Make more use of idx_t

Make more use of idx_t

[Bruno Haible]

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Here are a couple of proposed patches, to make use of idx_t instead of ptrdiff_t
when the value is always nonnegative. This helps understanding the code.


2020-12-05  Bruno Haible  <bruno@clisp.org>

	filenamecat-tests: Use idx_t for nonnegative ptrdiff_t variables.
	* tests/test-filenamecat.c: Include idx.h.
	(main): Mark prefixlen as nonnegative.
	* modules/filenamecat-tests (Depends-on): Add idx.

2020-12-05  Bruno Haible  <bruno@clisp.org>

	time_rz: Use idx_t for nonnegative ptrdiff_t variables.
	* lib/time_rz.c: Include idx.h.
	(save_abbr): Mark zone_size as nonnegative.
	* modules/time_rz (Depends-on): Add idx.

2020-12-05  Bruno Haible  <bruno@clisp.org>

	parse-datetime: Use idx_t for nonnegative ptrdiff_t variables.
	* lib/parse-datetime.y: Include idx.h.
	(textint): Mark digits as nonnegative.
	(parser_control): Mark dates_seen, days_seen, local_zones_seen,
	dsts_seen, times_seen, zones_seen as nonnegative.
	(lookup_word): Mark wordlen as nonnegative.
	(yylex): Mark count as nonnegative.
	(parse_datetime2): Mark tzsize as nonnegative.
	* modules/parse-datetime (Depends-on): Add idx.

2020-12-05  Bruno Haible  <bruno@clisp.org>

	fnmatch: Use idx_t for nonnegative ptrdiff_t variables.
	* lib/fnmatch.c: Include idx.h. In glibc, define idx_t directly.
	* lib/fnmatch_loop.c (EXT): Mark slen, new_used, plensize as
	nonnegative.
	* modules/fnmatch (Depends-on): Add idx.

2020-12-05  Bruno Haible  <bruno@clisp.org>

	c-stack: Use idx_t for nonnegative ptrdiff_t variables.
	* lib/c-stack.c: Include idx.h.
	(die): Mark buflen as nonnegative.
	* modules/c-stack (Depends-on): Add idx.

2020-12-05  Bruno Haible  <bruno@clisp.org>

	backupfile: Use idx_t for nonnegative ptrdiff_t variables.
	* lib/backupfile.c: Include idx.h.
	(numbered_backup): Mark base_offset as nonnegative.
	(backupfile_internal): Likewise.
	* modules/backup-rename (Depends-on): Add idx.
	* modules/backupfile (Depends-on): Likewise.


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From 873bdd8d5275e41f107daed3efcae16084f69d95 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 5 Dec 2020 11:43:25 +0100
Subject: [PATCH 1/6] backupfile: Use idx_t for nonnegative ptrdiff_t
 variables.

* lib/backupfile.c: Include idx.h.
(numbered_backup): Mark base_offset as nonnegative.
(backupfile_internal): Likewise.
* modules/backup-rename (Depends-on): Add idx.
* modules/backupfile (Depends-on): Likewise.
---
 ChangeLog             |  9 +++++++++
 lib/backupfile.c      | 17 +++++++++--------
 modules/backup-rename |  1 +
 modules/backupfile    |  1 +
 4 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 3de6368..3281f74 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2020-12-05  Bruno Haible  <bruno@clisp.org>
+
+	backupfile: Use idx_t for nonnegative ptrdiff_t variables.
+	* lib/backupfile.c: Include idx.h.
+	(numbered_backup): Mark base_offset as nonnegative.
+	(backupfile_internal): Likewise.
+	* modules/backup-rename (Depends-on): Add idx.
+	* modules/backupfile (Depends-on): Likewise.
+
 2020-12-04  Bruno Haible  <bruno@clisp.org>
 
 	utime: Fix a test failure on macOS 10.13.
diff --git a/lib/backupfile.c b/lib/backupfile.c
index 2a88248..dfc29e8 100644
--- a/lib/backupfile.c
+++ b/lib/backupfile.c
@@ -22,12 +22,6 @@
 
 #include "backup-internal.h"
 
-#include "attribute.h"
-#include "basename-lgpl.h"
-#include "opendirat.h"
-#include "renameatu.h"
-#include "xalloc-oversized.h"
-
 #include <errno.h>
 #include <fcntl.h>
 #include <stdbool.h>
@@ -36,6 +30,13 @@
 #include <string.h>
 #include <unistd.h>
 
