diff --git a/cmake/LLVM.cmake b/cmake/LLVM.cmake
index 9a9754684a8..3827a9b3e11 100644
--- a/cmake/LLVM.cmake
+++ b/cmake/LLVM.cmake
@@ -91,12 +91,6 @@ function(fetch_or_build_slang_llvm)
             target_compile_options(slang-llvm PRIVATE -wd4244 /Zc:preprocessor)
         endif()
 
-        if(NOT LLVM_ENABLE_RTTI)
-            # Make sure that we don't disable rtti if this library wasn't compiled with
-            # support
-            add_supported_cxx_flags(slang-llvm PRIVATE -fno-rtti /GR-)
-        endif()
-
         # TODO: Put a check here that libslang-llvm.so doesn't have a 'NEEDED'
         # directive for libLLVM-21.so, it's almost certainly going to break at
         # runtime in surprising ways when linked alongside Mesa (or anything else
diff --git a/external/build-llvm.sh b/external/build-llvm.sh
index 91bc56560a7..571f00ae2b1 100755
--- a/external/build-llvm.sh
+++ b/external/build-llvm.sh
@@ -134,6 +134,9 @@ cmake_arguments_for_slang=(
   -DLLVM_ENABLE_PROJECTS=clang
   "-DLLVM_TARGETS_TO_BUILD=X86;ARM;AArch64"
   -DLLVM_BUILD_TOOLS=0
+  # slang-llvm is built with RTTI enabled to support UndefinedBehaviorSanitizer's vptr checks, so
+  # LLVM should be built with RTTI as well
+  -DLLVM_ENABLE_RTTI=1
   # Get LLVM to use the static linked version of the msvc runtime
   "-DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded$<$<CONFIG:Debug>:Debug>"
   "-DLLVM_USE_CRT_RELEASE=MT"