_mjit_compile_ivar.erb: cancel on undefined ivar

I assumed somehow this check was not needed, but it did need.
By canceling this instead of just warning here, we didn't lose the
current performance so much.

test_jit.rb: test the case that reproduces SEGV by that.

TestGemStreamUI.rb: delete. This test on --jit-wait is fixed.

=== Optcarrot Benchmark ===

$ benchmark-driver benchmark.yml --rbenv 'before::before --disable-gems --jit;after::after --disable-gems --jit' -v --repeat-count 24
before: ruby 2.6.0dev (2018-10-24 trunk 65355) +JIT [x86_64-linux]
after: ruby 2.6.0dev (2018-10-24 trunk 65355) +JIT [x86_64-linux]
last_commit=_mjit_compile_ivar.erb: cancel on undefined ivar
Calculating -------------------------------------
                             before       after
Optcarrot Lan_Master.nes     85.344      84.849 fps

Comparison:
             Optcarrot Lan_Master.nes
                  before:        85.3 fps
                   after:        84.8 fps - 1.01x  slower

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65356 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/excludes/_wercker/test-mjit-wait/TestGemStreamUI.rb

@@ -1,2 +0,0 @@
-# debugging this right now
-exclude(/.*/, 'this fails with --jit-wait')

test/ruby/test_jit.rb

@@ -669,6 +669,27 @@ class TestJIT < Test::Unit::TestCase
     end;
   end
 
+  def test_inlined_undefined_ivar
+    assert_eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", stdout: "bbb", success_count: 2, min_calls: 3)
+    begin;
+      class Foo
+        def initialize
+          @a = :a
+        end
+
+        def bar
+          if @b.nil?
+            @b = :b
+          end
+        end
+      end
+
+      print(Foo.new.bar)
+      print(Foo.new.bar)
+      print(Foo.new.bar)
+    end;
+  end
+
   def test_attr_reader
     assert_eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", stdout: "4nil\nnil\n6", success_count: 2, min_calls: 2)
     begin;

tool/ruby_vm/views/_mjit_compile_ivar.erb

@@ -26,19 +26,19 @@
         fprintf(f, "    VALUE obj = GET_SELF();\n");
         fprintf(f, "    const rb_serial_t ic_serial = (rb_serial_t)%"PRI_SERIALT_PREFIX"u;\n", ic_copy->ic_serial);
         fprintf(f, "    const st_index_t index = %"PRIuSIZE";\n", ic_copy->ic_value.index);
+% # JIT: cache hit path of vm_getivar, or cancel JIT.
 % if insn.name == 'setinstancevariable'
         fprintf(f, "    VALUE val = stack[%d];\n", b->stack_size - 1);
-% end
-%
-% # JIT: cache hit path of vm_getivar, or cancel JIT.
         fprintf(f, "    if (LIKELY(RB_TYPE_P(obj, T_OBJECT) && ic_serial == RCLASS_SERIAL(RBASIC(obj)->klass) && index < ROBJECT_NUMIV(obj))) {\n");
-% if insn.name == 'setinstancevariable'
         fprintf(f, "        VALUE *ptr = ROBJECT_IVPTR(obj);\n");
         fprintf(f, "        RB_OBJ_WRITE(obj, &ptr[index], val);\n");
-% else
-        fprintf(f, "        stack[%d] = ROBJECT_IVPTR(obj)[index];\n", b->stack_size);
-% end
         fprintf(f, "    }\n");
+% else
+        fprintf(f, "    VALUE val;\n");
+        fprintf(f, "    if (LIKELY(RB_TYPE_P(obj, T_OBJECT) && ic_serial == RCLASS_SERIAL(RBASIC(obj)->klass) && index < ROBJECT_NUMIV(obj) && (val = ROBJECT_IVPTR(obj)[index]) != Qundef)) {\n");
+        fprintf(f, "        stack[%d] = val;\n", b->stack_size);
+        fprintf(f, "    }\n");
+% end
         fprintf(f, "    else {\n");
         fprintf(f, "        reg_cfp->pc = original_body_iseq + %d;\n", pos);
         fprintf(f, "        reg_cfp->sp = (VALUE *)reg_cfp->bp + %d;\n", b->stack_size + 1);