Optimize continuation pointer management

The BEAM instructions for calling a function don't save the
continuation pointer (return address) on the stack, but to a special
BEAM register called CP. It is the responsibility of the called
function to save CP to the stack frame before calling other functions.

In the earlier implementations of BEAM on Sparc, CP was located in a
CPU register. That meant that the continuation pointer was never
written to memory when calling simple functions that didn't call
other functions at all or ended in a tail-call to another function.

The modern BEAM no longer keeps CP in CPU register. Instead, it is
kept in the `process` struct (in `p->cp`). That means the continuation
pointer must be written to the memory on every call, and if the called
function will call other functions, it will must read the continuation
pointer from `p->cp` and store it on the stack.

This commit eliminates the concept of the CP register and modifies
the call instructions to directly store the continuation pointer on
the stack. That makes allocation and trimming of stack frames slightly
faster. A more important benefit is simplification of code that handles
continuation pointers. Because all continuation pointers are now stored
on the stack, the special case of handling `p->cp` disappears.

Co-authored-by: John Högberg <[email protected]>

1 files changed, 0 insertions, 1 deletions

diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
index 405611c584..a7a6528f92 100644
--- a/erts/emulator/beam/erl_process.h
+++ b/erts/emulator/beam/erl_process.h
@@ -975,7 +975,6 @@ struct process {
     unsigned max_arg_reg;	/* Maximum number of argument registers available. */
     Eterm def_arg_reg[6];	/* Default array for argument registers. */
 
-    BeamInstr* cp;		/* (untagged) Continuation pointer (for threaded code). */
     BeamInstr* i;		/* Program counter for threaded code. */
     Sint catches;		/* Number of catches on stack */
     Sint fcalls;		/*