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
@@ -2,6 +2,7 @@
   blake3.c
   blake3_dispatch.c
   blake3_portable.c
+  blake3_neon.c
 )
 
 if (LLVM_DISABLE_ASSEMBLY_FILES)
@@ -10,6 +11,10 @@
   set(CAN_USE_ASSEMBLER TRUE)
 endif()
 
+macro(disable_blake3_x86_simd)
+  add_definitions(-DBLAKE3_NO_AVX512 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_SSE2)
+endmacro()
+
 # The BLAKE3 team recommends using the assembly versions, from the README:
 #
 # "For each of the x86 SIMD instruction sets, four versions are available:
@@ -18,70 +23,62 @@
 # preferred. They perform better, they perform more consistently across
 # different compilers, and they build more quickly."
 
-if (MSVC)
-  check_symbol_exists(_M_X64 "" IS_X64)
-  check_symbol_exists(_M_ARM64 "" IS_ARM64)
-else()
-  check_symbol_exists(__x86_64__ "" IS_X64)
-  check_symbol_exists(__aarch64__ "" IS_ARM64)
-
-  if (IS_X64)
-    # Clang-6 needs this flag.
-    set_source_files_properties(blake3_avx512_x86-64_windows_gnu.S
-      PROPERTIES COMPILE_OPTIONS "-mavx512vl")
-    set_source_files_properties(blake3_avx512_x86-64_unix.S
-      PROPERTIES COMPILE_OPTIONS "-mavx512vl")
-  endif()
-endif()
-
-if (IS_X64 AND CAN_USE_ASSEMBLER)
+if (CAN_USE_ASSEMBLER)
   if (MSVC)
-    enable_language(ASM_MASM)
-    list(APPEND LLVM_BLAKE3_FILES
-      blake3_sse2_x86-64_windows_msvc.asm
-      blake3_sse41_x86-64_windows_msvc.asm
-      blake3_avx2_x86-64_windows_msvc.asm
-      blake3_avx512_x86-64_windows_msvc.asm
-    )
+    check_symbol_exists(_M_X64 "" IS_X64)
+    if (IS_X64)
+      enable_language(ASM_MASM)
+      list(APPEND LLVM_BLAKE3_FILES
+        blake3_sse2_x86-64_windows_msvc.asm
+        blake3_sse41_x86-64_windows_msvc.asm
+        blake3_avx2_x86-64_windows_msvc.asm
+        blake3_avx512_x86-64_windows_msvc.asm
+      )
+    else()
+      disable_blake3_x86_simd()
+    endif()
   elseif(WIN32 OR CYGWIN)
-    list(APPEND LLVM_BLAKE3_FILES
-      blake3_sse2_x86-64_windows_gnu.S
-      blake3_sse41_x86-64_windows_gnu.S
-      blake3_avx2_x86-64_windows_gnu.S
-      blake3_avx512_x86-64_windows_gnu.S
-    )
+    check_symbol_exists(__x86_64__ "" IS_X64)
+    if (IS_X64)
+      list(APPEND LLVM_BLAKE3_FILES
+        blake3_sse2_x86-64_windows_gnu.S
+        blake3_sse41_x86-64_windows_gnu.S
+        blake3_avx2_x86-64_windows_gnu.S
+        blake3_avx512_x86-64_windows_gnu.S
+      )
+      # Clang-6 needs this flag.
+      set_source_files_properties(blake3_avx512_x86-64_windows_gnu.S
+        PROPERTIES COMPILE_OPTIONS "-mavx512vl")
+    else()
+      disable_blake3_x86_simd()
+    endif()
   else()
-    list(APPEND LLVM_BLAKE3_FILES
-      blake3_sse2_x86-64_unix.S
-      blake3_sse41_x86-64_unix.S
-      blake3_avx2_x86-64_unix.S
-      blake3_avx512_x86-64_unix.S
-    )
+    check_symbol_exists(__i386__ "" IS_X32)
+    if (IS_X32)
+      # blake3 C code autoenables SIMD for i386, but those implementations are
+      # only available via the intrinsics sources which we don't enable here.
+      disable_blake3_x86_simd()
+    else()
+      # In a macOS Universal build (setting CMAKE_OSX_ARCHITECTURES to multiple
+      # values), compilation of the source files will target multiple architectures
+      # (each source file is internally compiled once for each architecture).
+      # To accomodate this configuration we include these assembly files without a
+      # CMake check but their source is guarded with architecture "#ifdef" checks.
+      list(APPEND LLVM_BLAKE3_FILES
+        blake3_sse2_x86-64_unix.S
+        blake3_sse41_x86-64_unix.S
+        blake3_avx2_x86-64_unix.S
+        blake3_avx512_x86-64_unix.S
+      )
+      # Clang-6 needs this flag. We also suppress '-Wunused-command-line-argument'
+      # in case the file is included with arm architecture.
+      set_source_files_properties(blake3_avx512_x86-64_unix.S
+        PROPERTIES COMPILE_OPTIONS "-mavx512vl;-Wno-unused-command-line-argument")
+    endif()
   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
-  )
-else()
-  add_definitions(-DBLAKE3_USE_NEON=0)
+  # CAN_USE_ASSEMBLER == FALSE
+  disable_blake3_x86_simd()
 endif()
 
