Remove interpreter’s byte stack

This improves performance overall on my benchmark on x86-64,
since the interpreted program-counter resides in a machine
register rather than in RAM.
* etc/DEBUG, src/.gdbinit: Remove xbytecode GDB command, as there
is no longer a byte stack to decode.
* src/bytecode.c (struct byte_stack, byte_stack_list)
(relocate_byte_stack): Remove.  All uses removed.
(FETCH): Simplify now that pc is now local (typically, in a
register) and no longer needs to be relocated.
(CHECK_RANGE): Remove.  All uses now done inline, in a different way.
(BYTE_CODE_QUIT): Remove; now done by op_relative_branch.
(exec_byte_code): Allocate a copy of the function’s bytecode,
so that there is no problem if GC moves it.
* src/lisp.h (struct handler): Remove byte_stack member.
All uses removed.
(SAFE_ALLOCA_LISP_EXTRA): New macro, a generalization of
SAFE_ALLOCA_LISP.
(SAFE_ALLOCA_LISP): Use it.

1 files changed, 44 insertions, 144 deletions

diff --git a/src/bytecode.c b/src/bytecode.c
index 0c5b8494d0c..995d37c09d5 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -281,59 +281,9 @@ enum byte_code_op
 #endif
 };
 
-/* Structure describing a value stack used during byte-code execution
-   in Fbyte_code.  */
-
-struct byte_stack
-{
-  /* Program counter.  This points into the byte_string below
-     and is relocated when that string is relocated.  */
-  const unsigned char *pc;
-
-  /* The string containing the byte-code, and its current address.
-     Storing this here protects it from GC.  */
-  Lisp_Object byte_string;
-  const unsigned char *byte_string_start;
-
-  /* Next entry in byte_stack_list.  */
-  struct byte_stack *next;
-};
-
-/* A list of currently active byte-code execution value stacks.
-   Fbyte_code adds an entry to the head of this list before it starts
-   processing byte-code, and it removes the entry again when it is
-   done.  Signaling an error truncates the list.  */
-
-struct byte_stack *byte_stack_list;
-
-
-/* Relocate program counters in the stacks on byte_stack_list.  Called
-   when GC has completed.  */
-
-void
-relocate_byte_stack (void)
-{
-  struct byte_stack *stack;
-
-  for (stack = byte_stack_list; stack; stack = stack->next)
-    {
-      if (stack->byte_string_start != SDATA (stack->byte_string))
-        {
-          ptrdiff_t offset = stack->pc - stack->byte_string_start;
-          stack->byte_string_start = SDATA (stack->byte_string);
-          stack->pc = stack->byte_string_start + offset;
-        }
-    }
-}
-
-
 /* Fetch the next byte from the bytecode stream.  */
 
-#if BYTE_CODE_SAFE
-#define FETCH (eassert (stack.byte_string_start == SDATA (stack.byte_string)), *stack.pc++)
-#else
-#define FETCH *stack.pc++
-#endif
+#define FETCH (*pc++)
 
 /* Fetch two bytes from the bytecode stream and make a 16-bit number
    out of them.  */
@@ -358,32 +308,6 @@ relocate_byte_stack (void)
 
 #define TOP (*top)
 
-/* Check for jumping out of range.  */
-
-#define CHECK_RANGE(ARG) \
-  (BYTE_CODE_SAFE && bytestr_length <= (ARG) ? emacs_abort () : (void) 0)
-
-/* A version of the QUIT macro which makes sure that the stack top is
-   set before signaling `quit'.  */
-
-#define BYTE_CODE_QUIT                                  \
-  do {                                                  \
-    if (quitcounter++)                                  \
-      break;                                            \
-    maybe_gc ();                                        \
-    if (!NILP (Vquit_flag) && NILP (Vinhibit_quit))     \
-      {                                                 \
-        Lisp_Object flag = Vquit_flag;                  \
-        Vquit_flag = Qnil;                              \
-        if (EQ (Vthrow_on_input, flag))                 \
-          Fthrow (Vthrow_on_input, Qt);                 \
-        quit ();                                        \
-      }                                                 \
-    else if (pending_signals)                           \
-      process_pending_signals ();                       \
-  } while (0)
-
-
 DEFUN ("byte-code", Fbyte_code, Sbyte_code, 3, 3, 0,
        doc: /* Function used internally in byte-compiled code.
 The first argument, BYTESTR, is a string of byte code;
@@ -423,7 +347,6 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
   Lisp_Object *vectorp;
   ptrdiff_t const_length;
   ptrdiff_t bytestr_length;
-  struct byte_stack stack;
   Lisp_Object *top;
   Lisp_Object result;
   enum handlertype type;
@@ -445,16 +368,16 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
   bytestr_length = SBYTES (bytestr);
   vectorp = XVECTOR (vector)->contents;
 
-  stack.byte_string = bytestr;
-  stack.pc = stack.byte_string_start = SDATA (bytestr);
-  unsigned char quitcounter = 0;
+  unsigned char quitcounter = 1;
   EMACS_INT stack_items = XFASTINT (maxdepth) + 1;
   Lisp_Object *stack_base;
-  SAFE_ALLOCA_LISP (stack_base, stack_items);
+  SAFE_ALLOCA_LISP_EXTRA (stack_base, stack_items, bytestr_length);
   Lisp_Object *stack_lim = stack_base + stack_items;
   top = stack_base;
-  stack.next = byte_stack_list;
-  byte_stack_list = &stack;
+  memcpy (stack_lim, SDATA (bytestr), bytestr_length);
+  void *void_stack_lim = stack_lim;
+  unsigned char const *bytestr_data = void_stack_lim;
+  unsigned char const *pc = bytestr_data;
   ptrdiff_t count = SPECPDL_INDEX ();
 
   if (!NILP (args_template))
@@ -608,11 +531,7 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
             op = FETCH2;
             v1 = POP;
             if (NILP (v1))
-              {
-                BYTE_CODE_QUIT;
-                CHECK_RANGE (op);
-                stack.pc = stack.byte_string_start + op;
-              }
+              goto op_branch;
             NEXT;
           }
 
