Introduce ARM Neon and SSE2 SIMD.

ext/json/ext/generator/extconf.rb

@@ -6,5 +6,40 @@
 else
   append_cflags("-std=c99")
   $defs << "-DJSON_GENERATOR"
+
+  if enable_config('generator-use-simd', default=true)
+    if RbConfig::CONFIG['host_cpu'] =~ /^(arm.*|aarch64.*)/
+      # Try to compile a small program using NEON instructions
+      if have_header('arm_neon.h')
+        have_type('uint8x16_t', headers=['arm_neon.h']) && try_compile(<<~'SRC')
+          #include <arm_neon.h>
+          int main() {
+              uint8x16_t test = vdupq_n_u8(32);
+              return 0;
+          }
+        SRC
+          $defs.push("-DENABLE_SIMD")
+
+          if enable_config('generator-use-neon-lut', default=false)
+            $defs.push('-DUSE_NEON_LUT')
+          end
+      end
+    end
+
+    if have_header('x86intrin.h') && have_type('__m128i', headers=['x86intrin.h']) && try_compile(<<~'SRC', opt='-msse2')
+      #include <x86intrin.h>
+      int main() {
+          __m128i test = _mm_set1_epi8(32);
+          return 0;
+      }
+      SRC
+        $defs.push("-DENABLE_SIMD")
+    end
+
+    have_header('cpuid.h')
+  end
+
+  create_header
+
   create_makefile 'json/ext/generator'
 end