diff --git a/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh b/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh
index 599f063b45c9..196a876a6e7e 100755
--- a/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh
+++ b/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh
@@ -140,6 +140,7 @@ if [[ ! -d ${LLVM_BUILD} ]]; then
     -DLLVM_ENABLE_ZLIB=ON \
     -DLLVM_ENABLE_TERMINFO=OFF \
     -DLLVM_ENABLE_THREADS=OFF \
+    -DLLVM_DISABLE_ASSEMBLY_FILES=ON \
   $LLVM_SRC
 fi
 cd ${LLVM_BUILD}
diff --git a/llvm/lib/Support/BLAKE3/CMakeLists.txt b/llvm/lib/Support/BLAKE3/CMakeLists.txt
index 718dd6421648..4374a840c7e3 100644
--- a/llvm/lib/Support/BLAKE3/CMakeLists.txt
+++ b/llvm/lib/Support/BLAKE3/CMakeLists.txt
@@ -4,6 +4,28 @@ set(LLVM_BLAKE3_FILES
   blake3_portable.c
 )
 
+if (LLVM_DISABLE_ASSEMBLY_FILES)
+  set(CAN_USE_ASSEMBLER FALSE)
+else()
+  set(CAN_USE_ASSEMBLER TRUE)
+endif()
+
+# Some LLVM builders set "CC=ccache /path/to/clang" as environment variable,
+# which confuses CMake thinking the assembler to use is the 'ccache' binary.
+# These builders need to also set "ASM=/path/to/clang" in environment, or use
+# LLVM_CCACHE_BUILD CMake variable instead, but until this happens check for
+# this case and disable building the assembly files.
+get_filename_component(ASM_BINARY ${CMAKE_ASM_COMPILER} NAME)
+if (ASM_BINARY STREQUAL "ccache")
+  message(WARNING "CMake is set to use '${CMAKE_ASM_COMPILER}' as assembler "
+    "which is likely due to having 'CC=ccache /path/to/clang' in the environment. "
+    "Building the BLAKE3 SIMD-optimized assembly files is disabled, set "
+    "'ASM=/path/to/clang' as environment variable and do a clean re-configure, "
+    "or unset CC/CXX and configure with '-DLLVM_CCACHE_BUILD=YES' instead, "
+    "to correct this.")
+  set(CAN_USE_ASSEMBLER FALSE)
+endif()
+
 # The BLAKE3 team recommends using the assembly versions, from the README:
 #
 # "For each of the x86 SIMD instruction sets, four versions are available:
@@ -11,13 +33,26 @@ set(LLVM_BLAKE3_FILES
 # version using C intrinsics. The assembly versions are generally
 # preferred. They perform better, they perform more consistently across
 # different compilers, and they build more quickly."
-# FIXME: Figure out what is wrong with the builders when using the assembly files and neon.
+
 if (MSVC)
-  enable_language(ASM_MASM)
+  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 (FALSE)#CMAKE_SYSTEM_PROCESSOR MATCHES "^(x86_64|AMD64)$")
+if (IS_X64 AND CAN_USE_ASSEMBLER)
   if (MSVC)
+    enable_language(ASM_MASM)
     list(APPEND LLVM_BLAKE3_FILES
       blake3_sse2_x86-64_windows_msvc.asm
       blake3_sse41_x86-64_windows_msvc.asm
@@ -41,14 +76,15 @@ if (FALSE)#CMAKE_SYSTEM_PROCESSOR MATCHES "^(x86_64|AMD64)$")
   endif()
 endif()
 
-if (FALSE)#CMAKE_SYSTEM_PROCESSOR MATCHES "^(arm|aarch)")
+if (IS_ARM64)
   list(APPEND LLVM_BLAKE3_FILES
     blake3_neon.c
   )
 endif()
 
-# FIXME: Figure out what is wrong with the builders when using the assembly files.
-add_definitions(-DBLAKE3_NO_AVX512 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_SSE2 -DBLAKE3_USE_NEON=0)
+if (IS_X64 AND NOT CAN_USE_ASSEMBLER)
+  add_definitions(-DBLAKE3_NO_AVX512 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_SSE2)
+endif()
 
 add_library(LLVMSupportBlake3 OBJECT EXCLUDE_FROM_ALL ${LLVM_BLAKE3_FILES})
 llvm_update_compile_flags(LLVMSupportBlake3)