diff --git a/llvm/lib/Support/BLAKE3/CMakeLists.txt b/llvm/lib/Support/BLAKE3/CMakeLists.txt
--- a/llvm/lib/Support/BLAKE3/CMakeLists.txt
+++ b/llvm/lib/Support/BLAKE3/CMakeLists.txt
@@ -74,16 +74,30 @@
       blake3_avx512_x86-64_unix.S
     )
   endif()
+else()
+  # In a macOS Universal build (setting CMAKE_OSX_ARCHITECTURES to multiple
+  # values), IS_X64 and IS_ARM64 won't be set, but compilation of the source
+  # files will consider targeting either of them (each source file is
+  # internally compiled once for each architecture). Thus, if we on the CMake
+  # level decide not to include the assembly files, tell the source to not
+  # expect it to be present either.
+  #
+  # Also, if targeting i386, then the blake3 source code automatically enables
+  # the SIMD implementations, but we don't provide those sources.
+  #
+  # FIXME: We could improve the CMAKE_OSX_ARCHITECTURES configuration by
+  # including all SIMD implementation files that might be relevant, and
+  # wrapping them in ifdefs like "#ifdef __x86_64__", to allow them to be
+  # included in a build for any architecture.
+  add_definitions(-DBLAKE3_NO_AVX512 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_SSE2)
 endif()
 
 if (IS_ARM64)
   list(APPEND LLVM_BLAKE3_FILES
     blake3_neon.c
   )
-endif()
-
-if (IS_X64 AND NOT CAN_USE_ASSEMBLER)
-  add_definitions(-DBLAKE3_NO_AVX512 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_SSE2)
+else()
+  add_definitions(-DBLAKE3_USE_NEON=0)
 endif()
 
 add_library(LLVMSupportBlake3 OBJECT EXCLUDE_FROM_ALL ${LLVM_BLAKE3_FILES})