[Support] [BLAKE3] Fix compilation with CMAKE_OSX_ARCHITECTURES

[mstorsjo]

Summary

With CMake, one can build for multiple macOS architectures
at the same time by setting CMAKE_OSX_ARCHITECTURES to multiple
architectures (avoiding needing to do two separate builds and
gluing the binaries together after the build).

In this case, while targeting x86_64 and arm64, neither IS_X64
nor IS_ARM64 is set, while compilation of the individual source
files will hit those cases (in either architecture mode).

Therefore, if we on the CMake level decide not to include the
architecture specific SIMD implementation files, also tell the
source this explicitly by passing the defines indicating that we
don't expect to use them.

Such a build clearly is less ideal than explicitly targeting one
architecture at a time if it won't include all the SIMD optimizations,
but that's a tradeoff that is up to the one deciding to do such an
universal build.

An alternative would be to include all potentially relevant source
files in the build, and wrap them in ifdefs, like #if BLAKE3_USE_NEON
within blake3_neon.c after including blake3_impl.h, or
#ifdef __x86_64__ in the .S assembly files.

[akyrtzi]

LGTM, but could you also include a FIXME note to indicate need to handle this kind of CMake setup for SIMD implementations, potentially using your alternative suggestion?

[mstorsjo]

In D122884#3422812, @akyrtzi wrote:

LGTM, but could you also include a FIXME note to indicate need to handle this kind of CMake setup for SIMD implementations, potentially using your alternative suggestion?

Sure, I can add such a todo.

FWIW I also noticed that this fixes builds for i386 (for any OS I'd believe) as the blake3 C code autoenables SIMD for i386 too, but those implementations are only available via the intrinsics sources (which are trickier to hook up in cmake, which we don't enable here).

llvm/lib/Support/BLAKE3/CMakeLists.txt

if (IS_X64 AND CAN_USE_ASSEMBLER)
   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
     )
   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})
 llvm_update_compile_flags(LLVMSupportBlake3)