maint: prefer attribute.h attributes

Prefer the macros used in attribute.h, and _Noreturn,
to the by-hand use of __attribute__, as this is more portable.
* bootstrap.conf (gnulib_modules): Add attribute.
* src/system.h: Include attribute.h.  All uses of
attributes changed to use the attribute.h macros.
Plus, use _Noreturn.
(FALLTHROUGH): Remove; attribute.h now defines this.

10 files changed, 29 insertions, 42 deletions

diff --git a/bootstrap.conf b/bootstrap.conf
index 9234a43..39908c0 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -24,6 +24,7 @@ avoided_gnulib_modules='
 # gnulib modules used by this package.
 gnulib_modules='
 announce-gen
+attribute
 argmatch
 binary-io
 c-stack
diff --git a/src/cmp.c b/src/cmp.c
index 2e89ac1..a96c9ee 100644
--- a/src/cmp.c
+++ b/src/cmp.c
@@ -53,7 +53,7 @@
 
 static int cmp (void);
 static off_t file_position (int);
-static size_t block_compare (word const *, word const *) _GL_ATTRIBUTE_PURE;
+static size_t block_compare (word const *, word const *) ATTRIBUTE_PURE;
 static size_t count_newlines (char *, size_t);
 static void sprintc (char *, unsigned char);
 
@@ -110,8 +110,7 @@ static struct option const long_options[] =
   {0, 0, 0, 0}
 };
 
-static void try_help (char const *, char const *) __attribute__((noreturn));
-static void
+static _Noreturn void
 try_help (char const *reason_msgid, char const *operand)
 {
   if (reason_msgid)
diff --git a/src/context.c b/src/context.c
index 30f8fd4..b3cfa7d 100644
--- a/src/context.c
+++ b/src/context.c
@@ -433,7 +433,7 @@ pr_unidiff_hunk (struct change *hunk)
    2*CONTEXT unchanged lines appear, and return a pointer
    to the 'struct change' for the last change before those lines.  */
 
-static struct change * _GL_ATTRIBUTE_PURE
+static struct change * ATTRIBUTE_PURE
 find_hunk (struct change *start)
 {
   struct change *prev;
diff --git a/src/diff.c b/src/diff.c
index ef9710a..fedc7df 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -72,7 +72,7 @@ static void summarize_regexp_list (struct regexp_list *);
 static void specify_style (enum output_style);
 static void specify_value (char const **, char const *, char const *);
 static void specify_colors_style (char const *);
-static void try_help (char const *, char const *) __attribute__((noreturn));
+static _Noreturn void try_help (char const *, char const *);
 static void check_stdout (void);
 static void usage (void);
 
diff --git a/src/diff.h b/src/diff.h
index 0228275..d65c1da 100644
--- a/src/diff.h
+++ b/src/diff.h
@@ -356,8 +356,8 @@ extern int diff_dirs (struct comparison const *,
                       int (*) (struct comparison const *,
                                char const *, char const *));
 extern char *find_dir_file_pathname (char const *, char const *)
-  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE
-  _GL_ATTRIBUTE_RETURNS_NONNULL;
+  ATTRIBUTE_MALLOC ATTRIBUTE_DEALLOC_FREE
+  ATTRIBUTE_RETURNS_NONNULL;
 
 /* ed.c */
 extern void print_ed_script (struct change *);
@@ -382,7 +382,7 @@ extern void print_sdiff_script (struct change *);
 /* util.c */
 extern char const change_letter[4];
 extern char const pr_program[];
-extern bool lines_differ (char const *, char const *) _GL_ATTRIBUTE_PURE;
+extern bool lines_differ (char const *, char const *) ATTRIBUTE_PURE;
 extern lin translate_line_number (struct file_data const *, lin);
 extern struct change *find_change (struct change *);
 extern struct change *find_reverse_change (struct change *);
@@ -390,13 +390,13 @@ extern enum changes analyze_hunk (struct change *, lin *, lin *, lin *, lin *);
 extern void begin_output (void);
 extern void cleanup_signal_handlers (void);
 extern void debug_script (struct change *);
-extern void fatal (char const *) __attribute__((noreturn));
+extern _Noreturn void fatal (char const *);
 extern void finish_output (void);
-extern void message (char const *, ...) _GL_ATTRIBUTE_FORMAT ((printf, 1, 2));
+extern void message (char const *, ...) ATTRIBUTE_FORMAT ((printf, 1, 2));
 extern void output_1_line (char const *, char const *, char const *,
                            char const *);
 extern void perror_with_name (char const *);
-extern void pfatal_with_name (char const *) __attribute__((noreturn));
+extern _Noreturn void pfatal_with_name (char const *);
 extern void print_1_line (char const *, char const * const *);
 extern void print_1_line_nl (char const *, char const * const *, bool);
 extern void print_message_queue (void);
diff --git a/src/diff3.c b/src/diff3.c
index 9e4ad54..2ae82f3 100644
--- a/src/diff3.c
+++ b/src/diff3.c
@@ -191,10 +191,10 @@ static struct diff3_block *reverse_diff3_blocklist (struct diff3_block *);
 static struct diff3_block *using_to_diff3_block (struct diff_block *[2], struct diff_block *[2], int, int, struct diff3_block const *);
 static struct diff_block *process_diff (char const *, char const *, char **);
 static void check_stdout (void);
-static void fatal (char const *) __attribute__((noreturn));
+static _Noreturn void fatal (char const *);
 static void output_diff3 (FILE *, struct diff3_block *, int const[3], int const[3]);
-static void perror_with_exit (char const *) __attribute__((noreturn));
-static void try_help (char const *, char const *) __attribute__((noreturn));
+static _Noreturn void perror_with_exit (char const *);
+static _Noreturn void try_help (char const *, char const *);
 static void usage (void);
 
 static char const *diff_program = DEFAULT_DIFF_PROGRAM;
@@ -1097,7 +1097,7 @@ process_diff (char const *filea,
 
