To build in fastdebug mode, configure with

  bash ./configure ... --with-debug-level=fastdebug

The patches here are for code that's only built in debug mode.

--- a/src/hotspot/share/opto/type.cpp	Fri Oct 11 20:42:42 2024
+++ b/src/hotspot/share/opto/type.cpp	Thu Nov 21 11:50:19 2024
@@ -912,6 +912,9 @@
   assert(Compile::current()->_type_verify == nullptr || Compile::current()->_type_verify->empty_cache(), "cache should have been discarded");
 }
 
+#ifdef _C
+#undef _C
+#endif
 class VerifyMeet {
 private:
   Compile* _C;
--- a/src/hotspot/share/utilities/vmError.cpp	Thu Nov 21 11:00:15 2024
+++ b/src/hotspot/share/utilities/vmError.cpp	Thu Nov 21 12:00:25 2024
@@ -80,6 +80,9 @@
 #ifndef PRODUCT
 #include <signal.h>
 #endif // PRODUCT
+#ifdef ASSERT
+# include <alloca.h>
+#endif // ASSERT
 
 bool              VMError::coredump_status;
 char              VMError::coredump_message[O_BUFLEN];