+#include "attribute.h"
+#include "basename-lgpl.h"
+#include "idx.h"
+#include "opendirat.h"
+#include "renameatu.h"
+#include "xalloc-oversized.h"
+
 #ifndef _D_EXACT_NAMLEN
 # define _D_EXACT_NAMLEN(dp) strlen ((dp)->d_name)
 #endif
@@ -198,7 +199,7 @@ enum numbered_backup_result
 
 static enum numbered_backup_result
 numbered_backup (int dir_fd, char **buffer, size_t buffer_size, size_t filelen,
-                 ptrdiff_t base_offset, DIR **dirpp, int *pnew_fd)
+                 idx_t base_offset, DIR **dirpp, int *pnew_fd)
 {
   enum numbered_backup_result result = BACKUP_IS_NEW;
   DIR *dirp = *dirpp;
@@ -307,7 +308,7 @@ char *
 backupfile_internal (int dir_fd, char const *file,
                      enum backup_type backup_type, bool rename)
 {
-  ptrdiff_t base_offset = last_component (file) - file;
+  idx_t base_offset = last_component (file) - file;
   size_t filelen = base_offset + strlen (file + base_offset);
 
   if (! simple_backup_suffix)
diff --git a/modules/backup-rename b/modules/backup-rename
index cdc96e6..d4bd339 100644
--- a/modules/backup-rename
+++ b/modules/backup-rename
@@ -17,6 +17,7 @@ closedir
 d-ino
 dirent-safer
 fcntl
+idx
 memcmp
 opendirat
 readdir
diff --git a/modules/backupfile b/modules/backupfile
index 2ca1e54..fd15ec5 100644
--- a/modules/backupfile
+++ b/modules/backupfile
@@ -17,6 +17,7 @@ closedir
 d-ino
 dirent-safer
 fcntl
+idx
 memcmp
 opendirat
 readdir
-- 
2.7.4


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From 0eec786d489ebaed246d6dbdc48c131742c90e42 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 5 Dec 2020 11:45:15 +0100
Subject: [PATCH 2/6] c-stack: Use idx_t for nonnegative ptrdiff_t variables.

* lib/c-stack.c: Include idx.h.
(die): Mark buflen as nonnegative.
* modules/c-stack (Depends-on): Add idx.
---
 ChangeLog       |  7 +++++++
 lib/c-stack.c   | 10 ++++++----
 modules/c-stack |  1 +
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 3281f74..6d32664 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2020-12-05  Bruno Haible  <bruno@clisp.org>
 
+	c-stack: Use idx_t for nonnegative ptrdiff_t variables.
+	* lib/c-stack.c: Include idx.h.
+	(die): Mark buflen as nonnegative.
+	* modules/c-stack (Depends-on): Add idx.
+
+2020-12-05  Bruno Haible  <bruno@clisp.org>
+
 	backupfile: Use idx_t for nonnegative ptrdiff_t variables.
 	* lib/backupfile.c: Include idx.h.
 	(numbered_backup): Mark base_offset as nonnegative.
diff --git a/lib/c-stack.c b/lib/c-stack.c
index 3aea16a..c5bb74a 100644
--- a/lib/c-stack.c
+++ b/lib/c-stack.c
@@ -37,9 +37,6 @@
 
 #include "c-stack.h"
 
-#include "gettext.h"
-#define _(msgid) gettext (msgid)
-
 #include <errno.h>
 #include <inttypes.h>
 
@@ -64,6 +61,11 @@ typedef struct sigaltstack stack_t;
 # include <stdio.h>
 #endif
 
+#include "idx.h"
+
+#include "gettext.h"
+#define _(msgid) gettext (msgid)
+
 /* Use libsigsegv only if needed; kernels like Solaris can detect
    stack overflow without the overhead of an external library.  */
 #define USE_LIBSIGSEGV (!HAVE_XSI_STACK_OVERFLOW_HEURISTIC && HAVE_LIBSIGSEGV)
@@ -134,7 +136,7 @@ die (int signo)
   size_t messagelen = strlen (message);
   static char const separator[] = {':', ' '};
   char buf[sizeof alternate_signal_stack / 16 + sizeof separator];
-  ptrdiff_t buflen;
+  idx_t buflen;
   if (prognamelen + messagelen < sizeof buf - sizeof separator)
     {
       char *p = mempcpy (buf, progname, prognamelen);
diff --git a/modules/c-stack b/modules/c-stack
index 77cf6aa..4c80f31 100644
--- a/modules/c-stack
+++ b/modules/c-stack
@@ -14,6 +14,7 @@ exitfail
 getprogname
 gettext-h
 havelib
+idx
 ignore-value
 intprops
 inttypes
-- 
2.7.4


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From 1e75316d5a581ad87d659bd6f4c22153d8640cc0 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 5 Dec 2020 11:47:51 +0100
Subject: [PATCH 3/6] fnmatch: Use idx_t for nonnegative ptrdiff_t variables.

