YJIT: Support empty splat and some block_arg calls to ivar getters

These seem odd at first glance, but they're used with `...` calls with
`Module#delegate` from Active Support. These account for ~3% of fallback
reasons in the `lobsters` benchmark.

test/ruby/test_yjit.rb

@@ -1514,6 +1514,24 @@ def test_disable_stats
     assert_in_out_err(%w[--yjit-stats --yjit-disable])
   end
 
+  def test_odd_calls_to_attr_reader
+    # Use of delegate from ActiveSupport use these kind of calls to getter methods.
+    assert_compiles(<<~RUBY, result: [1, 1, 1], no_send_fallbacks: true)
+      class One
+        attr_reader :one
+        def initialize
+          @one = 1
+        end
+      end
+
+      def calls(obj, empty, &)
+        [obj.one(*empty), obj.one(&), obj.one(*empty, &)]
+      end
+
+      calls(One.new, [])
+    RUBY
+  end
+
   private
 
   def code_gc_helpers
@@ -1537,7 +1555,17 @@ def assert_no_exits(script)
   end
 
   ANY = Object.new
-  def assert_compiles(test_script, insns: [], call_threshold: 1, stdout: nil, exits: {}, result: ANY, frozen_string_literal: nil, mem_size: nil, code_gc: false)
+  def assert_compiles(
+    test_script, insns: [],
+    call_threshold: 1,
+    stdout: nil,
+    exits: {},
+    result: ANY,
+    frozen_string_literal: nil,
+    mem_size: nil,
+    code_gc: false,
+    no_send_fallbacks: false
+  )
     reset_stats = <<~RUBY
       RubyVM::YJIT.runtime_stats
       RubyVM::YJIT.reset_stats!
@@ -1610,6 +1638,10 @@ def collect_insns(iseq)
       end
     end
 
+    if no_send_fallbacks
+      assert_equal(0, runtime_stats[:num_send_dynamic], "Expected no use of fallback implementation")
+    end
+
     # Only available when --enable-yjit=dev
     if runtime_stats[:all_stats]
       missed_insns = insns.dup

yjit/src/codegen.rs

@@ -7307,21 +7307,43 @@ fn gen_send_general(
                 );
             }
             VM_METHOD_TYPE_IVAR => {
-                if flags & VM_CALL_ARGS_SPLAT != 0 {
-                    gen_counter_incr(asm, Counter::send_args_splat_ivar);
+                // This is a .send call not supported right now for getters
+                if flags & VM_CALL_OPT_SEND != 0 {
+                    gen_counter_incr(asm, Counter::send_send_getter);
                     return None;
                 }
 
-                if argc != 0 {
-                    // Argument count mismatch. Getters take no arguments.
-                    gen_counter_incr(asm, Counter::send_getter_arity);
-                    return None;
+                if flags & VM_CALL_ARGS_BLOCKARG != 0 {
+                    match asm.ctx.get_opnd_type(StackOpnd(0)) {
+                        Type::Nil | Type::BlockParamProxy => {
+                            // Getters ignore the block arg, and these types of block args can be
+                            // passed without side-effect (never any `to_proc` call).
+                            asm.stack_pop(1);
+                        }
+                        _ => {
+                            gen_counter_incr(asm, Counter::send_getter_block_arg);
+                            return None;
+                        }
+                    }
                 }
 
-                // This is a .send call not supported right now for getters
-                if flags & VM_CALL_OPT_SEND != 0 {
-                    gen_counter_incr(asm, Counter::send_send_getter);
-                    return None;
+                if argc != 0 {
+                    // Guard for simple splat of empty array
+                    if VM_CALL_ARGS_SPLAT == flags & (VM_CALL_ARGS_SPLAT | VM_CALL_KWARG | VM_CALL_KW_SPLAT)
+                        && argc == 1 {
+                        // Not using chain guards since on failure these likely end up just raising
+                        // ArgumentError
+                        let splat = asm.stack_opnd(0);
+                        guard_object_is_array(asm, splat, splat.into(), Counter::guard_send_getter_splat_non_empty);
+                        let splat_len = get_array_len(asm, splat);
+                        asm.cmp(splat_len, 0.into());
+                        asm.jne(Target::side_exit(Counter::guard_send_getter_splat_non_empty));
+                        asm.stack_pop(1);
+                    } else {
+                        // Argument count mismatch. Getters take no arguments.
+                        gen_counter_incr(asm, Counter::send_getter_arity);
+                        return None;
+                    }
                 }
 
                 if c_method_tracing_currently_enabled(jit) {
@@ -7338,13 +7360,9 @@ fn gen_send_general(
                     return None;
                 }
 
+                let recv = asm.stack_opnd(0); // the receiver should now be the stack top
                 let ivar_name = unsafe { get_cme_def_body_attr_id(cme) };
 
-                if flags & VM_CALL_ARGS_BLOCKARG != 0 {
-                    gen_counter_incr(asm, Counter::send_getter_block_arg);
-                    return None;
-                }
-
                 return gen_get_ivar(
                     jit,
                     asm,
@@ -7353,7 +7371,7 @@ fn gen_send_general(
                     comptime_recv,
                     ivar_name,
                     recv,
-                    recv_opnd,
+                    recv.into(),
                 );
             }
             VM_METHOD_TYPE_ATTRSET => {

yjit/src/stats.rs

@@ -400,6 +400,7 @@ make_counters! {
 
     // Method calls that exit to the interpreter
     guard_send_block_arg_type,
+    guard_send_getter_splat_non_empty,
     guard_send_klass_megamorphic,
     guard_send_se_cf_overflow,
     guard_send_se_protected_check_failed,