Emit a performance warning when a class reached max variations

[Feature #19538]

This new `peformance` warning category is disabled by default.
It needs to be specifically enabled via `-W:performance` or `Warning[:performance] = true`

common.mk

@@ -14227,10 +14227,12 @@ shape.$(OBJEXT): $(top_srcdir)/internal/array.h
 shape.$(OBJEXT): $(top_srcdir)/internal/basic_operators.h
 shape.$(OBJEXT): $(top_srcdir)/internal/class.h
 shape.$(OBJEXT): $(top_srcdir)/internal/compilers.h
+shape.$(OBJEXT): $(top_srcdir)/internal/error.h
 shape.$(OBJEXT): $(top_srcdir)/internal/gc.h
 shape.$(OBJEXT): $(top_srcdir)/internal/imemo.h
 shape.$(OBJEXT): $(top_srcdir)/internal/serial.h
 shape.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
+shape.$(OBJEXT): $(top_srcdir)/internal/string.h
 shape.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 shape.$(OBJEXT): $(top_srcdir)/internal/variable.h
 shape.$(OBJEXT): $(top_srcdir)/internal/vm.h

error.c

@@ -77,6 +77,7 @@ static ID id_warn;
 static ID id_category;
 static ID id_deprecated;
 static ID id_experimental;
+static ID id_performance;
 static VALUE sym_category;
 static VALUE sym_highlight;
 static struct {
@@ -3147,6 +3148,7 @@ Init_Exception(void)
     id_category = rb_intern_const("category");
     id_deprecated = rb_intern_const("deprecated");
     id_experimental = rb_intern_const("experimental");
+    id_performance = rb_intern_const("performance");
     id_top = rb_intern_const("top");
     id_bottom = rb_intern_const("bottom");
     id_iseq = rb_make_internal_id();
@@ -3158,11 +3160,13 @@ Init_Exception(void)
     warning_categories.id2enum = rb_init_identtable();
     st_add_direct(warning_categories.id2enum, id_deprecated, RB_WARN_CATEGORY_DEPRECATED);
     st_add_direct(warning_categories.id2enum, id_experimental, RB_WARN_CATEGORY_EXPERIMENTAL);
+    st_add_direct(warning_categories.id2enum, id_performance, RB_WARN_CATEGORY_PERFORMANCE);
 
     warning_categories.enum2id = rb_init_identtable();
     st_add_direct(warning_categories.enum2id, RB_WARN_CATEGORY_NONE, 0);
     st_add_direct(warning_categories.enum2id, RB_WARN_CATEGORY_DEPRECATED, id_deprecated);
     st_add_direct(warning_categories.enum2id, RB_WARN_CATEGORY_EXPERIMENTAL, id_experimental);
+    st_add_direct(warning_categories.enum2id, RB_WARN_CATEGORY_PERFORMANCE, id_performance);
 }
 
 void

include/ruby/internal/error.h

@@ -50,6 +50,9 @@ typedef enum {
     /** Warning is for experimental features. */
     RB_WARN_CATEGORY_EXPERIMENTAL,
 
+    /** Warning is for performance issues (not enabled by -w). */
+    RB_WARN_CATEGORY_PERFORMANCE,
+
     RB_WARN_CATEGORY_ALL_BITS = (
         (1U << RB_WARN_CATEGORY_DEPRECATED) |
         (1U << RB_WARN_CATEGORY_EXPERIMENTAL) |

ruby.c

@@ -331,6 +331,7 @@ usage(const char *name, int help, int highlight, int columns)
     static const struct ruby_opt_message warn_categories[] = {
         M("deprecated", "",       "deprecated features"),
         M("experimental", "",     "experimental features"),
+        M("performance", "",      "performance issues"),
     };
 #if USE_YJIT
     static const struct ruby_opt_message yjit_options[] = {
@@ -1190,6 +1191,9 @@ proc_options(long argc, char **argv, ruby_cmdline_options_t *opt, int envopt)
                 else if (NAME_MATCH_P("experimental", s, len)) {
                     bits = 1U << RB_WARN_CATEGORY_EXPERIMENTAL;
                 }
+                else if (NAME_MATCH_P("performance", s, len)) {
+                    bits = 1U << RB_WARN_CATEGORY_PERFORMANCE;
+                }
                 else {
                     rb_warn("unknown warning category: `%s'", s);
                 }

shape.c

@@ -7,6 +7,7 @@
 #include "internal/gc.h"
 #include "internal/symbol.h"
 #include "internal/variable.h"
+#include "internal/error.h"
 #include "variable.h"
 #include <stdbool.h>
 
