Fix: enable EBO for simple_ilist on MSVC via LLVM_DECLARE_EMPTY_BASES

external/llvh/include/llvh/ADT/ilist.h

@@ -80,9 +80,16 @@ template <typename NodeTy> struct ilist_callback_traits {
 /// node related operations.
 ///
 /// TODO: Remove this layer of indirection.  It's not necessary.
+///
+/// `LLVM_DECLARE_EMPTY_BASES` is required so MSVC collapses the two empty
+/// helper bases via Empty Base Optimization. Without it `sizeof` jumps from
+/// 1 to 2, which then prevents EBO when `ilist_node_traits` is itself used
+/// as a base of `iplist_impl` — `iplist`/`ilist` end up carrying needless
+/// trait padding on Windows. See the symmetric fix on `simple_ilist` and
+/// `external/llvh/patches/simple-ilist-msvc-empty-bases.patch`.
 template <typename NodeTy>
-struct ilist_node_traits : ilist_alloc_traits<NodeTy>,
-                           ilist_callback_traits<NodeTy> {};
+struct LLVM_DECLARE_EMPTY_BASES ilist_node_traits
+    : ilist_alloc_traits<NodeTy>, ilist_callback_traits<NodeTy> {};
 
 /// Template traits for intrusive list.
 ///

external/llvh/include/llvh/ADT/simple_ilist.h

@@ -75,8 +75,15 @@ namespace llvh {
 /// equivalent to the last.
 ///
 /// See \a is_valid_option for steps on adding a new option.
+///
+/// `simple_ilist` inherits from two empty helpers (`list_base_type` and
+/// `SpecificNodeAccess`). Without `LLVM_DECLARE_EMPTY_BASES` MSVC fails to
+/// collapse them and shifts the embedded `Sentinel` from offset 0 to offset
+/// 8, breaking callers that rely on `&list == &Sentinel` — most visibly FFI
+/// consumers and any code comparing an iterator's stored node pointer
+/// against `&list` rather than against an iterator from `list.end()`.
 template <typename T, class... Options>
-class simple_ilist
+class LLVM_DECLARE_EMPTY_BASES simple_ilist
     : ilist_detail::compute_node_options<T, Options...>::type::list_base_type,
       ilist_detail::SpecificNodeAccess<
           typename ilist_detail::compute_node_options<T, Options...>::type> {
@@ -346,6 +353,23 @@ void simple_ilist<T, Options...>::sort(Compare comp) {
   merge(RHS, comp);
 }
 
+namespace ilist_detail {
+// EBO compile-time invariant for simple_ilist. Its two empty bases
+// (the configured list_base_type and SpecificNodeAccess) must collapse via
+// Empty Base Optimization, otherwise the embedded Sentinel shifts away from
+// offset 0 and FFI consumers that rely on `&list == &Sentinel` break.
+// On MSVC this requires LLVM_DECLARE_EMPTY_BASES applied to the class
+// template (defined in llvh/Support/Compiler.h). Catches accidental removal
+// of the macro, ABI-flag drift, or future MSVC layout changes.
+struct ebo_check_node : ilist_node<ebo_check_node> {};
+static_assert(
+    sizeof(simple_ilist<ebo_check_node>) ==
+        sizeof(ilist_sentinel<compute_node_options<ebo_check_node>::type>),
+    "simple_ilist must use Empty Base Optimization. If this fires on MSVC, "
+    "ensure LLVM_DECLARE_EMPTY_BASES is applied to the class template "
+    "(see external/llvh/patches/simple-ilist-msvc-empty-bases.patch).");
+} // end namespace ilist_detail
+
 } // end namespace llvh
 
 #endif // LLVM_ADT_SIMPLE_ILIST_H

external/llvh/include/llvh/Support/Compiler.h

@@ -108,6 +108,18 @@
 #define LLVM_LIBRARY_VISIBILITY
 #endif
 
+/// LLVM_DECLARE_EMPTY_BASES - Force MSVC to apply Empty Base Optimization
+/// across multiple empty bases. By default MSVC pads each empty base with
+/// one byte and pointer-aligns the result, which shifts member offsets and
+/// breaks any code that relies on a derived object's address coinciding
+/// with the address of its first member (e.g. layout-sensitive containers
+/// and FFI consumers).
+#ifdef _MSC_VER
+#define LLVM_DECLARE_EMPTY_BASES __declspec(empty_bases)
+#else
+#define LLVM_DECLARE_EMPTY_BASES
+#endif
+
 #if defined(__GNUC__)
 #define LLVM_PREFETCH(addr, rw, locality) __builtin_prefetch(addr, rw, locality)
 #else

