move second opt run to lto phase and cleanup code

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -606,10 +606,31 @@ pub(crate) fn run_pass_manager(
 
     // If this rustc version was build with enzyme/autodiff enabled, and if users applied the
     // `#[autodiff]` macro at least once, then we will later call llvm_optimize a second time.
-    let first_run = true;
     debug!("running llvm pm opt pipeline");
     unsafe {
-        write::llvm_optimize(cgcx, dcx, module, config, opt_level, opt_stage, first_run)?;
+        write::llvm_optimize(
+            cgcx,
+            dcx,
+            module,
+            config,
+            opt_level,
+            opt_stage,
+            write::AutodiffStage::DuringAD,
+        )?;
+    }
+    // FIXME(ZuseZ4): Make this more granular
+    if cfg!(llvm_enzyme) && !thin {
+        unsafe {
+            write::llvm_optimize(
+                cgcx,
+                dcx,
+                module,
+                config,
+                opt_level,
+                llvm::OptStage::FatLTO,
+                write::AutodiffStage::PostAD,
+            )?;
+        }
     }
     debug!("lto done");
     Ok(())

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -530,6 +530,16 @@ fn get_instr_profile_output_path(config: &ModuleConfig) -> Option<CString> {
     config.instrument_coverage.then(|| c"default_%m_%p.profraw".to_owned())
 }
 
+// PreAD will run llvm opts but disable size increasing opts (vectorization, loop unrolling)
+// DuringAD is the same as above, but also runs the enzyme opt and autodiff passes.
+// PostAD will run all opts, including size increasing opts.
+#[derive(Debug, Eq, PartialEq)]
+pub(crate) enum AutodiffStage {
+    PreAD,
+    DuringAD,
+    PostAD,
+}
+
 pub(crate) unsafe fn llvm_optimize(
     cgcx: &CodegenContext<LlvmCodegenBackend>,
     dcx: DiagCtxtHandle<'_>,
@@ -537,7 +547,7 @@ pub(crate) unsafe fn llvm_optimize(
     config: &ModuleConfig,
     opt_level: config::OptLevel,
     opt_stage: llvm::OptStage,
-    skip_size_increasing_opts: bool,
+    autodiff_stage: AutodiffStage,
 ) -> Result<(), FatalError> {
     // Enzyme:
     // The whole point of compiler based AD is to differentiate optimized IR instead of unoptimized
@@ -550,13 +560,16 @@ pub(crate) unsafe fn llvm_optimize(
     let unroll_loops;
     let vectorize_slp;
     let vectorize_loop;
+    let run_enzyme = cfg!(llvm_enzyme) && autodiff_stage == AutodiffStage::DuringAD;
 
-    let run_enzyme = cfg!(llvm_enzyme);
     // When we build rustc with enzyme/autodiff support, we want to postpone size-increasing
-    // optimizations until after differentiation. FIXME(ZuseZ4): Before shipping on nightly,
+    // 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 skip_size_increasing_opts && run_enzyme {
+    if cfg!(llvm_enzyme) && autodiff_stage != AutodiffStage::PostAD {
         unroll_loops = false;
         vectorize_slp = false;
         vectorize_loop = false;
@@ -566,7 +579,7 @@ pub(crate) unsafe fn llvm_optimize(
         vectorize_slp = config.vectorize_slp;
         vectorize_loop = config.vectorize_loop;
     }
-    trace!(?unroll_loops, ?vectorize_slp, ?vectorize_loop);
+    trace!(?unroll_loops, ?vectorize_slp, ?vectorize_loop, ?run_enzyme);
     let using_thin_buffers = opt_stage == llvm::OptStage::PreLinkThinLTO || config.bitcode_needed();
     let pgo_gen_path = get_pgo_gen_path(config);
     let pgo_use_path = get_pgo_use_path(config);
@@ -686,18 +699,14 @@ pub(crate) unsafe fn optimize(
             _ => llvm::OptStage::PreLinkNoLTO,
         };
 
