Patch: malloc function attribute

[Anthony Green <green at cygnus dot com>]

The java runtime library has many functions for allocating memory in
different ways.  They all behave like `malloc' in that they always
return pointers that cannot alias anything.  The compiler should be
taking advantage of this information.  It already tries to identify
malloc-like functions by looking for special function names (see
special_function_p in calls.c).  Rather than add more magic function
names, I've added a `malloc' function attribute for annotating source
- as is suggested by this comment in special_function_p:

      /* XXX should have "malloc" attribute on functions instead
	 of recognizing them by name.  */

I have correspoding patches to annotate jc1's builtin decls, and the
libgcj header files.  It definitely results in better code.

1999-11-27  Anthony Green  <green@cygnus.com>

	* tree.h (struct tree_decl): Add malloc_flag.
	(DECL_IS_MALLOC): Define.
	* c-common.c (attrs): Add A_MALLOC attribute.
	(init_attributes): Add this attribute to the table.
	(decl_attributes): Handle malloc attribute.
	* calls.c (special_function_p): Check for the malloc attribute.
	* extend.texi (Function Attributes): Document malloc attribute.

Index: tree.h
===================================================================
RCS file: /cvs/gcc/egcs/gcc/tree.h,v
retrieving revision 1.104
diff -u -r1.104 tree.h
--- tree.h	1999/11/24 01:19:04	1.104
+++ tree.h	1999/11/28 03:24:09
@@ -1238,6 +1238,11 @@
    to redefine for any purpose whatever.  */
 #define DECL_BUILT_IN_NONANSI(NODE) ((NODE)->common.unsigned_flag)
 
+/* Nonzero in a FUNCTION_DECL means this function should be treated
+   as if it were a malloc, meaning it returns a pointer that is
+   not an alias.  */
+#define DECL_IS_MALLOC(NODE) (DECL_CHECK (NODE)->decl.malloc_flag)
+
 /* Nonzero in a FIELD_DECL means it is a bit field, and must be accessed
    specially.  */
 #define DECL_BIT_FIELD(NODE) (DECL_CHECK (NODE)->decl.bit_field_flag)
@@ -1347,6 +1352,7 @@
   unsigned virtual_flag : 1;
   unsigned ignored_flag : 1;
   unsigned abstract_flag : 1;
+  unsigned malloc_flag : 1;
 
   unsigned in_system_header_flag : 1;
   unsigned common_flag : 1;
Index: c-common.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/c-common.c,v
retrieving revision 1.76
diff -u -r1.76 c-common.c
--- c-common.c	1999/11/25 16:58:31	1.76
+++ c-common.c	1999/11/28 03:24:10
@@ -140,7 +140,7 @@
 enum attrs {A_PACKED, A_NOCOMMON, A_COMMON, A_NORETURN, A_CONST, A_T_UNION,
 	    A_NO_CHECK_MEMORY_USAGE, A_NO_INSTRUMENT_FUNCTION,
 	    A_CONSTRUCTOR, A_DESTRUCTOR, A_MODE, A_SECTION, A_ALIGNED,
-	    A_UNUSED, A_FORMAT, A_FORMAT_ARG, A_WEAK, A_ALIAS};
+	    A_UNUSED, A_FORMAT, A_FORMAT_ARG, A_WEAK, A_ALIAS, A_MALLOC};
 
 enum format_type { printf_format_type, scanf_format_type,
 		   strftime_format_type };
@@ -481,6 +481,7 @@
   add_attribute (A_ALIAS, "alias", 1, 1, 1);
   add_attribute (A_NO_INSTRUMENT_FUNCTION, "no_instrument_function", 0, 0, 1);
   add_attribute (A_NO_CHECK_MEMORY_USAGE, "no_check_memory_usage", 0, 0, 1);
+  add_attribute (A_MALLOC, "malloc", 0, 0, 1);
 }
 
 /* Default implementation of valid_lang_attribute, below.  By default, there
@@ -613,6 +614,13 @@
 	      = build_pointer_type
 		(build_type_variant (TREE_TYPE (type),
 				     TREE_READONLY (TREE_TYPE (type)), 1));
+	  else
+	    warning ("`%s' attribute ignored", IDENTIFIER_POINTER (name));
+	  break;
+
+	case A_MALLOC:
+	  if (TREE_CODE (decl) == FUNCTION_DECL)
+	    DECL_IS_MALLOC (decl) = 1;
 	  else
 	    warning ("`%s' attribute ignored", IDENTIFIER_POINTER (name));
 	  break;
Index: calls.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/calls.c,v
retrieving revision 1.69
diff -u -r1.69 calls.c
--- calls.c	1999/11/16 17:43:39	1.69
+++ calls.c	1999/11/28 03:24:12
@@ -546,10 +546,14 @@
 {
   *returns_twice = 0;
   *is_longjmp = 0;
-  *is_malloc = 0;
   *may_be_alloca = 0;
 
-  if (name != 0 && IDENTIFIER_LENGTH (DECL_NAME (fndecl)) <= 17
+  /* The function decl may have the `malloc' attribute.  */
+  *is_malloc = fndecl && DECL_IS_MALLOC (fndecl);
+
+  if (! is_malloc 
+      && name != 0 
+      && IDENTIFIER_LENGTH (DECL_NAME (fndecl)) <= 17
       /* Exclude functions not at the file scope, or not `extern',
 	 since they are not the magic functions we would otherwise
 	 think they are.  */
Index: extend.texi
===================================================================
RCS file: /cvs/gcc/egcs/gcc/extend.texi,v
retrieving revision 1.40
diff -u -r1.40 extend.texi
--- extend.texi	1999/11/19 13:02:59	1.40
+++ extend.texi	1999/11/28 03:24:15
@@ -1310,6 +1310,7 @@
 @cindex functions that never return
 @cindex functions that have no side effects
 @cindex functions in arbitrary sections
+@cindex functions that bahave like malloc
 @cindex @code{volatile} applied to function
 @cindex @code{const} applied to function
 @cindex functions with @code{printf}, @code{scanf} or @code{strftime} style arguments
@@ -1323,10 +1324,10 @@
 
 The keyword @code{__attribute__} allows you to specify special
 attributes when making a declaration.  This keyword is followed by an
-attribute specification inside double parentheses.  Nine attributes,
+attribute specification inside double parentheses.  Ten attributes,
 @code{noreturn}, @code{const}, @code{format},
-@code{no_instrument_function}, @code{section},
-@code{constructor}, @code{destructor}, @code{unused} and @code{weak} are
+@code{no_instrument_function}, @code{section}, @code{constructor},
+@code{destructor}, @code{unused}, @code{weak} and @code{malloc} are
 currently defined for functions.  Other attributes, including
 @code{section} are supported for variables declarations (@pxref{Variable
 Attributes}) and for types (@pxref{Type Attributes}).
@@ -1539,6 +1540,13 @@
 also be used with non-function declarations.  Weak symbols are supported
 for ELF targets, and also for a.out targets when using the GNU assembler
 and linker.
+
+@item malloc
+@cindex @code{malloc} attribute
+The @code{malloc} attribute is used to tell the compiler that a function
+may be treated as if it were the malloc function.  The compiler assumes
+that calls to malloc result in pointers that cannot alias anything.
+This will often improve optimization.
 
 @item alias ("target")
 @cindex @code{alias} attribute

-- 
Anthony Green                                               Cygnus Solutions
                                                       Sunnyvale, California