external/llvh/patches/ilist-node-traits-msvc-empty-bases.patch

@@ -0,0 +1,23 @@
+diff --git a/include/llvh/ADT/ilist.h b/include/llvh/ADT/ilist.h
+index 95edfb8b..518f3ab6 100644
+--- a/include/llvh/ADT/ilist.h
++++ b/include/llvh/ADT/ilist.h
+@@ -80,9 +80,16 @@ template <typename NodeTy> struct ilist_callback_traits {
+ /// node related operations.
+ ///
+ /// TODO: Remove this layer of indirection.  It's not necessary.
++///
++/// `LLVM_DECLARE_EMPTY_BASES` is required so MSVC collapses the two empty
++/// helper bases via Empty Base Optimization. Without it `sizeof` jumps from
++/// 1 to 2, which then prevents EBO when `ilist_node_traits` is itself used
++/// as a base of `iplist_impl` — `iplist`/`ilist` end up carrying needless
++/// trait padding on Windows. See the symmetric fix on `simple_ilist` and
++/// `external/llvh/patches/simple-ilist-msvc-empty-bases.patch`.
+ template <typename NodeTy>
+-struct ilist_node_traits : ilist_alloc_traits<NodeTy>,
+-                           ilist_callback_traits<NodeTy> {};
++struct LLVM_DECLARE_EMPTY_BASES ilist_node_traits
++    : ilist_alloc_traits<NodeTy>, ilist_callback_traits<NodeTy> {};
+
+ /// Template traits for intrusive list.
+ ///

external/llvh/patches/simple-ilist-msvc-empty-bases.patch

@@ -0,0 +1,68 @@
+diff --git a/include/llvh/ADT/simple_ilist.h b/include/llvh/ADT/simple_ilist.h
+index 6a59ddc7..8c1bc68d 100644
+--- a/include/llvh/ADT/simple_ilist.h
++++ b/include/llvh/ADT/simple_ilist.h
+@@ -75,8 +75,15 @@ namespace llvh {
+ /// equivalent to the last.
+ ///
+ /// See \a is_valid_option for steps on adding a new option.
++///
++/// `simple_ilist` inherits from two empty helpers (`list_base_type` and
++/// `SpecificNodeAccess`). Without `LLVM_DECLARE_EMPTY_BASES` MSVC fails to
++/// collapse them and shifts the embedded `Sentinel` from offset 0 to offset
++/// 8, breaking callers that rely on `&list == &Sentinel` — most visibly FFI
++/// consumers and any code comparing an iterator's stored node pointer
++/// against `&list` rather than against an iterator from `list.end()`.
+ template <typename T, class... Options>
+-class simple_ilist
++class LLVM_DECLARE_EMPTY_BASES simple_ilist
+     : ilist_detail::compute_node_options<T, Options...>::type::list_base_type,
+       ilist_detail::SpecificNodeAccess<
+           typename ilist_detail::compute_node_options<T, Options...>::type> {
+@@ -346,6 +353,23 @@ void simple_ilist<T, Options...>::sort(Compare comp) {
+   merge(RHS, comp);
+ }
+
++namespace ilist_detail {
++// EBO compile-time invariant for simple_ilist. Its two empty bases
++// (the configured list_base_type and SpecificNodeAccess) must collapse via
++// Empty Base Optimization, otherwise the embedded Sentinel shifts away from
++// offset 0 and FFI consumers that rely on `&list == &Sentinel` break.
++// On MSVC this requires LLVM_DECLARE_EMPTY_BASES applied to the class
++// template (defined in llvh/Support/Compiler.h). Catches accidental removal
++// of the macro, ABI-flag drift, or future MSVC layout changes.
++struct ebo_check_node : ilist_node<ebo_check_node> {};
++static_assert(
++    sizeof(simple_ilist<ebo_check_node>) ==
++        sizeof(ilist_sentinel<compute_node_options<ebo_check_node>::type>),
++    "simple_ilist must use Empty Base Optimization. If this fires on MSVC, "
++    "ensure LLVM_DECLARE_EMPTY_BASES is applied to the class template "
++    "(see external/llvh/patches/simple-ilist-msvc-empty-bases.patch).");
++} // end namespace ilist_detail
++
+ } // end namespace llvh
+
+ #endif // LLVM_ADT_SIMPLE_ILIST_H
+diff --git a/include/llvh/Support/Compiler.h b/include/llvh/Support/Compiler.h
+index 34173110..1dc671e5 100644
+--- a/include/llvh/Support/Compiler.h
++++ b/include/llvh/Support/Compiler.h
+@@ -108,6 +108,18 @@
+ #define LLVM_LIBRARY_VISIBILITY
+ #endif
+
++/// LLVM_DECLARE_EMPTY_BASES - Force MSVC to apply Empty Base Optimization
++/// across multiple empty bases. By default MSVC pads each empty base with
++/// one byte and pointer-aligns the result, which shifts member offsets and
++/// breaks any code that relies on a derived object's address coinciding
++/// with the address of its first member (e.g. layout-sensitive containers
++/// and FFI consumers).
++#ifdef _MSC_VER
++#define LLVM_DECLARE_EMPTY_BASES __declspec(empty_bases)
++#else
++#define LLVM_DECLARE_EMPTY_BASES
++#endif
++
+ #if defined(__GNUC__)
+ #define LLVM_PREFETCH(addr, rw, locality) __builtin_prefetch(addr, rw, locality)
+ #else