 /* Skip tabs and spaces, and return the first character after them.  */
 
-static char * _GL_ATTRIBUTE_PURE
+static char * ATTRIBUTE_PURE
 skipwhite (char *s)
 {
   while (*s == ' ' || *s == '\t')
diff --git a/src/dir.c b/src/dir.c
index 409eee4..b38ce73 100644
--- a/src/dir.c
+++ b/src/dir.c
@@ -322,7 +322,7 @@ diff_dirs (struct comparison const *cmp,
 
 /* Return nonzero if CMP is looping recursively in argument I.  */
 
-static bool _GL_ATTRIBUTE_PURE
+static bool ATTRIBUTE_PURE
 dir_loop (struct comparison const *cmp, int i)
 {
   struct comparison const *p = cmp;
diff --git a/src/sdiff.c b/src/sdiff.c
index f6011ce..9dd4960 100644
--- a/src/sdiff.c
+++ b/src/sdiff.c
@@ -63,8 +63,8 @@ static bool edit (struct line_filter *, char const *, lin, lin, struct line_filt
 static bool interact (struct line_filter *, struct line_filter *, char const *, struct line_filter *, char const *, FILE *);
 static void checksigs (void);
 static void diffarg (char const *);
-static void fatal (char const *) __attribute__((noreturn));
-static void perror_fatal (char const *) __attribute__((noreturn));
+static _Noreturn void fatal (char const *);
+static _Noreturn void perror_fatal (char const *);
 static void trapsigs (void);
 static void untrapsig (int);
 
@@ -151,8 +151,7 @@ static struct option const longopts[] =
   {0, 0, 0, 0}
 };
 
-static void try_help (char const *, char const *) __attribute__((noreturn));
-static void
+static _Noreturn void
 try_help (char const *reason_msgid, char const *operand)
 {
   if (reason_msgid)
@@ -225,7 +224,7 @@ Mandatory arguments to long options are mandatory for short options too.\n\
 /* Clean up after a signal or other failure.  This function is
    async-signal-safe.  */
 static void
-cleanup (int signo __attribute__((unused)))
+cleanup (_GL_UNUSED int signo)
 {
 #if HAVE_WORKING_FORK
   if (0 < diffpid)
@@ -235,8 +234,7 @@ cleanup (int signo __attribute__((unused)))
     unlink (tmpname);
 }
 
-static void exiterr (void) __attribute__((noreturn));
-static void
+static _Noreturn void
 exiterr (void)
 {
   cleanup (0);
diff --git a/src/system.h b/src/system.h
index b6d86a9..7b71932 100644
--- a/src/system.h
+++ b/src/system.h
@@ -27,18 +27,12 @@
 # define IF_LINT(Code) /* empty */
 #endif
 
-/* Define '__attribute__' and 'volatile' first
-   so that they're used consistently in all system includes.  */
-#if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 6) || __STRICT_ANSI__
-# define __attribute__(x)
-#endif
-
 #include <verify.h>
 
 #include <sys/types.h>
 
 #include <sys/stat.h>
-#include "stat-macros.h"
+#include <stat-macros.h>
 
 #ifndef STAT_BLOCKSIZE
 # if HAVE_STRUCT_STAT_ST_BLKSIZE
@@ -115,8 +109,11 @@ int strcasecmp (char const *, char const *);
 #define MAX(a, b) ((a) >= (b) ? (a) : (b))
 
 #include <stdbool.h>
+
+#include <attribute.h>
 #include <intprops.h>
-#include "propername.h"
+#include <propername.h>
+
 #include "version.h"
 
 /* Type used for fast comparison of several bytes at a time.
@@ -211,11 +208,3 @@ typedef ptrdiff_t lin;
 #endif
 
 #define STREQ(a, b) (strcmp (a, b) == 0)
-
-#ifndef FALLTHROUGH
-# if __GNUC__ < 7
-#  define FALLTHROUGH ((void) 0)
-# else
-#  define FALLTHROUGH __attribute__ ((__fallthrough__))
-# endif
-#endif
diff --git a/src/util.c b/src/util.c
index 1cb98d4..7e77522 100644
--- a/src/util.c
+++ b/src/util.c
@@ -1225,13 +1225,13 @@ lines_differ (char const *s1, char const *s2)
 /* Find the consecutive changes at the start of the script START.
    Return the last link before the first gap.  */
 
-struct change * _GL_ATTRIBUTE_CONST
+struct change * ATTRIBUTE_CONST
 find_change (struct change *start)
 {
   return start;
 }
 
-struct change * _GL_ATTRIBUTE_CONST
+struct change * ATTRIBUTE_CONST
 find_reverse_change (struct change *start)
 {
   return start;
@@ -1473,7 +1473,7 @@ char const change_letter[] = { 0, 'd', 'a', 'c' };
    Internal line numbers count from 0 starting after the prefix.
    Actual line numbers count from 1 within the entire file.  */
 
-lin _GL_ATTRIBUTE_PURE
+lin ATTRIBUTE_PURE
 translate_line_number (struct file_data const *file, lin i)
 {
   return i + file->prefix_lines + 1;