cpplib: improve detection of once-only headers

[Zack Weinberg]

Jim Meyering <meyering@ascend.com> sent me this patch, which makes
cpplib detect once-only headers which have a guard of the form

#if !defined MACRO
#define MACRO

...

#endif

Before, we only detected them if they used #ifndef.

I've vetted it for style and tested that it does what it claims to and
doesn't break anything.  There are more elegant ways to do what it
does, but they require major work that neither I nor he has time for
at the moment.  This is clean enough for me.

Jim maintains {file,text,shell}utils, so I would assume he's OK
copyright-assignment-wise, but I no longer have the ability to check
this.

zw

1999-08-25 13:51 -0700  Jim Meyering  <meyering@ascend.com>

	* cpplib.c (detect_if_not_defined): New function.
	(do_if): Use it to detect potential once-only headers.

===================================================================
Index: cpplib.c
--- cpplib.c	1999/08/14 00:42:07	1.87
+++ cpplib.c	1999/08/25 20:57:57
@@ -1577,6 +1577,81 @@ do_sccs (pfile, keyword)
 }
 #endif
 
+
+/* We've found an `#if' directive.  If the only thing before it in
+   this file is white space, and if it is of the form
+   `#if ! defined SYMBOL', then SYMBOL is a possible controlling macro
+   for inclusion of this file.  (See redundant_include_p in cppfiles.c
+   for an explanation of controlling macros.)  If so, return a
+   malloc'd copy of SYMBOL.  Otherwise, return NULL.  */
+
+static U_CHAR *
+detect_if_not_defined (pfile)
+     cpp_reader *pfile;
+{
+  U_CHAR *control_macro = 0;
+
+  if (pfile->only_seen_white == 2)
+    {
+      char *ident;
+      enum cpp_token token;
+      int base_offset;
+      int token_offset;
+      int need_rparen = 0;
+
+      /* Save state required for restore.  */
+      pfile->no_macro_expand++;
+      parse_set_mark (pfile);
+      base_offset = CPP_WRITTEN (pfile);
+
+      /* Look for `!', */
+      if (get_directive_token (pfile) != CPP_OTHER
+	  || CPP_WRITTEN (pfile) != (size_t) base_offset + 1
+	  || CPP_PWRITTEN (pfile)[-1] != '!')
+	goto restore;
+
+      /* ...then `defined', */
+      token_offset = CPP_WRITTEN (pfile);
+      token = get_directive_token (pfile);
+      if (token != CPP_NAME)
+	goto restore;
+      ident = pfile->token_buffer + token_offset;
+      CPP_NUL_TERMINATE (pfile);
+      if (strcmp (ident, "defined"))
+	goto restore;
+
+      /* ...then an optional '(' and the name, */
+      token_offset = CPP_WRITTEN (pfile);
+      token = get_directive_token (pfile);
+      if (token == CPP_LPAREN)
+	{
+	  token_offset = CPP_WRITTEN (pfile);
+	  token = get_directive_token (pfile);
+	  if (token != CPP_NAME)
+	    goto restore;
+	  need_rparen = 1;
+	}
+      else if (token != CPP_NAME)
+	goto restore;
+
+      ident = pfile->token_buffer + token_offset;
+      CPP_NUL_TERMINATE (pfile);
+
+      /* ...then the ')', if necessary, */
+      if ((!need_rparen || get_directive_token (pfile) == CPP_RPAREN)
+	  /* ...and make sure there's nothing else on the line.  */
+	  && get_directive_token (pfile) == CPP_VSPACE)
+	control_macro = xstrdup (ident);
+
+    restore:
+      CPP_SET_WRITTEN (pfile, base_offset);
+      pfile->no_macro_expand--;
+      parse_goto_mark (pfile);
+    }
+
+  return control_macro;
+}
+
 /*
  * handle #if command by
  *   1) inserting special `defined' keyword into the hash table
@@ -1595,8 +1670,9 @@ do_if (pfile, keyword)
      cpp_reader *pfile;
      struct directive *keyword ATTRIBUTE_UNUSED;
 {
+  U_CHAR *control_macro = detect_if_not_defined (pfile);
   HOST_WIDEST_INT value = eval_if_expression (pfile);
-  conditional_skip (pfile, value == 0, T_IF, NULL_PTR);
+  conditional_skip (pfile, value == 0, T_IF, control_macro);
   return 0;
 }