YJIT: Leave cfp->pc uninitialized for VM_FRAME_MAGIC_CFUNC

C function frames don't need to use the VM-specific pc field to run
properly. When pushing a control frame from output code, save one
instruction by leaving the field uninitialized.

Fix-up rb_vm_svar_lep(), which is used while setting local variables via
Regexp#=~. Use cfp->iseq as a secondary signal so it can stop assuming
that all CFUNC frames always have zero pc's.

1 files changed, 5 insertions, 1 deletions

diff --git a/yjit/src/codegen.rs b/yjit/src/codegen.rs
index 577031c86e..2ea64871b3 100644
--- a/yjit/src/codegen.rs
+++ b/yjit/src/codegen.rs
@@ -5095,7 +5095,11 @@ fn gen_send_cfunc(
         cme,
         recv,
         sp,
-        pc: Some(0),
+        pc: if cfg!(debug_assertions) {
+            Some(!0) // Poison value. Helps to fail fast.
+        } else {
+            None     // Leave PC uninitialized as cfuncs shouldn't read it
+        },
         iseq: None,
         local_size: 0,
     });