From 9689a4f58a09cf8c3f9094085a8243b8db4b0ded Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Thu, 31 Oct 2024 21:29:13 -0400
Subject: [PATCH] Internals: Support VL_UNREACHABLE in C++23/MSVC. No
 functional change intended.

---
 include/verilatedos.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/verilatedos.h b/include/verilatedos.h
index 6cb83732f..f633a0f32 100644
--- a/include/verilatedos.h
+++ b/include/verilatedos.h
@@ -67,11 +67,21 @@
 # endif
 # define VL_LIKELY(x) __builtin_expect(!!(x), 1)  // Prefer over C++20 [[likely]]
 # define VL_UNLIKELY(x) __builtin_expect(!!(x), 0)  // Prefer over C++20 [[unlikely]]
-# define VL_UNREACHABLE __builtin_unreachable()  // C++23 std::unreachable()
 # define VL_PREFETCH_RD(p) __builtin_prefetch((p), 0)
 # define VL_PREFETCH_RW(p) __builtin_prefetch((p), 1)
 #endif
 
+#ifdef __cpp_lib_unreachable
+/// Statement that may never be reached (for coverage etc)
+# define VL_UNREACHABLE std::unreachable()  // C++23
+#elif defined(__GNUC__)
+# define VL_UNREACHABLE __builtin_unreachable()
+#elif defined(_MSC_VER)  // MSVC
+# define VL_UNREACHABLE __assume(false)
+#else
+# define VL_UNREACHABLE
+#endif
+
 // Function acquires a capability/lock (-fthread-safety)
 #define VL_ACQUIRE(...) \
         VL_CLANG_ATTR(annotate("ACQUIRE")) \
@@ -179,9 +189,6 @@
 #endif
 /// Boolean expression never hit by users (branch coverage disabled)
 # define VL_UNCOVERABLE(x) VL_UNLIKELY(x)
-#ifndef VL_UNREACHABLE
-# define VL_UNREACHABLE  ///< Statement that may never be reached (for coverage etc)
-#endif
 #ifndef VL_PREFETCH_RD
 # define VL_PREFETCH_RD(p)  ///< Prefetch pointer argument with read intent
 #endif