RFC: Improvements to alternate-entry-point handling

[Zack Weinberg <zack@codesourcery.com>]

This patch accomplishes some improvements to the alternate entry point
handling code (not currently used for anything, but I have plans for
it in the near future).

First, the LABEL_NAME and LABEL_ALTERNATE_NAME fields are mutually
exclusive; they will never be set at the same time.  Collapse them
together, and use a flag bit to indicate whether a CODE_LABEL is an
alternate entry point.  This also obviates the need for the special
ASM_OUTPUT_ALTERNATE_LABEL_NAME macro.  (It is not currently a space
win; a CODE_LABEL rtx is still one word too big for the size-32 GC
bucket.)

Second, add the ability to generate global and weak symbol definitions
for alternate entry points.  This is needed for any use of alternate
entries other than optimizing intra-compilation-unit call sequences.
For instance, C++ would like to use alternate entry points to generate
this-pointer adjustment thunks, and Fortran has a general facility for
creating multiple-entry functions.  Both are useless if the alternate
entries are only visible to a single file.

I have compiled this, but I have not tested it at all - to do that I
have to write some code that _uses_ it, first.  At this point I'm only
looking for criticism of the patch.  Am I being too clever building a
two-bit field out of two RTL flags?  Is there some non-obvious reason
why we need ASM_OUTPUT_ALTERNATE_LABEL_NAME?  That sort of thing.

zw

	* rtl.def (CODE_LABEL): Remove slot 8.
	* rtl.h (struct rtx_def): Document new uses of jump and call fields.
	(LABEL_ALTERNATE_NAME): Delete.
	(LABEL_KIND, SET_LABEL_KIND, LABEL_ALT_ENTRY_P): New.
	* defaults.h: Remove default for ASM_OUTPUT_ALTERNATE_LABEL_NAME.

	* final.c (output_alternate_entry_point): New.
	(final_scan_insn): Use it instead of
	ASM_OUTPUT_ALTERNATE_LABEL_NAME.  Do not consider possibility
	of a case label being an alternate entry point.

	* cfgbuild.c (make_edges, find_bb_boundaries): Use LABEL_ALT_ENTRY_P.
	* emit-rtl.c (gen_label_rtx): Adjust call to gen_rtx_CODE_LABEL.
	Do not clear LABEL_NUSES (unnecessary) or LABEL_ALTERNATE_NAME
	(field deleted).
	* print-rtl.c, ra-debug.c: Update code to output CODE_LABELs.

	* doc/rtl.texi: Document LABEL_KIND, SET_LABEL_KIND, and
	LABEL_ALT_ENTRY_P; not LABEL_ALTERNATE_NAME.
	* doc/tm.texi: Delete documentation of
	ASM_OUTPUT_ALTERNATE_LABEL_NAME.

===================================================================
Index: rtl.def
--- rtl.def	18 Jun 2002 16:18:12 -0000	1.57
+++ rtl.def	18 Jul 2002 00:38:34 -0000
@@ -573,10 +573,9 @@ DEF_RTL_EXPR(BARRIER, "barrier", "iuu", 
    4: is used in jump.c for the use-count of the label.
    5: is used in flow.c to point to the chain of label_ref's to this label.
    6: is a number that is unique in the entire compilation.
-   7: is the user-given name of the label, if any.
-   8: is the alternate label name.  */
-DEF_RTL_EXPR(CODE_LABEL, "code_label", "iuuB00iss", 'x')
-     
+   7: is the user-given name of the label, if any.  */
+DEF_RTL_EXPR(CODE_LABEL, "code_label", "iuuB00is", 'x')
+
 /* Say where in the code a source line starts, for symbol table's sake.
    Operand:
    4: filename, if line number > 0, note-specific data otherwise.
===================================================================
Index: rtl.h
--- rtl.h	11 Jul 2002 10:32:53 -0000	1.364
+++ rtl.h	18 Jul 2002 00:38:34 -0000
@@ -130,9 +130,10 @@ struct rtx_def
   /* 1 in a MEM if we should keep the alias set for this mem unchanged
      when we access a component.
      1 in a CALL_INSN if it is a sibling call.
-     1 in a SET that is for a return.  */
+     1 in a SET that is for a return.
+     In a CODE_LABEL, part of the two-bit alternate entry field.  */
   unsigned int jump : 1;
-  /* This flag is currently unused.  */
+  /* In a CODE_LABEL, part of the two-bit alternate entry field.  */
   unsigned int call : 1;
   /* 1 in a REG, MEM, or CONCAT if the value is set at most once, anywhere.
      1 in a SUBREG if it references an unsigned object whose mode has been
@@ -876,8 +877,55 @@ extern const char * const note_insn_name
    of LABEL_REFs that point at it, so unused labels can be deleted.  */
 #define LABEL_NUSES(RTX) XCINT (RTX, 4, CODE_LABEL)
 