-        // If we know that we will later run AD, then we disable vectorization and loop unrolling
+        // If we know that we will later run AD, then we disable vectorization and loop unrolling.
-        let skip_size_increasing_opts = cfg!(llvm_enzyme);
+        // 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 };
         return unsafe {
-            llvm_optimize(
+            llvm_optimize(cgcx, dcx, module, config, opt_level, opt_stage, autodiff_stage)
-                cgcx,
-                dcx,
-                module,
-                config,
-                opt_level,
-                opt_stage,
-                skip_size_increasing_opts,
-            )
         };
     }
     Ok(())

compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -4,10 +4,9 @@ use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, AutoDiffItem, DiffActivit
 use rustc_codegen_ssa::ModuleCodegen;
 use rustc_codegen_ssa::back::write::ModuleConfig;
 use rustc_errors::FatalError;
-use rustc_session::config::Lto;
 use tracing::{debug, trace};
 
-use crate::back::write::{llvm_err, llvm_optimize};
+use crate::back::write::llvm_err;
 use crate::builder::SBuilder;
 use crate::context::SimpleCx;
 use crate::declare::declare_simple_fn;
@@ -153,7 +152,7 @@ fn generate_enzyme_call<'ll>(
             _ => {}
         }
 
-        trace!("matching autodiff arguments");
+        debug!("matching autodiff arguments");
         // We now handle the issue that Rust level arguments not always match the llvm-ir level
         // arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
         // llvm-ir level. The number of activities matches the number of Rust level arguments, so we
@@ -222,7 +221,10 @@ fn generate_enzyme_call<'ll>(
                     // A duplicated pointer will have the following two outer_fn arguments:
                     // (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
                     // (..., metadata! enzyme_dup, ptr, ptr, ...).
-                    if matches!(diff_activity, DiffActivity::Duplicated | DiffActivity::DuplicatedOnly) {
+                    if matches!(
+                        diff_activity,
+                        DiffActivity::Duplicated | DiffActivity::DuplicatedOnly
+                    ) {
                         assert!(
                             llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Pointer
                         );
@@ -282,7 +284,7 @@ pub(crate) fn differentiate<'ll>(
     module: &'ll ModuleCodegen<ModuleLlvm>,
     cgcx: &CodegenContext<LlvmCodegenBackend>,
     diff_items: Vec<AutoDiffItem>,
-    config: &ModuleConfig,
+    _config: &ModuleConfig,
 ) -> Result<(), FatalError> {
     for item in &diff_items {
         trace!("{}", item);
@@ -317,29 +319,6 @@ pub(crate) fn differentiate<'ll>(
 
     // FIXME(ZuseZ4): support SanitizeHWAddress and prevent illegal/unsupported opts
 
-    if let Some(opt_level) = config.opt_level {
-        let opt_stage = match cgcx.lto {
-            Lto::Fat => llvm::OptStage::PreLinkFatLTO,
-            Lto::Thin | Lto::ThinLocal => llvm::OptStage::PreLinkThinLTO,
-            _ if cgcx.opts.cg.linker_plugin_lto.enabled() => llvm::OptStage::PreLinkThinLTO,
-            _ => llvm::OptStage::PreLinkNoLTO,
-        };
-        // This is our second opt call, so now we run all opts,
-        // to make sure we get the best performance.
-        let skip_size_increasing_opts = false;
-        trace!("running Module Optimization after differentiation");
-        unsafe {
-            llvm_optimize(
-                cgcx,
-                diag_handler.handle(),
-                module,
-                config,
-                opt_level,
-                opt_stage,
-                skip_size_increasing_opts,
-            )?
-        };
-    }
     trace!("done with differentiate()");
 
     Ok(())

compiler/rustc_llvm/build.rs

@@ -193,6 +193,10 @@ fn main() {
         cfg.define(&flag, None);
     }
 
+    if tracked_env_var_os("LLVM_ENZYME").is_some() {
+        cfg.define("ENZYME", None);
+    }
+
     if tracked_env_var_os("LLVM_RUSTLLVM").is_some() {
         cfg.define("LLVM_RUSTLLVM", None);
     }

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -689,7 +689,9 @@ struct LLVMRustSanitizerOptions {
 };
 