* lib/fnmatch.c: Include idx.h. In glibc, define idx_t directly.
* lib/fnmatch_loop.c (EXT): Mark slen, new_used, plensize as
nonnegative.
* modules/fnmatch (Depends-on): Add idx.
---
 ChangeLog          | 8 ++++++++
 lib/fnmatch.c      | 6 ++++++
 lib/fnmatch_loop.c | 6 +++---
 modules/fnmatch    | 1 +
 4 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 6d32664..797f6d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 2020-12-05  Bruno Haible  <bruno@clisp.org>
 
+	fnmatch: Use idx_t for nonnegative ptrdiff_t variables.
+	* lib/fnmatch.c: Include idx.h. In glibc, define idx_t directly.
+	* lib/fnmatch_loop.c (EXT): Mark slen, new_used, plensize as
+	nonnegative.
+	* modules/fnmatch (Depends-on): Add idx.
+
+2020-12-05  Bruno Haible  <bruno@clisp.org>
+
 	c-stack: Use idx_t for nonnegative ptrdiff_t variables.
 	* lib/c-stack.c: Include idx.h.
 	(die): Mark buflen as nonnegative.
diff --git a/lib/fnmatch.c b/lib/fnmatch.c
index 3014428..c80809e 100644
--- a/lib/fnmatch.c
+++ b/lib/fnmatch.c
@@ -76,6 +76,12 @@ extern int fnmatch (const char *pattern, const char *string, int flags);
 #include <intprops.h>
 #include <flexmember.h>
 
+#ifdef _LIBC
+typedef ptrdiff_t idx_t;
+#else
+# include "idx.h"
+#endif
+
 /* We often have to test for FNM_FILE_NAME and FNM_PERIOD being both set.  */
 #define NO_LEADING_PERIOD(flags) \
   ((flags & (FNM_FILE_NAME | FNM_PERIOD)) == (FNM_FILE_NAME | FNM_PERIOD))
diff --git a/lib/fnmatch_loop.c b/lib/fnmatch_loop.c
index d9be16f..c533107 100644
--- a/lib/fnmatch_loop.c
+++ b/lib/fnmatch_loop.c
@@ -1036,9 +1036,9 @@ EXT (INT opt, const CHAR *pattern, const CHAR *string, const CHAR *string_end,
             struct patternlist *newp;                                         \
             size_t plen = (opt == L_('?') || opt == L_('@')                   \
                            ? pattern_len : (p - startp + 1UL));               \
-            ptrdiff_t slen = FLEXSIZEOF (struct patternlist, str, 0);         \
-            ptrdiff_t new_used = alloca_used + slen;                          \
-            ptrdiff_t plensize;                                               \
+            idx_t slen = FLEXSIZEOF (struct patternlist, str, 0);             \
+            idx_t new_used = alloca_used + slen;                              \
+            idx_t plensize;                                                   \
             if (INT_MULTIPLY_WRAPV (plen, sizeof (CHAR), &plensize)           \
                 || INT_ADD_WRAPV (new_used, plensize, &new_used))             \
               {                                                               \
diff --git a/modules/fnmatch b/modules/fnmatch
index f547670..2cb69a7 100644
--- a/modules/fnmatch
+++ b/modules/fnmatch
@@ -14,6 +14,7 @@ attribute       [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
 btowc           [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
 builtin-expect  [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
 flexmember      [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
+idx             [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
 intprops        [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
 isblank         [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
 iswctype        [test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1]
-- 
2.7.4


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From 08d7f5cb9d2d087d17541256868219384693b645 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 5 Dec 2020 11:51:23 +0100
Subject: [PATCH 4/6] parse-datetime: Use idx_t for nonnegative ptrdiff_t
 variables.

