YJIT: Relax --yjit-verify-ctx after singleton class creation

Types like `Type::CString` really only assert that at one point the object had
its class field equal to `String`. Once a singleton class is created for any
strings, the type makes no assertion about any class field anymore, and becomes
the same as `Type::TString`.

Previously, the `--yjit-verify-ctx` option wasn't allowing objects of these
kind that have have singleton classes to pass verification even though the code
generators handle it just fine.

Found through `ruby/spec`.

bootstraptest/test_yjit.rb

@@ -4830,6 +4830,20 @@ assert_equal "abc", %q{
   str
 }
 
+# test --yjit-verify-ctx for arrays with a singleton class
+assert_equal "ok", %q{
+  class Array
+    def foo
+      self.singleton_class.define_method(:first) { :ok }
+      first
+    end
+  end
+
+  def test = [].foo
+
+  test
+}
+
 assert_equal '["raised", "Module", "Object"]', %q{
   def foo(obj)
     obj.superclass.name

yjit/src/codegen.rs

@@ -567,14 +567,36 @@ fn verify_ctx(jit: &JITState, ctx: &Context) {
         unsafe { CStr::from_ptr(rb_obj_info(val)).to_str().unwrap() }
     }
 
+    // Some types such as CString only assert the class field of the object
+    // when there has never been a singleton class created for objects of that class.
+    // Once there is a singleton class created they become their weaker
+    // `T*` variant, and we more objects should pass the verification.
+    fn relax_type_with_singleton_class_assumption(ty: Type) -> Type {
+        if let Type::CString | Type::CArray | Type::CHash = ty {
+            if has_singleton_class_of(ty.known_class().unwrap()) {
+                match ty {
+                    Type::CString => return Type::TString,
+                    Type::CArray => return Type::TArray,
+                    Type::CHash => return Type::THash,
+                    _ => (),
+                }
+            }
+        }
+
+        ty
+    }
+
     // Only able to check types when at current insn
     assert!(jit.at_current_insn());
 
     let self_val = jit.peek_at_self();
     let self_val_type = Type::from(self_val);
+    let learned_self_type = ctx.get_opnd_type(SelfOpnd);
+    let learned_self_type = relax_type_with_singleton_class_assumption(learned_self_type);
+
 
     // Verify self operand type
-    if self_val_type.diff(ctx.get_opnd_type(SelfOpnd)) == TypeDiff::Incompatible {
+    if self_val_type.diff(learned_self_type) == TypeDiff::Incompatible {
         panic!(
             "verify_ctx: ctx self type ({:?}) incompatible with actual value of self {}",
             ctx.get_opnd_type(SelfOpnd),
@@ -587,6 +609,7 @@ fn verify_ctx(jit: &JITState, ctx: &Context) {
     for i in 0..top_idx {
         let learned_mapping = ctx.get_opnd_mapping(StackOpnd(i));
         let learned_type = ctx.get_opnd_type(StackOpnd(i));
+        let learned_type = relax_type_with_singleton_class_assumption(learned_type);
 
         let stack_val = jit.peek_at_stack(ctx, i as isize);
         let val_type = Type::from(stack_val);
@@ -632,6 +655,7 @@ fn verify_ctx(jit: &JITState, ctx: &Context) {
     let top_idx: usize = cmp::min(local_table_size as usize, MAX_TEMP_TYPES);
     for i in 0..top_idx {
         let learned_type = ctx.get_local_type(i);
+        let learned_type = relax_type_with_singleton_class_assumption(learned_type);
         let local_val = jit.peek_at_local(i as i32);
         let local_type = Type::from(local_val);
 

yjit/src/core.rs

@@ -53,11 +53,11 @@ pub enum Type {
     ImmSymbol,
 
     TString, // An object with the T_STRING flag set, possibly an rb_cString
-    CString, // An un-subclassed string of type rb_cString (can have instance vars in some cases)
+    CString, // An object that at one point had its class field equal rb_cString (creating a singleton class changes it)
     TArray, // An object with the T_ARRAY flag set, possibly an rb_cArray
-    CArray, // An un-subclassed array of type rb_cArray (can have instance vars in some cases)
+    CArray, // An object that at one point had its class field equal rb_cArray (creating a singleton class changes it)
     THash, // An object with the T_HASH flag set, possibly an rb_cHash
-    CHash, // An un-subclassed hash of type rb_cHash (can have instance vars in some cases)
+    CHash, // An object that at one point had its class field equal rb_cHash (creating a singleton class changes it)
 
     BlockParamProxy, // A special sentinel value indicating the block parameter should be read from
                      // the current surrounding cfp