@@ -407,6 +408,16 @@ rb_shape_get_next(rb_shape_t* shape, VALUE obj, ID id)
 
         if (variation_created) {
             RCLASS_EXT(klass)->variation_count++;
+            if (rb_warning_category_enabled_p(RB_WARN_CATEGORY_PERFORMANCE)) {
+                if (RCLASS_EXT(klass)->variation_count >= SHAPE_MAX_VARIATIONS) {
+                    rb_category_warning(
+                        RB_WARN_CATEGORY_PERFORMANCE,
+                        "Maximum shapes variations (%d) reached by %"PRIsVALUE", instance variables accesses will be slower.",
+                        SHAPE_MAX_VARIATIONS,
+                        rb_class_path(klass)
+                    );
+                }
+            }
         }
     }
 

test/ruby/test_object.rb

@@ -422,6 +422,18 @@ class TestObject < Test::Unit::TestCase
     assert_equal(1+3+5+7+9, n)
   end
 
+  def test_max_shape_variation_with_performance_warnings
+    assert_in_out_err([], <<-INPUT, %w(), /Maximum shapes variations \(8\) reached by Foo, instance variables accesses will be slower\.$/)
+      $VERBOSE = true
+      Warning[:performance] = true
+
+      class Foo; end
+      10.times do |i|
+        Foo.new.instance_variable_set(:"@a\#{i}", nil)
+      end
+    INPUT
+  end
+
   def test_redefine_method_under_verbose
     assert_in_out_err([], <<-INPUT, %w(2), /warning: method redefined; discarding old foo$/)
       $VERBOSE = true

test/ruby/test_rubyoptions.rb

@@ -99,19 +99,24 @@ class TestRubyOptions < Test::Unit::TestCase
     assert_in_out_err(%w(-W:no-deprecated -e) + ['p Warning[:deprecated]'], "", %w(false), [])
     assert_in_out_err(%w(-W:experimental -e) + ['p Warning[:experimental]'], "", %w(true), [])
     assert_in_out_err(%w(-W:no-experimental -e) + ['p Warning[:experimental]'], "", %w(false), [])
+    assert_in_out_err(%w(-W -e) + ['p Warning[:performance]'], "", %w(false), [])
+    assert_in_out_err(%w(-W:performance -e) + ['p Warning[:performance]'], "", %w(true), [])
     assert_in_out_err(%w(-W:qux), "", [], /unknown warning category: `qux'/)
     assert_in_out_err(%w(-w -e) + ['p Warning[:deprecated]'], "", %w(true), [])
     assert_in_out_err(%w(-W -e) + ['p Warning[:deprecated]'], "", %w(true), [])
     assert_in_out_err(%w(-We) + ['p Warning[:deprecated]'], "", %w(true), [])
     assert_in_out_err(%w(-e) + ['p Warning[:deprecated]'], "", %w(false), [])
-    code = 'puts "#{$VERBOSE}:#{Warning[:deprecated]}:#{Warning[:experimental]}"'
+    assert_in_out_err(%w(-w -e) + ['p Warning[:performance]'], "", %w(false), [])
+    assert_in_out_err(%w(-W -e) + ['p Warning[:performance]'], "", %w(false), [])
+    code = 'puts "#{$VERBOSE}:#{Warning[:deprecated]}:#{Warning[:experimental]}:#{Warning[:performance]}"'
     Tempfile.create(["test_ruby_test_rubyoption", ".rb"]) do |t|
       t.puts code
       t.close
-      assert_in_out_err(["-r#{t.path}", '-e', code], "", %w(false:false:true false:false:true), [])
-      assert_in_out_err(["-r#{t.path}", '-w', '-e', code], "", %w(true:true:true true:true:true), [])
-      assert_in_out_err(["-r#{t.path}", '-W:deprecated', '-e', code], "", %w(false:true:true false:true:true), [])
-      assert_in_out_err(["-r#{t.path}", '-W:no-experimental', '-e', code], "", %w(false:false:false false:false:false), [])
+      assert_in_out_err(["-r#{t.path}", '-e', code], "", %w(false:false:true:false false:false:true:false), [])
+      assert_in_out_err(["-r#{t.path}", '-w', '-e', code], "", %w(true:true:true:false true:true:true:false), [])
+      assert_in_out_err(["-r#{t.path}", '-W:deprecated', '-e', code], "", %w(false:true:true:false false:true:true:false), [])
+      assert_in_out_err(["-r#{t.path}", '-W:no-experimental', '-e', code], "", %w(false:false:false:false false:false:false:false), [])
+      assert_in_out_err(["-r#{t.path}", '-W:performance', '-e', code], "", %w(false:false:true:true false:false:true:true), [])
     end
   ensure
     ENV['RUBYOPT'] = save_rubyopt