@@ -791,10 +710,22 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
           NEXT;
 
         CASE (Bgoto):
-          BYTE_CODE_QUIT;
-          op = FETCH2;    /* pc = FETCH2 loses since FETCH2 contains pc++ */
-          CHECK_RANGE (op);
-          stack.pc = stack.byte_string_start + op;
+          op = FETCH2;
+        op_branch:
+          op -= pc - bytestr_data;
+        op_relative_branch:
+          if (BYTE_CODE_SAFE
+              && ! (bytestr_data - pc <= op
+                    && op < bytestr_data + bytestr_length - pc))
+            emacs_abort ();
+          quitcounter += op < 0;
+          if (!quitcounter)
+            {
+              quitcounter = 1;
+              maybe_gc ();
+              QUIT;
+            }
+          pc += op;
           NEXT;
 
         CASE (Bgotoifnonnil):
@@ -803,51 +734,35 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
             op = FETCH2;
             v1 = POP;
             if (!NILP (v1))
-              {
-                BYTE_CODE_QUIT;
-                CHECK_RANGE (op);
-                stack.pc = stack.byte_string_start + op;
-              }
+              goto op_branch;
             NEXT;
           }
 
         CASE (Bgotoifnilelsepop):
           op = FETCH2;
           if (NILP (TOP))
-            {
-              BYTE_CODE_QUIT;
-              CHECK_RANGE (op);
-              stack.pc = stack.byte_string_start + op;
-            }
-          else DISCARD (1);
+            goto op_branch;
+          DISCARD (1);
           NEXT;
 
         CASE (Bgotoifnonnilelsepop):
           op = FETCH2;
           if (!NILP (TOP))
-            {
-              BYTE_CODE_QUIT;
-              CHECK_RANGE (op);
-              stack.pc = stack.byte_string_start + op;
-            }
-          else DISCARD (1);
+            goto op_branch;
+          DISCARD (1);
           NEXT;
 
         CASE (BRgoto):
-          BYTE_CODE_QUIT;
-          stack.pc += (int) *stack.pc - 127;
-          NEXT;
+          op = FETCH - 128;
+          goto op_relative_branch;
 
         CASE (BRgotoifnil):
           {
             Lisp_Object v1;
             v1 = POP;
+            op = FETCH - 128;
             if (NILP (v1))
-              {
-                BYTE_CODE_QUIT;
-                stack.pc += (int) *stack.pc - 128;
-              }
-            stack.pc++;
+              goto op_relative_branch;
             NEXT;
           }
 
@@ -855,33 +770,24 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
           {
             Lisp_Object v1;
             v1 = POP;
+            op = FETCH - 128;
             if (!NILP (v1))
-              {
-                BYTE_CODE_QUIT;
-                stack.pc += (int) *stack.pc - 128;
-              }
-            stack.pc++;
+              goto op_relative_branch;
             NEXT;
           }
 
         CASE (BRgotoifnilelsepop):
-          op = *stack.pc++;
+          op = FETCH - 128;
           if (NILP (TOP))
-            {
-              BYTE_CODE_QUIT;
-              stack.pc += op - 128;
-            }
-          else DISCARD (1);
+            goto op_relative_branch;
+          DISCARD (1);
           NEXT;
 
         CASE (BRgotoifnonnilelsepop):
-          op = *stack.pc++;
+          op = FETCH - 128;
           if (!NILP (TOP))
-            {
-              BYTE_CODE_QUIT;
-              stack.pc += op - 128;
-            }
-          else DISCARD (1);
+            goto op_relative_branch;
+          DISCARD (1);
           NEXT;
 
         CASE (Breturn):
@@ -946,15 +852,11 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
             if (sys_setjmp (c->jmp))
               {
                 struct handler *c = handlerlist;
-                int dest;
                 top = c->bytecode_top;
-                dest = c->bytecode_dest;
+                op = c->bytecode_dest;
                 handlerlist = c->next;
                 PUSH (c->val);
-                CHECK_RANGE (dest);
-                /* Might have been re-set by longjmp!  */
-                stack.byte_string_start = SDATA (stack.byte_string);
-                stack.pc = stack.byte_string_start + dest;
+                goto op_branch;
               }
 
             NEXT;
@@ -1629,7 +1531,7 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
           call3 (Qerror,
                  build_string ("Invalid byte opcode: op=%s, ptr=%d"),
                  make_number (op),
-                 make_number ((stack.pc - 1) - stack.byte_string_start));
+                 make_number (pc - 1 - bytestr_data));
 
           /* Handy byte-codes for lexical binding.  */
         CASE (Bstack_ref1):
@@ -1689,8 +1591,6 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
 
  exit:
 
-  byte_stack_list = byte_stack_list->next;
-
   /* Binds and unbinds are supposed to be compiled balanced.  */
   if (SPECPDL_INDEX () != count)
     {