Process.warmup: precompute strings coderange

This both save time for when it will be eventually needed,
and avoid mutating heap pages after a potential fork.

Instrumenting some large Rails app, I've witnessed up to
58% of String instances having their coderange still unknown.

gc.c

@@ -9596,6 +9596,11 @@ gc_set_candidate_object_i(void *vstart, void *vend, size_t stride, void *data)
           case T_NONE:
           case T_ZOMBIE:
             break;
+          case T_STRING:
+            // precompute the string coderange. This both save time for when it will be
+            // eventually needed, and avoid mutating heap pages after a potential fork.
+            rb_enc_str_coderange(v);
+            // fall through
           default:
             if (!RVALUE_OLD_P(v) && !RVALUE_WB_UNPROTECTED(v)) {
                 RVALUE_AGE_SET_CANDIDATE(objspace, v);

process.c

@@ -8555,6 +8555,7 @@ static VALUE rb_mProcID_Syscall;
  *  * Perform a major GC.
  *  * Compacts the heap.
  *  * Promotes all surviving objects to the old generation.
+ *  * Precompute the coderange of all strings.
  */
 
 static VALUE
@@ -8566,7 +8567,6 @@ proc_warmup(VALUE _)
     return Qtrue;
 }
 
-
 /*
  * Document-module: Process
  *

test/ruby/test_process.rb

@@ -2709,4 +2709,16 @@ EOS
       assert_equal compact_count + 1, GC.stat(:compact_count)
     end;
   end
+
+  def test_warmup_precompute_string_coderange
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    require 'objspace'
+    begin;
+      obj = "a" * 12
+      obj.force_encoding(Encoding::BINARY)
+      assert_include(ObjectSpace.dump(obj), '"coderange":"unknown"')
+      Process.warmup
+      assert_include(ObjectSpace.dump(obj), '"coderange":"7bit"')
+    end;
+  end
 end