* lib/parse-datetime.y: Include idx.h.
(textint): Mark digits as nonnegative.
(parser_control): Mark dates_seen, days_seen, local_zones_seen,
dsts_seen, times_seen, zones_seen as nonnegative.
(lookup_word): Mark wordlen as nonnegative.
(yylex): Mark count as nonnegative.
(parse_datetime2): Mark tzsize as nonnegative.
* modules/parse-datetime (Depends-on): Add idx.
---
 ChangeLog              | 12 ++++++++++++
 lib/parse-datetime.y   | 21 +++++++++++----------
 modules/parse-datetime |  1 +
 3 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 797f6d0..57dea1d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,17 @@
 2020-12-05  Bruno Haible  <bruno@clisp.org>
 
+	parse-datetime: Use idx_t for nonnegative ptrdiff_t variables.
+	* lib/parse-datetime.y: Include idx.h.
+	(textint): Mark digits as nonnegative.
+	(parser_control): Mark dates_seen, days_seen, local_zones_seen,
+	dsts_seen, times_seen, zones_seen as nonnegative.
+	(lookup_word): Mark wordlen as nonnegative.
+	(yylex): Mark count as nonnegative.
+	(parse_datetime2): Mark tzsize as nonnegative.
+	* modules/parse-datetime (Depends-on): Add idx.
+
+2020-12-05  Bruno Haible  <bruno@clisp.org>
+
 	fnmatch: Use idx_t for nonnegative ptrdiff_t variables.
 	* lib/fnmatch.c: Include idx.h. In glibc, define idx_t directly.
 	* lib/fnmatch_loop.c (EXT): Mark slen, new_used, plensize as
diff --git a/lib/parse-datetime.y b/lib/parse-datetime.y
index e8ed691..2b56db4 100644
--- a/lib/parse-datetime.y
+++ b/lib/parse-datetime.y
@@ -35,6 +35,7 @@
 
 #include "parse-datetime.h"
 
+#include "idx.h"
 #include "intprops.h"
 #include "timespec.h"
 #include "verify.h"
@@ -139,7 +140,7 @@ typedef struct
 {
   bool negative;
   intmax_t value;
-  ptrdiff_t digits;
+  idx_t digits;
 } textint;
 
 /* An entry in the lexical lookup table.  */
@@ -212,12 +213,12 @@ typedef struct
   /* Presence or counts of nonterminals of various flavors parsed so far.  */
   bool timespec_seen;
   bool rels_seen;
-  ptrdiff_t dates_seen;
-  ptrdiff_t days_seen;
-  ptrdiff_t local_zones_seen;
-  ptrdiff_t dsts_seen;
-  ptrdiff_t times_seen;
-  ptrdiff_t zones_seen;
+  idx_t dates_seen;
+  idx_t days_seen;
+  idx_t local_zones_seen;
+  idx_t dsts_seen;
+  idx_t times_seen;
+  idx_t zones_seen;
   bool year_seen;
 
   /* Print debugging output to stderr.  */
@@ -1334,7 +1335,7 @@ lookup_word (parser_control const *pc, char *word)
 {
   char *p;
   char *q;
-  ptrdiff_t wordlen;
+  idx_t wordlen;
   table const *tp;
   bool period_found;
   bool abbrev;
@@ -1514,7 +1515,7 @@ yylex (union YYSTYPE *lvalp, parser_control *pc)
       if (c != '(')
         return to_uchar (*pc->input++);
 
-      ptrdiff_t count = 0;
+      idx_t count = 0;
       do
         {
           c = *pc->input++;
@@ -1750,7 +1751,7 @@ parse_datetime2 (struct timespec *result, char const *p,
   if (strncmp (p, "TZ=\"", 4) == 0)
     {
       char const *tzbase = p + 4;
-      ptrdiff_t tzsize = 1;
+      idx_t tzsize = 1;
       char const *s;
 
       for (s = tzbase; *s; s++, tzsize++)
diff --git a/modules/parse-datetime b/modules/parse-datetime
index a16b90f..820bc6c 100644
--- a/modules/parse-datetime
+++ b/modules/parse-datetime
@@ -15,6 +15,7 @@ c-ctype
 stdbool
 gettime
 gettext-h
+idx
 intprops
 inttypes
 mktime
-- 
2.7.4


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From f09bd3c4dfa248da10dbec6ac5708f61239dca09 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 5 Dec 2020 11:53:00 +0100
Subject: [PATCH 5/6] time_rz: Use idx_t for nonnegative ptrdiff_t variables.

