RJIT: Upgrade type to String after guard

lib/ruby_vm/rjit/insn_compiler.rb

@@ -2995,7 +2995,7 @@ module RubyVM::RJIT
 
       # Guard that the concat argument is a string
       asm.mov(:rax, ctx.stack_opnd(0))
-      guard_object_is_string(asm, :rax, :rcx, side_exit)
+      guard_object_is_string(jit, ctx, asm, :rax, :rcx, StackOpnd[0])
 
       # Guard buffers from GC since rb_str_buf_append may allocate. During the VM lock on GC,
       # other Ractors may trigger global invalidation, so we need ctx.clear_local_types.
@@ -3499,7 +3499,20 @@ module RubyVM::RJIT
       end
     end
 
-    def guard_object_is_string(asm, object_reg, flags_reg, side_exit)
+    # @param jit [RubyVM::RJIT::JITState]
+    # @param ctx [RubyVM::RJIT::Context]
+    # @param asm [RubyVM::RJIT::Assembler]
+    def guard_object_is_string(jit, ctx, asm, object_reg, flags_reg, object_opnd, counter = nil)
+      object_type = ctx.get_opnd_type(object_opnd)
+      if object_type.string?
+        return
+      end
+
+      guard_object_is_heap(jit, ctx, asm, object_reg, object_opnd, counter)
+
+      side_exit = side_exit(jit, ctx)
+      side_exit = counted_exit(side_exit, counter) if counter
+
       asm.comment('guard object is string')
       # Pull out the type mask
       asm.mov(flags_reg, [object_reg, C.RBasic.offsetof(:flags)])
@@ -3508,6 +3521,10 @@ module RubyVM::RJIT
       # Compare the result with T_STRING
       asm.cmp(flags_reg, C::RUBY_T_STRING)
       asm.jne(side_exit)
+
+      if object_type.diff(Type::TString) != TypeDiff::Incompatible
+        ctx.upgrade_opnd_type(object_opnd, Type::TString)
+      end
     end
 
     # clobbers object_reg

lib/ruby_vm/rjit/type.rb

@@ -39,6 +39,15 @@ module RubyVM::RJIT
       end
     end
 
+    # Check if it's a T_STRING object (both TString and CString are T_STRING)
+    def string?
+      case self
+      in Type::TString then true
+      in Type::CString then true
+      else false
+      end
+    end
+
     # Returns a boolean representing whether the value is truthy if known, otherwise nil
     def known_truthy
       case self