YJIT: Fastpath for Module#name

Module#name shows up as a top C method callee in lobsters so probably
common enough. It's also easy to substitute thanks to rb_mod_name()
already having no GC yield points.

    klass = BasicObject
    50_000_000.times { klass.name }

    Benchmark 1: /.rubies/post/bin/ruby --yjit mod_name.rb
      Time (mean ± σ):      1.433 s ±  0.010 s    [User: 1.410 s, System: 0.010 s]
      Range (min … max):    1.421 s …  1.449 s    10 runs

    Benchmark 2: /.rubies/mstr/bin/ruby --yjit mod_name.rb
      Time (mean ± σ):      1.491 s ±  0.012 s    [User: 1.468 s, System: 0.010 s]
      Range (min … max):    1.470 s …  1.511 s    10 runs

    Summary
      /.rubies/post/bin/ruby --yjit mod_name.rb ran
        1.04 ± 0.01 times faster than /.rubies/mstr/bin/ruby --yjit mod_name.rb

variable.c

@@ -97,6 +97,8 @@ rb_namespace_p(VALUE obj)
  * to not be anonymous. <code>*permanent</code> is set to 1
  * if +classpath+ has no anonymous components. There is no builtin
  * Ruby level APIs that can change a permanent +classpath+.
+ *
+ * YJIT needs this function to not allocate.
  */
 static VALUE
 classname(VALUE klass, bool *permanent)
@@ -127,6 +129,7 @@ rb_mod_name0(VALUE klass, bool *permanent)
 VALUE
 rb_mod_name(VALUE mod)
 {
+    // YJIT needs this function to not allocate.
     bool permanent;
     return classname(mod, &permanent);
 }

yjit/bindgen/src/main.rs

@@ -378,6 +378,7 @@ fn main() {
         .allowlist_function("rb_attr_get")
         .allowlist_function("rb_ivar_defined")
         .allowlist_function("rb_ivar_get")
+        .allowlist_function("rb_mod_name")
 
         // From internal/vm.h
         .allowlist_var("rb_vm_insns_count")

yjit/src/codegen.rs

@@ -5135,6 +5135,33 @@ fn jit_rb_mod_eqq(
     return true;
 }
 
+// Substitution for rb_mod_name(). Returns the name of a module/class.
+fn jit_rb_mod_name(
+    _jit: &mut JITState,
+    asm: &mut Assembler,
+    _ci: *const rb_callinfo,
+    _cme: *const rb_callable_method_entry_t,
+    _block: Option<BlockHandler>,
+    argc: i32,
+    _known_recv_class: Option<VALUE>,
+) -> bool {
+    if argc != 0 {
+        return false;
+    }
+
+    asm_comment!(asm, "Module#name");
+
+    // rb_mod_name() never allocates, so no preparation needed.
+    let name = asm.ccall(rb_mod_name as _, vec![asm.stack_opnd(0)]);
+
+    let _ = asm.stack_pop(1); // pop self
+    // call-seq: mod.name -> string or nil
+    let ret = asm.stack_push(Type::Unknown);
+    asm.mov(ret, name);
+
+    true
+}
+
 // Codegen for rb_obj_equal()
 // object identity comparison
 fn jit_rb_obj_equal(
@@ -10373,6 +10400,7 @@ pub fn yjit_reg_method_codegen_fns() {
         yjit_reg_method(rb_mKernel, "eql?", jit_rb_obj_equal);
         yjit_reg_method(rb_cModule, "==", jit_rb_obj_equal);
         yjit_reg_method(rb_cModule, "===", jit_rb_mod_eqq);
+        yjit_reg_method(rb_cModule, "name", jit_rb_mod_name);
         yjit_reg_method(rb_cSymbol, "==", jit_rb_obj_equal);
         yjit_reg_method(rb_cSymbol, "===", jit_rb_obj_equal);
         yjit_reg_method(rb_cInteger, "==", jit_rb_int_equal);

yjit/src/cruby_bindings.inc.rs

@@ -1036,6 +1036,7 @@ extern "C" {
     pub fn rb_str_buf_append(dst: VALUE, src: VALUE) -> VALUE;
     pub fn rb_str_dup(str_: VALUE) -> VALUE;
     pub fn rb_str_intern(str_: VALUE) -> VALUE;
+    pub fn rb_mod_name(mod_: VALUE) -> VALUE;
     pub fn rb_ivar_get(obj: VALUE, name: ID) -> VALUE;
     pub fn rb_ivar_defined(obj: VALUE, name: ID) -> VALUE;
     pub fn rb_attr_get(obj: VALUE, name: ID) -> VALUE;