-/* Associate a name with a CODE_LABEL.  */
-#define LABEL_ALTERNATE_NAME(RTX) XCSTR (RTX, 8, CODE_LABEL)
+/* Labels carry a two-bit field composed of the ->jump and ->call
+   bits.  This field indicates whether the label is an alternate
+   entry point, and if so, what kind.  */
+enum label_kind
+{
+  LABEL_NORMAL = 0,	/* ordinary label */
+  LABEL_STATIC_ENTRY,	/* alternate entry point, not exported */
+  LABEL_GLOBAL_ENTRY,	/* alternate entry point, exported */
+  LABEL_WEAK_ENTRY	/* alternate entry point, exported as weak symbol */
+};
+
+#if defined ENABLE_RTL_FLAG_CHECKING && (GCC_VERSION > 2007)
+
+/* Retrieve the kind of LABEL.  */
+#define LABEL_KIND(LABEL) __extension__					\
+({ rtx const _label = (LABEL);						\
+   if (GET_CODE (_label) != CODE_LABEL)					\
+     rtl_check_failed_flag ("LABEL_KIND", _label, __FILE__, __LINE__,	\
+			    __FUNCTION__);				\
+   (enum label_kind) ((_label->jump << 1) & _label->call); })
+
+/* Set the kind of LABEL.  */
+#define SET_LABEL_KIND(LABEL, KIND) do {				\
+   rtx _label = (LABEL);						\
+   unsigned int _kind = (KIND);						\
+   if (GET_CODE (_label) != CODE_LABEL)					\
+     rtl_check_failed_flag ("SET_LABEL_KIND", _label, __FILE__, __LINE__, \
+			    __FUNCTION__);				\
+   _label->jump = ((kind >> 1) & 1);					\
+   _label->call = (kind & 1);						\
+} while (0)
+
+#else
+
+/* Retrieve the kind of LABEL.  */
+#define LABEL_KIND(LABEL) \
+   (enum label_kind) (((LABEL)->jump << 1) & (LABEL)->call); })
+
+/* Set the kind of LABEL.  */
+#define SET_LABEL_KIND(LABEL, KIND) do {				\
+   rtx _label = (LABEL);						\
+   unsigned int _kind = (KIND);						\
+   _label->jump = ((kind >> 1) & 1);					\
+   _label->call = (kind & 1);						\
+} while (0)
+
+#endif /* rtl flag checking */
+
+#define LABEL_ALT_ENTRY_P(LABEL) (LABEL_KIND (LABEL) != LABEL_NORMAL)
 
 /* The original regno this ADDRESSOF was built for.  */
 #define ADDRESSOF_REGNO(RTX) XCUINT (RTX, 1, ADDRESSOF)
===================================================================
Index: defaults.h
--- defaults.h	18 Jun 2002 01:35:30 -0000	1.80
+++ defaults.h	18 Jul 2002 00:38:31 -0000
@@ -80,12 +80,6 @@ do { fputs (integer_asm_op (POINTER_SIZE
    } while (0)
 #endif
 
-/* Provide default for ASM_OUTPUT_ALTERNATE_LABEL_NAME.  */
-#ifndef ASM_OUTPUT_ALTERNATE_LABEL_NAME
-#define ASM_OUTPUT_ALTERNATE_LABEL_NAME(FILE,INSN) \
-do { ASM_OUTPUT_LABEL(FILE,LABEL_ALTERNATE_NAME (INSN)); } while (0)
-#endif
-
 /* choose a reasonable default for ASM_OUTPUT_ASCII.  */
 
 #ifndef ASM_OUTPUT_ASCII
===================================================================
Index: cfgbuild.c
--- cfgbuild.c	12 Jun 2002 14:51:02 -0000	1.25
+++ cfgbuild.c	18 Jul 2002 00:38:30 -0000
@@ -321,7 +321,7 @@ make_edges (label_value_list, min, max, 
       enum rtx_code code;
       int force_fallthru = 0;
 
-      if (GET_CODE (bb->head) == CODE_LABEL && LABEL_ALTERNATE_NAME (bb->head))
+      if (GET_CODE (bb->head) == CODE_LABEL && LABEL_ALT_ENTRY_P (bb->head))
 	cached_make_edge (NULL, ENTRY_BLOCK_PTR, bb, 0);
 
       /* Examine the last instruction of the block, and discover the
@@ -699,7 +699,7 @@ find_bb_boundaries (bb)
 	  bb = fallthru->dest;
 	  remove_edge (fallthru);
 	  flow_transfer_insn = NULL_RTX;
-	  if (LABEL_ALTERNATE_NAME (insn))
+	  if (LABEL_ALT_ENTRY_P (insn))
 	    make_edge (ENTRY_BLOCK_PTR, bb, 0);
 	}
 