 add_library(LLVMSupportBlake3 OBJECT EXCLUDE_FROM_ALL ${LLVM_BLAKE3_FILES})
diff --git a/llvm/lib/Support/BLAKE3/blake3_avx2_x86-64_unix.S b/llvm/lib/Support/BLAKE3/blake3_avx2_x86-64_unix.S
--- a/llvm/lib/Support/BLAKE3/blake3_avx2_x86-64_unix.S
+++ b/llvm/lib/Support/BLAKE3/blake3_avx2_x86-64_unix.S
@@ -1,3 +1,5 @@
+#if defined(__x86_64__)
+
 #if defined(__ELF__) && defined(__linux__)
 .section .note.GNU-stack,"",%progbits
 #endif
@@ -1821,3 +1823,4 @@
 BLAKE3_IV:
         .long  0x6A09E667, 0xBB67AE85, 0x3C6EF372, 0xA54FF53A
 
+#endif
diff --git a/llvm/lib/Support/BLAKE3/blake3_avx512_x86-64_unix.S b/llvm/lib/Support/BLAKE3/blake3_avx512_x86-64_unix.S
--- a/llvm/lib/Support/BLAKE3/blake3_avx512_x86-64_unix.S
+++ b/llvm/lib/Support/BLAKE3/blake3_avx512_x86-64_unix.S
@@ -1,3 +1,5 @@
+#if defined(__x86_64__)
+
 #if defined(__ELF__) && defined(__linux__)
 .section .note.GNU-stack,"",%progbits
 #endif
@@ -2595,3 +2597,5 @@
         .long   0x3C6EF372
 BLAKE3_IV_3:
         .long   0xA54FF53A
+
+#endif
diff --git a/llvm/lib/Support/BLAKE3/blake3_neon.c b/llvm/lib/Support/BLAKE3/blake3_neon.c
--- a/llvm/lib/Support/BLAKE3/blake3_neon.c
+++ b/llvm/lib/Support/BLAKE3/blake3_neon.c
@@ -1,5 +1,7 @@
 #include "blake3_impl.h"
 
+#if BLAKE3_USE_NEON
+
 #include <arm_neon.h>
 
 #ifdef __ARM_BIG_ENDIAN
@@ -350,3 +352,5 @@
     out = &out[BLAKE3_OUT_LEN];
   }
 }
+
+#endif // BLAKE3_USE_NEON
diff --git a/llvm/lib/Support/BLAKE3/blake3_sse2_x86-64_unix.S b/llvm/lib/Support/BLAKE3/blake3_sse2_x86-64_unix.S
--- a/llvm/lib/Support/BLAKE3/blake3_sse2_x86-64_unix.S
+++ b/llvm/lib/Support/BLAKE3/blake3_sse2_x86-64_unix.S
@@ -1,3 +1,5 @@
+#if defined(__x86_64__)
+
 #if defined(__ELF__) && defined(__linux__)
 .section .note.GNU-stack,"",%progbits
 #endif
@@ -2301,3 +2303,5 @@
 	.long  0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x00000000
 PBLENDW_0xC0_MASK:
 	.long  0x00000000, 0x00000000, 0x00000000, 0xFFFFFFFF
+
+#endif
diff --git a/llvm/lib/Support/BLAKE3/blake3_sse41_x86-64_unix.S b/llvm/lib/Support/BLAKE3/blake3_sse41_x86-64_unix.S
--- a/llvm/lib/Support/BLAKE3/blake3_sse41_x86-64_unix.S
+++ b/llvm/lib/Support/BLAKE3/blake3_sse41_x86-64_unix.S
@@ -1,3 +1,5 @@
+#if defined(__x86_64__)
+
 #if defined(__ELF__) && defined(__linux__)
 .section .note.GNU-stack,"",%progbits
 #endif
@@ -2038,3 +2040,5 @@
 	.long  64, 64, 64, 64
 CMP_MSB_MASK:
 	.long  0x80000000, 0x80000000, 0x80000000, 0x80000000
+
+#endif