ZJIT: Annotate NilClass#nil? and Kernel#nil?

These methods return fixed `true` or `false` so we can be certain about their return types.

Author: st0012

test/ruby/test_zjit.rb

@@ -840,6 +840,20 @@ def test(x)
     }, call_threshold: 1, insns: [:branchnil]
   end
 
+  def test_nil_nil
+    assert_compiles 'true', %q{
+      def test = nil.nil?
+      test
+    }, insns: [:opt_nil_p]
+  end
+
+  def test_non_nil_nil
+    assert_compiles 'false', %q{
+      def test = 1.nil?
+      test
+    }, insns: [:opt_nil_p]
+  end
+
   # tool/ruby_vm/views/*.erb relies on the zjit instructions a) being contiguous and
   # b) being reliably ordered after all the other instructions.
   def test_instruction_order

zjit/src/cruby_methods.rs

@@ -80,6 +80,8 @@ pub fn init() -> Annotations {
     annotate!(rb_cModule, "===", types::BoolExact, no_gc, leaf);
     annotate!(rb_cArray, "length", types::Fixnum, no_gc, leaf, elidable);
     annotate!(rb_cArray, "size", types::Fixnum, no_gc, leaf, elidable);
+    annotate!(rb_cNilClass, "nil?", types::TrueClassExact, no_gc, leaf, elidable);
+    annotate!(rb_mKernel, "nil?", types::FalseClassExact, no_gc, leaf, elidable);
 
     Annotations {
         cfuncs: std::mem::take(cfuncs)

zjit/src/hir.rs

@@ -6368,4 +6368,68 @@ mod opt_tests {
               Return v2
         "#]]);
     }
+
+    #[test]
+    fn test_nil_nil_specialized_to_ccall() {
+        eval("
+            def test = nil.nil?
+        ");
+        assert_optimized_method_hir("test", expect![[r#"
+            fn test:
+            bb0(v0:BasicObject):
+              v2:NilClassExact = Const Value(nil)
+              PatchPoint MethodRedefined(NilClass@0x1000, nil?@0x1008)
+              v7:TrueClassExact = CCall nil?@0x1010, v2
+              Return v7
+        "#]]);
+    }
+
+    #[test]
+    fn test_eliminate_nil_nil_specialized_to_ccall() {
+        eval("
+            def test
+              nil.nil?
+              1
+            end
+        ");
+        assert_optimized_method_hir("test", expect![[r#"
+            fn test:
+            bb0(v0:BasicObject):
+              PatchPoint MethodRedefined(NilClass@0x1000, nil?@0x1008)
+              v5:Fixnum[1] = Const Value(1)
+              Return v5
+        "#]]);
+    }
+
+    #[test]
+    fn test_non_nil_nil_specialized_to_ccall() {
+        eval("
+            def test = 1.nil?
+        ");
+        assert_optimized_method_hir("test", expect![[r#"
+            fn test:
+            bb0(v0:BasicObject):
+              v2:Fixnum[1] = Const Value(1)
+              PatchPoint MethodRedefined(Integer@0x1000, nil?@0x1008)
+              v7:FalseClassExact = CCall nil?@0x1010, v2
+              Return v7
+        "#]]);
+    }
+
+    #[test]
+    fn test_eliminate_non_nil_nil_specialized_to_ccall() {
+        eval("
+            def test
+              1.nil?
+              2
+            end
+        ");
+        assert_optimized_method_hir("test", expect![[r#"
+            fn test:
+            bb0(v0:BasicObject):
+              PatchPoint MethodRedefined(Integer@0x1000, nil?@0x1008)
+              v5:Fixnum[2] = Const Value(2)
+              Return v5
+        "#]]);
+    }
 }