autodiff: typetree recursive depth query from enzyme with fallback

Signed-off-by: Karan Janthe <[email protected]>

Author: KMJ-007

compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs

@@ -127,7 +127,6 @@ pub(crate) mod Enzyme_AD {
         );
         pub(crate) fn EnzymeTypeTreeToString(arg1: CTypeTreeRef) -> *const c_char;
         pub(crate) fn EnzymeTypeTreeToStringFree(arg1: *const c_char);
-        pub(crate) fn EnzymeGetMaxTypeDepth() -> ::std::os::raw::c_uint;
     }
 
     unsafe extern "C" {

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1847,3 +1847,15 @@ extern "C" void LLVMRustSetNoSanitizeHWAddress(LLVMValueRef Global) {
   MD.NoHWAddress = true;
   GV.setSanitizerMetadata(MD);
 }
+
+#ifdef ENZYME
+extern "C" {
+extern llvm::cl::opt<unsigned> EnzymeMaxTypeDepth;
+}
+
+extern "C" size_t LLVMRustEnzymeGetMaxTypeDepth() { return EnzymeMaxTypeDepth; }
+#else
+extern "C" size_t LLVMRustEnzymeGetMaxTypeDepth() {
+  return 6; // Default fallback depth
+}
+#endif

compiler/rustc_middle/src/ty/mod.rs

@@ -2256,26 +2256,19 @@ pub fn typetree_from_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> TypeTree {
     typetree_from_ty_inner(tcx, ty, 0, &mut visited)
 }
 
+/// Maximum recursion depth for TypeTree generation to prevent stack overflow
+/// from pathological deeply nested types. Combined with cycle detection.
+const MAX_TYPETREE_DEPTH: usize = 6;
+
 /// Internal recursive function for TypeTree generation with cycle detection and depth limiting.
 fn typetree_from_ty_inner<'tcx>(
     tcx: TyCtxt<'tcx>,
     ty: Ty<'tcx>,
     depth: usize,
     visited: &mut Vec<Ty<'tcx>>,
 ) -> TypeTree {
-    #[cfg(llvm_enzyme)]
-    {
-        unsafe extern "C" {
-            fn EnzymeGetMaxTypeDepth() -> ::std::os::raw::c_uint;
-        }
-        let max_depth = unsafe { EnzymeGetMaxTypeDepth() } as usize;
-        if depth > max_depth {
-            return TypeTree::new();
-        }
-    }
-
-    #[cfg(not(llvm_enzyme))]
-    if depth > 6 {
+    if depth >= MAX_TYPETREE_DEPTH {
+        trace!("typetree depth limit {} reached for type: {}", MAX_TYPETREE_DEPTH, ty);
         return TypeTree::new();
     }