fix non-enzyme builds

Author: ZuseZ4

Diff for: compiler/rustc_codegen_llvm/src/back/write.rs

@@ -551,11 +551,12 @@ pub(crate) unsafe fn llvm_optimize(
     let vectorize_slp;
     let vectorize_loop;
 
+    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,
     // 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 && cfg!(llvm_enzyme) {
+    if skip_size_increasing_opts && run_enzyme {
         unroll_loops = false;
         vectorize_slp = false;
         vectorize_loop = false;
@@ -633,6 +634,7 @@ pub(crate) unsafe fn llvm_optimize(
             vectorize_loop,
             config.no_builtins,
             config.emit_lifetime_markers,
+            run_enzyme,
             sanitizer_options.as_ref(),
             pgo_gen_path.as_ref().map_or(std::ptr::null(), |s| s.as_ptr()),
             pgo_use_path.as_ref().map_or(std::ptr::null(), |s| s.as_ptr()),

Diff for: compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2346,6 +2346,7 @@ unsafe extern "C" {
         LoopVectorize: bool,
         DisableSimplifyLibCalls: bool,
         EmitLifetimeMarkers: bool,
+        RunEnzyme: bool,
         SanitizerOptions: Option<&SanitizerOptions>,
         PGOGenPath: *const c_char,
         PGOUsePath: *const c_char,

Diff for: compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -688,15 +688,16 @@ struct LLVMRustSanitizerOptions {
   bool SanitizeKernelAddressRecover;
 };
 
-extern "C" void registerEnzyme(llvm::PassBuilder &PB);
+// This symbol won't be available or used when Enzyme is not enabled
+extern "C" void registerEnzyme(llvm::PassBuilder &PB) __attribute__((weak));
 
 extern "C" LLVMRustResult LLVMRustOptimize(
     LLVMModuleRef ModuleRef, LLVMTargetMachineRef TMRef,
     LLVMRustPassBuilderOptLevel OptLevelRust, LLVMRustOptStage OptStage,
     bool IsLinkerPluginLTO, bool NoPrepopulatePasses, bool VerifyIR,
     bool LintIR, bool UseThinLTOBuffers, bool MergeFunctions, bool UnrollLoops,
     bool SLPVectorize, bool LoopVectorize, bool DisableSimplifyLibCalls,
-    bool EmitLifetimeMarkers, LLVMRustSanitizerOptions *SanitizerOptions,
+    bool EmitLifetimeMarkers, bool RunEnzyme, LLVMRustSanitizerOptions *SanitizerOptions,
     const char *PGOGenPath, const char *PGOUsePath, bool InstrumentCoverage,
     const char *InstrProfileOutput, const char *PGOSampleUsePath,
     bool DebugInfoForProfiling, void *LlvmSelfProfiler,
@@ -1013,11 +1014,13 @@ extern "C" LLVMRustResult LLVMRustOptimize(
   }
 
   // now load "-enzyme" pass:
-  registerEnzyme(PB);
-  if (auto Err = PB.parsePassPipeline(MPM, "enzyme")) {
-    std::string ErrMsg = toString(std::move(Err));
-    LLVMRustSetLastError(ErrMsg.c_str());
-    return LLVMRustResult::Failure;
+  if (RunEnzyme) {
+    registerEnzyme(PB);
+    if (auto Err = PB.parsePassPipeline(MPM, "enzyme")) {
+      std::string ErrMsg = toString(std::move(Err));
+      LLVMRustSetLastError(ErrMsg.c_str());
+      return LLVMRustResult::Failure;
+    }
   }
 
   // Upgrade all calls to old intrinsics first.