===================================================================
Index: emit-rtl.c
--- emit-rtl.c	11 Jul 2002 10:32:54 -0000	1.284
+++ emit-rtl.c	18 Jul 2002 00:38:32 -0000
@@ -2185,14 +2185,8 @@ widen_memory_access (memref, mode, offse
 rtx
 gen_label_rtx ()
 {
-  rtx label;
-
-  label = gen_rtx_CODE_LABEL (VOIDmode, 0, NULL_RTX, NULL_RTX,
-		  	      NULL, label_num++, NULL, NULL);
-
-  LABEL_NUSES (label) = 0;
-  LABEL_ALTERNATE_NAME (label) = NULL;
-  return label;
+  return gen_rtx_CODE_LABEL (VOIDmode, 0, NULL_RTX, NULL_RTX,
+		  	     NULL, label_num++, NULL);
 }
 \f
 /* For procedure integration.  */
===================================================================
Index: final.c
--- final.c	17 Jun 2002 17:47:20 -0000	1.262
+++ final.c	18 Jul 2002 00:38:32 -0000
@@ -222,6 +222,7 @@ static void profile_after_prologue PARAM
 static void notice_source_line	PARAMS ((rtx));
 static rtx walk_alter_subreg	PARAMS ((rtx *));
 static void output_asm_name	PARAMS ((void));
+static void output_alternate_entry_point PARAMS ((FILE *, rtx));
 static tree get_mem_expr_from_op	PARAMS ((rtx, int *));
 static void output_asm_operand_names PARAMS ((rtx *, int *, int));
 static void output_operand	PARAMS ((rtx, int));
@@ -1952,6 +1953,37 @@ get_insn_template (code, insn)
     }
 }
 
+/* Emit the appropriate declaration for an alternate-entry-point
+   symbol represented by INSN, to FILE.  INSN is a CODE_LABEL with
+   LABEL_KIND != LABEL_NORMAL.
+
+   The case fall-through in this function is intentional.  */
+static void
+output_alternate_entry_point (file, insn)
+     FILE *file;
+     rtx insn;
+{
+  const char *name = LABEL_NAME (insn);
+
+  switch (LABEL_KIND (insn))
+    {
+    case LABEL_WEAK_ENTRY:
+#ifdef ASM_WEAKEN_LABEL
+      ASM_WEAKEN_LABEL (file, name);
+#endif
+    case LABEL_GLOBAL_ENTRY:
+      ASM_GLOBALIZE_LABEL (file, name);
+    case LABEL_STATIC_ENTRY:
+      /* FIXME output a .type directive here if appropriate.  */
+      ASM_OUTPUT_LABEL (file, name);
+      break;
+
+    case LABEL_NORMAL:
+    default:
+      abort ();
+    }
+}
+
 /* The final scan for one insn, INSN.
    Args are same as in `final', except that INSN
    is the insn being scanned.
@@ -2242,17 +2274,14 @@ final_scan_insn (insn, file, optimize, p
 	      ASM_OUTPUT_CASE_LABEL (file, "L", CODE_LABEL_NUMBER (insn),
 				     NEXT_INSN (insn));
 #else
-	      if (LABEL_ALTERNATE_NAME (insn))
-		ASM_OUTPUT_ALTERNATE_LABEL_NAME (file, insn);
-	      else
-		ASM_OUTPUT_INTERNAL_LABEL (file, "L", CODE_LABEL_NUMBER (insn));
+	      ASM_OUTPUT_INTERNAL_LABEL (file, "L", CODE_LABEL_NUMBER (insn));
 #endif
 #endif
 	      break;
 	    }
 	}
-      if (LABEL_ALTERNATE_NAME (insn))
-	ASM_OUTPUT_ALTERNATE_LABEL_NAME (file, insn);
+      if (LABEL_ALT_ENTRY_P (insn))
+	output_alternate_entry_point (file, insn);
       else
 	ASM_OUTPUT_INTERNAL_LABEL (file, "L", CODE_LABEL_NUMBER (insn));
       break;
===================================================================
Index: print-rtl.c
--- print-rtl.c	18 Jun 2002 20:12:13 -0000	1.84
+++ print-rtl.c	18 Jul 2002 00:38:33 -0000
@@ -526,9 +526,14 @@ print_rtx (in_rtx)
 
     case CODE_LABEL:
       fprintf (outfile, " [%d uses]", LABEL_NUSES (in_rtx));
-      if (LABEL_ALTERNATE_NAME (in_rtx))
-	fprintf (outfile, " [alternate name: %s]",
-		 LABEL_ALTERNATE_NAME (in_rtx));
+      switch (LABEL_KIND (in_rtx))
+	{
+	  case LABEL_NORMAL: break;
+	  case LABEL_STATIC_ENTRY: fputs (" [entry]", outfile); break;
+	  case LABEL_GLOBAL_ENTRY: fputs (" [global entry]", outfile); break;
+	  case LABEL_WEAK_ENTRY: fputs (" [weak entry]", outfile); break;
+	  default: abort();
+	}
       break;
 
