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, 4 insertions, 11 deletions

diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
index e8c83276f5..4b4337ce17 100644
--- a/erts/emulator/beam/erl_process.c
+++ b/erts/emulator/beam/erl_process.c
@@ -11458,7 +11458,7 @@ erl_create_process(Process* parent, /* Parent of process (default group leader).
 #else
     arg_size = size_object_litopt(args, &litarea);
 #endif
-    heap_need = arg_size;
+    heap_need = arg_size + 1;   /* Reserve place for continuation pointer */
 
     p->flags = flags;
 
@@ -11507,7 +11507,8 @@ erl_create_process(Process* parent, /* Parent of process (default group leader).
     p->old_hend = p->old_htop = p->old_heap = NULL;
     p->high_water = p->heap;
     p->gen_gcs = 0;
-    p->stop = p->hend = p->heap + sz;
+    p->hend = p->heap + sz;
+    p->stop = p->hend - 1;     /* Reserve place for continuation pointer */
     p->htop = p->heap;
     p->heap_sz = sz;
     p->abandoned_heap = NULL;
@@ -11525,7 +11526,7 @@ erl_create_process(Process* parent, /* Parent of process (default group leader).
     p->current = &p->u.initial;
 
     p->i = (BeamInstr *) beam_apply;
-    p->cp = (BeamInstr *) beam_apply+1;
+    p->stop[0] = make_cp(beam_apply + 1);
 
     p->arg_reg = p->def_arg_reg;
     p->max_arg_reg = sizeof(p->def_arg_reg)/sizeof(p->def_arg_reg[0]);
@@ -11839,7 +11840,6 @@ void erts_init_empty_process(Process *p)
     p->u.initial.function = 0;
     p->u.initial.arity = 0;
     p->catches = 0;
-    p->cp = NULL;
     p->i = NULL;
     p->current = NULL;
 
@@ -11917,7 +11917,6 @@ erts_debug_verify_clean_empty_process(Process* p)
     ASSERT(p->bif_timers == NULL);
     ASSERT(p->dictionary == NULL);
     ASSERT(p->catches == 0);
-    ASSERT(p->cp == NULL);
     ASSERT(p->i == NULL);
     ASSERT(p->current == NULL);
 
@@ -13146,9 +13145,6 @@ erts_program_counter_info(fmtfn_t to, void *to_arg, Process *p)
     erts_print(to, to_arg, "Program counter: %p (", p->i);
     print_function_from_pc(to, to_arg, p->i);
     erts_print(to, to_arg, ")\n");
-    erts_print(to, to_arg, "CP: %p (", p->cp);
-    print_function_from_pc(to, to_arg, p->cp);
-    erts_print(to, to_arg, ")\n");
     state = erts_atomic32_read_acqb(&p->state);
     if (!(state & (ERTS_PSFLG_RUNNING
 		   | ERTS_PSFLG_RUNNING_SYS
@@ -13425,9 +13421,6 @@ static void print_current_process_info(fmtfn_t to, void *to_arg,
 	erts_print(to, to_arg, "Current Process Program counter: %p (", p->i);
 	print_function_from_pc(to, to_arg, p->i);
 	erts_print(to, to_arg, ")\n");
-	erts_print(to, to_arg, "Current Process CP: %p (", p->cp);
-	print_function_from_pc(to, to_arg, p->cp);
-	erts_print(to, to_arg, ")\n");
 
 	/* Getting this stacktrace can segfault if we are very very
 	   unlucky if called while a process is being garbage collected.