 // This symbol won't be available or used when Enzyme is not enabled
-extern "C" void registerEnzyme(llvm::PassBuilder &PB) __attribute__((weak));
+#ifdef ENZYME
+extern "C" void registerEnzyme(llvm::PassBuilder &PB);
+#endif
 
 extern "C" LLVMRustResult LLVMRustOptimize(
     LLVMModuleRef ModuleRef, LLVMTargetMachineRef TMRef,
@@ -697,8 +699,9 @@ extern "C" LLVMRustResult LLVMRustOptimize(
     bool IsLinkerPluginLTO, bool NoPrepopulatePasses, bool VerifyIR,
     bool LintIR, bool UseThinLTOBuffers, bool MergeFunctions, bool UnrollLoops,
     bool SLPVectorize, bool LoopVectorize, bool DisableSimplifyLibCalls,
-    bool EmitLifetimeMarkers, bool RunEnzyme, LLVMRustSanitizerOptions *SanitizerOptions,
+    bool EmitLifetimeMarkers, bool RunEnzyme,
-    const char *PGOGenPath, const char *PGOUsePath, bool InstrumentCoverage,
+    LLVMRustSanitizerOptions *SanitizerOptions, const char *PGOGenPath,
+    const char *PGOUsePath, bool InstrumentCoverage,
     const char *InstrProfileOutput, const char *PGOSampleUsePath,
     bool DebugInfoForProfiling, void *LlvmSelfProfiler,
     LLVMRustSelfProfileBeforePassCallback BeforePassCallback,
@@ -1014,6 +1017,7 @@ extern "C" LLVMRustResult LLVMRustOptimize(
   }
 
   // now load "-enzyme" pass:
+#ifdef ENZYME
   if (RunEnzyme) {
     registerEnzyme(PB);
     if (auto Err = PB.parsePassPipeline(MPM, "enzyme")) {
@@ -1022,6 +1026,7 @@ extern "C" LLVMRustResult LLVMRustOptimize(
       return LLVMRustResult::Failure;
     }
   }
+#endif
 
   // Upgrade all calls to old intrinsics first.
   for (Module::iterator I = TheModule->begin(), E = TheModule->end(); I != E;)

src/bootstrap/src/core/build_steps/compile.rs

@@ -1049,9 +1049,9 @@ pub fn rustc_cargo(
     // <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Internal.20lint.20for.20raw.20.60print!.60.20and.20.60println!.60.3F>.
     cargo.rustflag("-Zon-broken-pipe=kill");
 
-    // We temporarily disable linking here as part of some refactoring.
+    // We want to link against registerEnzyme and in the future we want to use additional
-    // This way, people can manually use -Z llvm-plugins and -C passes=enzyme for now.
+    // functionality from Enzyme core. For that we need to link against Enzyme.
-    // In a follow-up PR, we will re-enable linking here and load the pass for them.
+    // FIXME(ZuseZ4): Get the LLVM version number automatically instead of hardcoding it.
     if builder.config.llvm_enzyme {
         cargo.rustflag("-l").rustflag("Enzyme-19");
     }
@@ -1234,6 +1234,9 @@ fn rustc_llvm_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetSelect
     if builder.is_rust_llvm(target) {
         cargo.env("LLVM_RUSTLLVM", "1");
     }
+    if builder.config.llvm_enzyme {
+        cargo.env("LLVM_ENZYME", "1");
+    }
     let llvm::LlvmResult { llvm_config, .. } = builder.ensure(llvm::Llvm { target });
     cargo.env("LLVM_CONFIG", &llvm_config);
 

tests/codegen/autodiff.rs

@@ -15,9 +15,9 @@ fn square(x: &f64) -> f64 {
 // CHECK-NEXT:invertstart:
 // CHECK-NEXT:  %_0 = fmul double %x.0.val, %x.0.val
 // CHECK-NEXT:  %0 = fadd fast double %x.0.val, %x.0.val
-// CHECK-NEXT:  %1 = load double, ptr %"x'", align 8, !alias.scope !17816, !noalias !17819
+// CHECK-NEXT:  %1 = load double, ptr %"x'", align 8
 // CHECK-NEXT:  %2 = fadd fast double %1, %0
-// CHECK-NEXT:  store double %2, ptr %"x'", align 8, !alias.scope !17816, !noalias !17819
+// CHECK-NEXT:  store double %2, ptr %"x'", align 8
 // CHECK-NEXT:  ret double %_0
 // CHECK-NEXT:}