     case CALL_PLACEHOLDER:
===================================================================
Index: ra-debug.c
--- ra-debug.c	15 Jul 2002 14:07:05 -0000	1.2
+++ ra-debug.c	18 Jul 2002 00:38:33 -0000
@@ -381,8 +381,14 @@ ra_print_rtx (file, x, with_pn)
 	  fprintf (file, "L%d:\t; ", CODE_LABEL_NUMBER (x));
 	  if (LABEL_NAME (x))
 	    fprintf (file, "(%s) ", LABEL_NAME (x));
-	  if (LABEL_ALTERNATE_NAME (x))
-	    fprintf (file, "(alternate: %s) ", LABEL_ALTERNATE_NAME (x));
+	  switch (LABEL_KIND (x))
+	    {
+	    case LABEL_NORMAL: break;
+	    case LABEL_STATIC_ENTRY: fputs (" (entry)", file); break;
+	    case LABEL_GLOBAL_ENTRY: fputs (" (global entry)", file); break;
+	    case LABEL_WEAK_ENTRY: fputs (" (weak entry)", file); break;
+	    default: abort();
+	    }
 	  fprintf (file, " [%d uses] uid=(", LABEL_NUSES (x));
 	}
       fprintf (file, "%d", INSN_UID (x));
===================================================================
Index: doc/rtl.texi
--- doc/rtl.texi	14 Jun 2002 18:58:10 -0000	1.39
+++ doc/rtl.texi	18 Jul 2002 00:45:22 -0000
@@ -2830,13 +2830,32 @@ Besides as a @code{code_label}, a label 
 
 @findex LABEL_NUSES
 The field @code{LABEL_NUSES} is only defined once the jump optimization
-phase is completed and contains the number of times this label is
+phase is completed.  It contains the number of times this label is
 referenced in the current function.
 
-@findex LABEL_ALTERNATE_NAME
-The field @code{LABEL_ALTERNATE_NAME} is used to associate a name with
-a @code{code_label}.  If this field is defined, the alternate name will
-be emitted instead of an internally generated label name.
+@findex LABEL_KIND
+@findex SET_LABEL_KIND
+@findex LABEL_ALT_ENTRY_P
+@cindex alternate entry points
+The field @code{LABEL_KIND} differentiates four different types of
+labels: @code{LABEL_NORMAL}, @code{LABEL_STATIC_ENTRY},
+@code{LABEL_GLOBAL_ENTRY}, and @code{LABEL_WEAK_ENTRY}.  The only labels
+that do not have type @code{LABEL_NORMAL} are @dfn{alternate entry
+points} to the current function.  These may be static (visible only in
+the containing translation unit), global (exposed to all translation
+units), or weak (global, but can be overriden by another symbol with the
+same name).
+
+Much of the compiler treats all four kinds of label identically.  Some
+of it needs to know whether or not a label is an alternate entry point;
+for this purpose, the macro @code{LABEL_ALT_ENTRY_P} is provided.  It is
+equivalent to testing whether @samp{LABEL_KIND (label) == LABEL_NORMAL}.
+The only place that cares about the distinction between static, global,
+and weak alternate entry points, besides the front-end code that creates
+them, is the function @code{output_alternate_entry_point}, in
+@file{final.c}.
+
+To set the kind of a label, use the @code{SET_LABEL_KIND} macro.
 
 @findex barrier
 @item barrier
===================================================================
Index: doc/tm.texi
--- doc/tm.texi	17 Jul 2002 21:31:42 -0000	1.146
+++ doc/tm.texi	18 Jul 2002 00:38:39 -0000
@@ -6597,17 +6597,6 @@ bundles.
 If this macro is not defined, then @code{ASM_OUTPUT_INTERNAL_LABEL} will be
 used.
 
-@findex ASM_OUTPUT_ALTERNATE_LABEL_NAME
-@item ASM_OUTPUT_ALTERNATE_LABEL_NAME (@var{stream}, @var{string})
-A C statement to output to the stdio stream @var{stream} the string
-@var{string}.
-
-The default definition of this macro is as follows:
-
-@example
-fprintf (@var{stream}, "%s:\n", LABEL_ALTERNATE_NAME (INSN))
-@end example
-
 @findex ASM_GENERATE_INTERNAL_LABEL
 @item ASM_GENERATE_INTERNAL_LABEL (@var{string}, @var{prefix}, @var{num})
 A C statement to store into the string @var{string} a label whose name