YJIT: Speculate block arg for c_func_method(&nil) calls

A good amount of call sites always pass nil as block argument, but the
nil doesn't show up in the context. Put a runtime guard for those
cases to handle it. Particular relevant for the `ruby-lsp` benchmark in
`yjit-bench`. Up to a 2% speedup across headline benchmarks.

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>
Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>
Co-authored-by: Kevin Menard <kevin@nirvdrum.com>
Co-authored-by: Randy Stauner <randy.stauner@shopify.com>

yjit/src/codegen.rs

@@ -6949,41 +6949,33 @@ fn gen_send_cfunc(
         return None;
     }
 
-    let block_arg_type = if block_arg {
+    let mut block_arg_type = if block_arg {
         Some(asm.ctx.get_opnd_type(StackOpnd(0)))
     } else {
         None
     };
 
     match block_arg_type {
         Some(Type::Nil | Type::BlockParamProxy) => {
-            // We'll handle this later
-        }
-        None => {
-            // Nothing to do
-        }
-        _ => {
-            gen_counter_incr(jit, asm, Counter::send_cfunc_block_arg);
-            return None;
-        }
-    }
-
-    match block_arg_type {
-        Some(Type::Nil) => {
-            // We have a nil block arg, so let's pop it off the args
+            // We don't need the actual stack value for these
             asm.stack_pop(1);
         }
-        Some(Type::BlockParamProxy) => {
-            // We don't need the actual stack value
+        Some(Type::Unknown | Type::UnknownImm) if jit.peek_at_stack(&asm.ctx, 0).nil_p() => {
+            // The sample blockarg is nil, so speculate that's the case.
+            asm.cmp(asm.stack_opnd(0), Qnil.into());
+            asm.jne(Target::side_exit(Counter::guard_send_cfunc_block_not_nil));
+            block_arg_type = Some(Type::Nil);
             asm.stack_pop(1);
         }
         None => {
             // Nothing to do
         }
         _ => {
-            assert!(false);
+            gen_counter_incr(jit, asm, Counter::send_cfunc_block_arg);
+            return None;
         }
     }
+    let block_arg_type = block_arg_type; // drop `mut`
 
     // Pop the empty kw_splat hash
     if kw_splat {

yjit/src/stats.rs

@@ -470,6 +470,7 @@ make_counters! {
     guard_send_str_aref_not_fixnum,
 
     guard_send_cfunc_bad_splat_vargs,
+    guard_send_cfunc_block_not_nil,
 
     guard_invokesuper_me_changed,