Introduce ARM Neon and SSE2 SIMD. (#743)

See the pull request for the long development history: #743

```
== Encoding activitypub.json (52595 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     2.913k i/100ms
Calculating -------------------------------------
               after     29.377k (± 2.0%) i/s   (34.04 μs/i) -    148.563k in   5.059169s

Comparison:
              before:    23314.1 i/s
               after:    29377.3 i/s - 1.26x  faster


== Encoding citm_catalog.json (500298 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   152.000 i/100ms
Calculating -------------------------------------
               after      1.569k (± 0.8%) i/s  (637.49 μs/i) -      7.904k in   5.039001s

Comparison:
              before:     1485.6 i/s
               after:     1568.7 i/s - 1.06x  faster


== Encoding twitter.json (466906 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   309.000 i/100ms
Calculating -------------------------------------
               after      3.115k (± 3.1%) i/s  (321.01 μs/i) -     15.759k in   5.063776s

Comparison:
              before:     2508.3 i/s
               after:     3115.2 i/s - 1.24x  faster
```

Author: samyron

.github/workflows/ci.yml

@@ -14,26 +14,29 @@ jobs:
 
   host:
     needs: ruby-versions
-    name: ${{ matrix.os }} ${{ matrix.ruby }}
+    name: ${{ matrix.os }} ${{ matrix.ruby }} ${{ matrix.env }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
         os:
-        - ubuntu-latest
-        - macos-14
-        - windows-latest
+          - ubuntu-latest
+          - macos-14
+          - windows-latest
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
+        env:
+          - ""
         include:
-        - { os: ubuntu-24.04-arm, ruby: 3.4 }
-        - { os: macos-13, ruby: 3.4 }
-        - { os: windows-latest , ruby: mswin     } # ruby/ruby windows CI
-        - { os: ubuntu-latest  , ruby: jruby-9.4 } # Ruby 3.1
-        - { os: macos-latest   , ruby: truffleruby-head }
-        - { os: ubuntu-latest  , ruby: truffleruby-head }
+          - { os: ubuntu-24.04-arm, ruby: 3.4 }
+          - { os: ubuntu-latest  , ruby: 3.4, env: "JSON_DISABLE_SIMD=1" }
+          - { os: macos-13, ruby: 3.4 }
+          - { os: windows-latest , ruby: mswin     } # ruby/ruby windows CI
+          - { os: ubuntu-latest  , ruby: jruby-9.4 } # Ruby 3.1
+          - { os: macos-latest   , ruby: truffleruby-head }
+          - { os: ubuntu-latest  , ruby: truffleruby-head }
         exclude:
-        - { os: windows-latest, ruby: jruby }
-        - { os: windows-latest, ruby: jruby-head }
+          - { os: windows-latest, ruby: jruby }
+          - { os: windows-latest, ruby: jruby-head }
 
     steps:
       - uses: actions/checkout@v4
@@ -49,9 +52,9 @@ jobs:
           bundle config --without benchmark
           bundle install
 
-      - run: rake compile
+      - run: rake compile ${{ matrix.env }}
 
-      - run: rake test JSON_COMPACT=1
+      - run: rake test JSON_COMPACT=1 ${{ matrix.env }}
 
       - run: rake build
 

ext/json/ext/generator/extconf.rb

@@ -6,5 +6,36 @@
 else
   append_cflags("-std=c99")
   $defs << "-DJSON_GENERATOR"
+
+  if enable_config('generator-use-simd', default=!ENV["JSON_DISABLE_SIMD"])
+    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")
+      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