle<'_>,
         modules: Vec<ModuleCodegen<Self::Module>>,
     ) -> Result<ModuleCodegen<Self::Module>, FatalError>;
-    /// Performs fat LTO by merging all modules into a single one and returning it
-    /// for further optimization.
-    fn run_fat_lto(
+    /// Performs fat LTO by merging all modules into a single one, running autodiff
+    /// if necessary and running any further optimizations
+    fn run_and_optimize_fat_lto(
         cgcx: &CodegenContext<Self>,
         modules: Vec<FatLtoInput<Self>>,
         cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
+        diff_fncs: Vec<AutoDiffItem>,
     ) -> Result<ModuleCodegen<Self::Module>, FatalError>;
     /// Performs thin LTO by performing necessary global analysis and returning two
     /// lists, one of the modules that need optimization and another for modules that
@@ -43,10 +44,6 @@ pub trait WriteBackendMethods: Clone + 'static {
         module: &mut ModuleCodegen<Self::Module>,
         config: &ModuleConfig,
     ) -> Result<(), FatalError>;
-    fn optimize_fat(
-        cgcx: &CodegenContext<Self>,
-        llmod: &mut ModuleCodegen<Self::Module>,
-    ) -> Result<(), FatalError>;
     fn optimize_thin(
         cgcx: &CodegenContext<Self>,
         thin: ThinModule<Self>,
@@ -61,11 +58,6 @@ pub trait WriteBackendMethods: Clone + 'static {
         want_summary: bool,
     ) -> (String, Self::ThinBuffer);
     fn serialize_module(module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer);
-    fn autodiff(
-        cgcx: &CodegenContext<Self>,
-        module: &ModuleCodegen<Self::Module>,
-        diff_fncs: Vec<AutoDiffItem>,
-    ) -> Result<(), FatalError>;
 }
 
 pub trait ThinBufferMethods: Send + Sync {