Refactor getclassvariable (#5137)

* Refactor getclassvariable

We only need the cref when we have a cache miss so don't look it up until we
need it. This speeds up class variable reads in the interpreter but
also simplifies the jit code.

Benchmarks for master vs this branch (without yjit):

Before:

```
Warming up --------------------------------------
         read a cvar     1.276M i/100ms
Calculating -------------------------------------
         read a cvar     12.596M (± 1.7%) i/s -     63.781M in   5.064902s
```

After:

```
Warming up --------------------------------------
         read a cvar     1.336M i/100ms
Calculating -------------------------------------
         read a cvar     13.114M (± 3.6%) i/s -     65.488M in   5.000584s
```

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

* Clean up function signatures / remove dead code

rb_vm_getclassvariable signature has changed and we don't need
rb_vm_get_cref.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/insns.def b/insns.def
index 7dfeed202b..018e3112ba 100644
--- a/insns.def
+++ b/insns.def
@@ -236,9 +236,8 @@ getclassvariable
 /* "class variable access from toplevel" warning can be hooked. */
 // attr bool leaf = false; /* has rb_warning() */
 {
-    rb_cref_t * cref = vm_get_cref(GET_EP());
     rb_control_frame_t *cfp = GET_CFP();
-    val = vm_getclassvariable(GET_ISEQ(), cref, cfp, id, (ICVARC)ic);
+    val = vm_getclassvariable(GET_ISEQ(), cfp, id, (ICVARC)ic);
 }
 
 /* Set value of class variable id of klass as val. */