clean up autodiff code/comments

Author: ZuseZ4

compiler/rustc_ast/src/expand/autodiff_attrs.rs

@@ -17,7 +17,6 @@ use crate::{Ty, TyKind};
 /// functions. The proper solution is to recognize and resolve this DAG of autodiff invocations,
 /// as it's already done in the C++ and Julia frontend of Enzyme.
 ///
-/// (FIXME) remove *First variants.
 /// Documentation for using [reverse](https://enzyme.mit.edu/rust/rev.html) and
 /// [forward](https://enzyme.mit.edu/rust/fwd.html) mode is available online.
 #[derive(Clone, Copy, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -564,19 +564,16 @@ pub(crate) unsafe fn llvm_optimize(
     // FIXME(ZuseZ4): In a future update we could figure out how to only optimize individual functions getting
     // differentiated.
 
+    let consider_ad = cfg!(llvm_enzyme) && config.autodiff.contains(&config::AutoDiff::Enable);
+    let run_enzyme = autodiff_stage == AutodiffStage::DuringAD;
     let unroll_loops;
     let vectorize_slp;
     let vectorize_loop;
-    let run_enzyme = cfg!(llvm_enzyme) && autodiff_stage == AutodiffStage::DuringAD;
 
     // When we build rustc with enzyme/autodiff support, we want to postpone size-increasing
     // optimizations until after differentiation. Our pipeline is thus: (opt + enzyme), (full opt).
     // We therefore have two calls to llvm_optimize, if autodiff is used.
-    //
-    // FIXME(ZuseZ4): Before shipping on nightly,
-    // we should make this more granular, or at least check that the user has at least one autodiff
-    // call in their code, to justify altering the compilation pipeline.
-    if cfg!(llvm_enzyme) && autodiff_stage != AutodiffStage::PostAD {
+    if consider_ad && autodiff_stage != AutodiffStage::PostAD {
         unroll_loops = false;
         vectorize_slp = false;
         vectorize_loop = false;
@@ -706,10 +703,8 @@ pub(crate) unsafe fn optimize(
 
         // If we know that we will later run AD, then we disable vectorization and loop unrolling.
         // Otherwise we pretend AD is already done and run the normal opt pipeline (=PostAD).
-        // FIXME(ZuseZ4): Make this more granular, only set PreAD if we actually have autodiff
-        // usages, not just if we build rustc with autodiff support.
-        let autodiff_stage =
-            if cfg!(llvm_enzyme) { AutodiffStage::PreAD } else { AutodiffStage::PostAD };
+        let consider_ad = cfg!(llvm_enzyme) && config.autodiff.contains(&config::AutoDiff::Enable);
+        let autodiff_stage = if consider_ad { AutodiffStage::PreAD } else { AutodiffStage::PostAD };
         return unsafe {
             llvm_optimize(cgcx, dcx, module, config, opt_level, opt_stage, autodiff_stage)
         };

compiler/rustc_monomorphize/src/partitioning/autodiff.rs

@@ -66,7 +66,7 @@ pub(crate) fn find_autodiff_source_functions<'tcx>(
     let mut autodiff_items: Vec<AutoDiffItem> = vec![];
     for (item, instance) in autodiff_mono_items {
         let target_id = instance.def_id();
-        let cg_fn_attr = tcx.codegen_fn_attrs(target_id).autodiff_item.clone();
+        let cg_fn_attr = &tcx.codegen_fn_attrs(target_id).autodiff_item;
         let Some(target_attrs) = cg_fn_attr else {
             continue;
         };