5 files changed, 23 insertions, 15 deletions
diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index 11be7041167..61fadad6066 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -246,6 +246,7 @@ impl<'ll, 'tcx> ArgAbiExt<'ll, 'tcx> for ArgAbi<'tcx, Ty<'tcx>> {
                     scratch_align,
                     bx.const_usize(copy_bytes),
                     MemFlags::empty(),
+                    None,
                 );
                 bx.lifetime_end(llscratch, scratch_size);
             }
diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 0f17cc9063a..83a9cf620f1 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -2,6 +2,7 @@ use std::borrow::{Borrow, Cow};
 use std::ops::Deref;
 use std::{iter, ptr};
 
+use rustc_ast::expand::typetree::FncTree;
 pub(crate) mod autodiff;
 pub(crate) mod gpu_offload;
 
@@ -1107,11 +1108,12 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         src_align: Align,
         size: &'ll Value,
         flags: MemFlags,
+        tt: Option<FncTree>,
     ) {
         assert!(!flags.contains(MemFlags::NONTEMPORAL), "non-temporal memcpy not supported");
         let size = self.intcast(size, self.type_isize(), false);
         let is_volatile = flags.contains(MemFlags::VOLATILE);
-        unsafe {
+        let memcpy = unsafe {
             llvm::LLVMRustBuildMemCpy(
                 self.llbuilder,
                 dst,
@@ -1120,7 +1122,16 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
                 src_align.bytes() as c_uint,
                 size,
                 is_volatile,
-            );
+            )
+        };
+
+        // TypeTree metadata for memcpy is especially important: when Enzyme encounters
+        // a memcpy during autodiff, it needs to know the structure of the data being
+        // copied to properly track derivatives. For example, copying an array of floats
+        // vs. copying a struct with mixed types requires different derivative handling.
+        // The TypeTree tells Enzyme exactly what memory layout to expect.
+        if let Some(tt) = tt {
+            crate::typetree::add_tt(self.cx().llmod, self.cx().llcx, memcpy, tt);
         }
     }
 
diff --git a/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
index 12b6ffa3c0b..9dd84ad9a4d 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
@@ -25,6 +25,7 @@ pub(crate) enum CConcreteType {
     DT_Half = 3,
     DT_Float = 4,
     DT_Double = 5,
+    // FIXME(KMJ-007): handle f128 using long double here(https://github.com/EnzymeAD/Enzyme/issues/1600)
     DT_Unknown = 6,
 }
 
diff --git a/compiler/rustc_codegen_llvm/src/typetree.rs b/compiler/rustc_codegen_llvm/src/typetree.rs
index 434316464e6..8e6272a186b 100644
--- a/compiler/rustc_codegen_llvm/src/typetree.rs
+++ b/compiler/rustc_codegen_llvm/src/typetree.rs
@@ -1,8 +1,11 @@
-use std::ffi::{CString, c_char, c_uint};
-
-use rustc_ast::expand::typetree::{FncTree, TypeTree as RustTypeTree};
+use rustc_ast::expand::typetree::FncTree;
+#[cfg(llvm_enzyme)]
+use {
+    crate::attributes,
+    rustc_ast::expand::typetree::TypeTree as RustTypeTree,
+    std::ffi::{CString, c_char, c_uint},
+};
 
-use crate::attributes;
 use crate::llvm::{self, Value};
 
 /// Converts a Rust TypeTree to Enzyme's internal TypeTree format
@@ -50,15 +53,6 @@ fn to_enzyme_typetree(
     enzyme_tt
 }
 
-#[cfg(not(llvm_enzyme))]
-fn to_enzyme_typetree(
-    _rust_typetree: RustTypeTree,
-    _data_layout: &str,
-    _llcx: &llvm::Context,
-) -> ! {
-    unimplemented!("TypeTree conversion not available without llvm_enzyme support")
-}
-
 // Attaches TypeTree information to LLVM function as enzyme_type attributes.
 #[cfg(llvm_enzyme)]
 pub(crate) fn add_tt<'ll>(
diff --git a/compiler/rustc_codegen_llvm/src/va_arg.rs b/compiler/rustc_codegen_llvm/src/va_arg.rs
index ab08125217f..d48c7cf874a 100644
--- a/compiler/rustc_codegen_llvm/src/va_arg.rs
+++ b/compiler/rustc_codegen_llvm/src/va_arg.rs
@@ -738,6 +738,7 @@ fn copy_to_temporary_if_more_aligned<'ll, 'tcx>(
             src_align,
             bx.const_u32(layout.layout.size().bytes() as u32),
             MemFlags::empty(),
+            None,
         );
         tmp
     } else {