Add SIMD support for various platforms

We need extconf to detect which SIMD features the compiler and processor
support, then we can compile in SIMD support for BLAKE3

Co-Authored-By: Jean Boussier <jean.boussier@gmail.com>

Author: tenderlove

ext/digest/blake3/extconf.rb

@@ -3,23 +3,79 @@
 
 require "mkmf"
 
-# Build the portable (no-SIMD) BLAKE3 implementation: only blake3.c,
-# blake3_dispatch.c and blake3_portable.c are compiled.  The SIMD
-# backends (SSE2/SSE4.1/AVX2/AVX512/NEON) are intentionally left out to
-# keep the build simple and dependency-free.  Each instruction set has
-# to be disabled explicitly, otherwise blake3_dispatch.c references
-# symbols from the omitted backends.  In particular NEON auto-enables on
-# AArch64 (see blake3_impl.h), so BLAKE3_USE_NEON must be forced off too.
-$defs << "-DBLAKE3_NO_SSE2"
-$defs << "-DBLAKE3_NO_SSE41"
-$defs << "-DBLAKE3_NO_AVX2"
-$defs << "-DBLAKE3_NO_AVX512"
-$defs << "-DBLAKE3_USE_NEON=0"
-
-$objs = %w[blake3init blake3 blake3_dispatch blake3_portable].map do |o|
-  "#{o}.#{$OBJEXT}"
+# BLAKE3 build configuration.
+#
+# The binding is always built from the portable C code (blake3.c,
+# blake3_dispatch.c, blake3_portable.c).  On architectures BLAKE3 ships
+# optimized backends for, we additionally compile the SIMD implementations
+# and let blake3_dispatch.c select the fastest one supported by the CPU the
+# program is actually running on (via CPUID on x86).  This means a binary
+# built on an AVX-512-capable machine still runs correctly on an older CPU.
+#
+# Each SIMD translation unit must be compiled with its own instruction-set
+# flag, which a single global $CFLAGS can't express, so we emit one explicit
+# object rule per backend at the end (see below).  Any x86 backend we do NOT
+# compile is disabled with -DBLAKE3_NO_<ISA>; that macro is honoured both by
+# the dispatcher and by sibling backends (e.g. blake3_avx2.c falls back to
+# SSE4.1 helpers), keeping the set of referenced symbols consistent.
+
+objs = %w[blake3init blake3 blake3_dispatch blake3_portable]
+
+# Extra per-object compiler flags, keyed by object basename.
+simd_cflags = {}
+
+# Probe used to confirm the compiler both accepts +flag+ and can compile the
+# intrinsics the backend relies on.
+def blake3_have_isa?(name, flag, snippet)
+  checking_for("#{name} intrinsics (#{flag})") do
+    try_compile(snippet, flag)
+  end
 end
 
+case RbConfig::CONFIG["host_cpu"]
+when /\A(x86_64|amd64|x64)\z/i
+  # Try to detect which SIMD features this x86 machine and compiler has
+  x86_backends = [
+    ["blake3_sse2",   "SSE2",    "-msse2",
+     "#include <immintrin.h>\nint main(void){ volatile __m128i x = _mm_setzero_si128(); (void)x; return 0; }\n",
+     "BLAKE3_NO_SSE2"],
+    ["blake3_sse41",  "SSE4.1",  "-msse4.1",
+     "#include <immintrin.h>\nint main(void){ volatile __m128i x = _mm_setzero_si128(); return _mm_testz_si128(x, x); }\n",
+     "BLAKE3_NO_SSE41"],
+    ["blake3_avx2",   "AVX2",    "-mavx2",
+     "#include <immintrin.h>\nint main(void){ volatile __m256i x = _mm256_setzero_si256(); (void)x; return 0; }\n",
+     "BLAKE3_NO_AVX2"],
+    ["blake3_avx512", "AVX-512", "-mavx512f -mavx512vl",
+     "#include <immintrin.h>\nint main(void){ volatile __m512i x = _mm512_setzero_si512(); (void)x; return 0; }\n",
+     "BLAKE3_NO_AVX512"],
+  ]
+
+  x86_backends.each do |obj, name, flag, snippet, no_macro|
+    if blake3_have_isa?(name, flag, snippet)
+      objs << obj
+      simd_cflags[obj] = flag
+    else
+      $defs << "-D#{no_macro}"
+    end
+  end
+when /\A(aarch64|arm64)\z/i
+  # NEON is part of the AArch64 baseline, so no runtime detection or special
+  # compiler flag is needed.  Leave BLAKE3_USE_NEON to auto-detect, which is
+  # 1 on little-endian AArch64 (see blake3_impl.h).
+  objs << "blake3_neon"
+else
+  # No optimized backend wired up for this architecture (e.g. 32-bit x86,
+  # ppc): build portable-only.  Disabling every x86 ISA keeps the dispatcher
+  # from referencing backends we didn't compile, and NEON is forced off.
+  $defs << "-DBLAKE3_NO_SSE2"
+  $defs << "-DBLAKE3_NO_SSE41"
+  $defs << "-DBLAKE3_NO_AVX2"
+  $defs << "-DBLAKE3_NO_AVX512"
+  $defs << "-DBLAKE3_USE_NEON=0"
+end
+
+$objs = objs.map { |o| "#{o}.#{$OBJEXT}" }
+
 have_header("sys/cdefs.h")
 
 $preload = %w[digest]