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, 6 insertions, 24 deletions

diff --git a/erts/etc/unix/etp-commands.in b/erts/etc/unix/etp-commands.in
index 20809d61e8..283f859651 100644
--- a/erts/etc/unix/etp-commands.in
+++ b/erts/etc/unix/etp-commands.in
@@ -1187,12 +1187,14 @@ document etp-cp
 %---------------------------------------------------------------------------
 % etp-cp Eterm
 % 
-% Take a code continuation pointer and print 
+% Take a code continuation pointer or instruction pointer and print 
 % module, function, arity and offset. 
 % 
-% Code continuation pointers can be found in the process structure e.g
-% process_tab[i]->cp and process_tab[i]->i, the second is the
-% program counter, which is the same thing as a continuation pointer.
+% Code continuation pointers can be found on the stack. The instruction
+% pointer is found in the process struct. For example:
+%
+%    c_p->i
+%    process_tab[i]->i
 %---------------------------------------------------------------------------
 end
 
@@ -1462,10 +1464,6 @@ define etp-stack-preamble
     printf "I: "
     etp ((Eterm)($arg0)->i)
   end
-  if ($arg0)->cp != 0
-    printf "cp:"
-    etp ((Eterm)($arg0)->cp)
-  end
 end
 
 define etp-stack-preamble-emu
@@ -1474,10 +1472,6 @@ define etp-stack-preamble-emu
   printf "%% Stacktrace (%u)\n", $etp_stack_end-$etp_stack_p
   printf "I: "
   etp ((BeamInstr)I)
-  if ($arg0)->cp != 0
-    printf "cp: "
-    etp ((Eterm)($arg0)->cp)
-  end
 end
 
 define etp-stacktrace-1
@@ -2183,13 +2177,6 @@ define etp-process-info-int
   else
     printf "unknown\n"
   end
-  printf "  CP: "
-  if ($etp_proc->cp)
-    etp-cp-1 $etp_proc->cp
-    printf "\n"
-  else
-    printf "unknown\n"
-  end
   printf "  I: "
   if ($etp_proc->i)
     etp-cp-1 $etp_proc->i
@@ -2419,11 +2406,6 @@ define etp-process-memory-info
       end
     end
 
-    if ($etp_pmem_proc->cp)
-      printf " CP: "
-      etp-cp-1 $etp_pmem_proc->cp
-      printf " "
-    end
     printf "\n"
   end
 end