* lib/time_rz.c: Include idx.h.
(save_abbr): Mark zone_size as nonnegative.
* modules/time_rz (Depends-on): Add idx.
---
 ChangeLog       | 7 +++++++
 lib/time_rz.c   | 3 ++-
 modules/time_rz | 1 +
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 57dea1d..9c0fef9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2020-12-05  Bruno Haible  <bruno@clisp.org>
 
+	time_rz: Use idx_t for nonnegative ptrdiff_t variables.
+	* lib/time_rz.c: Include idx.h.
+	(save_abbr): Mark zone_size as nonnegative.
+	* modules/time_rz (Depends-on): Add idx.
+
+2020-12-05  Bruno Haible  <bruno@clisp.org>
+
 	parse-datetime: Use idx_t for nonnegative ptrdiff_t variables.
 	* lib/parse-datetime.y: Include idx.h.
 	(textint): Mark digits as nonnegative.
diff --git a/lib/time_rz.c b/lib/time_rz.c
index a33b807..6e70ca9 100644
--- a/lib/time_rz.c
+++ b/lib/time_rz.c
@@ -33,6 +33,7 @@
 #include <string.h>
 
 #include "flexmember.h"
+#include "idx.h"
 #include "time-internal.h"
 
 /* The approximate size to use for small allocation requests.  This is
@@ -120,7 +121,7 @@ save_abbr (timezone_t tz, struct tm *tm)
         {
           if (! (*zone_copy || (zone_copy == tz->abbrs && tz->tz_is_set)))
             {
-              ptrdiff_t zone_size = strlen (zone) + 1;
+              idx_t zone_size = strlen (zone) + 1;
               if (zone_size < tz->abbrs + ABBR_SIZE_MIN - zone_copy)
                 extend_abbrs (zone_copy, zone, zone_size);
               else
diff --git a/modules/time_rz b/modules/time_rz
index 699a61d..6a2b23c 100644
--- a/modules/time_rz
+++ b/modules/time_rz
@@ -22,6 +22,7 @@ c99
 extensions
 time
 flexmember     [test $HAVE_TIMEZONE_T = 0]
+idx            [test $HAVE_TIMEZONE_T = 0]
 setenv         [test $HAVE_TIMEZONE_T = 0]
 stdbool        [test $HAVE_TIMEZONE_T = 0]
 time_r         [test $HAVE_TIMEZONE_T = 0]
-- 
2.7.4


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From fe6ed193c3eb8bf99aa6440158bbd4f7ae8122f0 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 5 Dec 2020 11:54:20 +0100
Subject: [PATCH 6/6] filenamecat-tests: Use idx_t for nonnegative ptrdiff_t
 variables.

* tests/test-filenamecat.c: Include idx.h.
(main): Mark prefixlen as nonnegative.
* modules/filenamecat-tests (Depends-on): Add idx.
---
 ChangeLog                 | 7 +++++++
 modules/filenamecat-tests | 1 +
 tests/test-filenamecat.c  | 4 +++-
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 9c0fef9..5364c15 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2020-12-05  Bruno Haible  <bruno@clisp.org>
 
+	filenamecat-tests: Use idx_t for nonnegative ptrdiff_t variables.
+	* tests/test-filenamecat.c: Include idx.h.
+	(main): Mark prefixlen as nonnegative.
+	* modules/filenamecat-tests (Depends-on): Add idx.
+
+2020-12-05  Bruno Haible  <bruno@clisp.org>
+
 	time_rz: Use idx_t for nonnegative ptrdiff_t variables.
 	* lib/time_rz.c: Include idx.h.
 	(save_abbr): Mark zone_size as nonnegative.
diff --git a/modules/filenamecat-tests b/modules/filenamecat-tests
index 14d3c17..7b2a98f 100644
--- a/modules/filenamecat-tests
+++ b/modules/filenamecat-tests
@@ -2,6 +2,7 @@ Files:
 tests/test-filenamecat.c
 
 Depends-on:
+idx
 stdbool
 
 configure.ac:
diff --git a/tests/test-filenamecat.c b/tests/test-filenamecat.c
index a6f236e..40259dc 100644
--- a/tests/test-filenamecat.c
+++ b/tests/test-filenamecat.c
@@ -21,6 +21,8 @@
 
 #include "filenamecat.h"
 
+#include "idx.h"
+
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -54,7 +56,7 @@ main (int argc _GL_UNUSED, char *argv[])
       char *base_in_result;
       char const *const *t = tests[i];
       char *res = file_name_concat (t[0], t[1], &base_in_result);
-      ptrdiff_t prefixlen = base_in_result - res;
+      idx_t prefixlen = base_in_result - res;
       size_t t0len = strlen (t[0]);
       size_t reslen = strlen (res);
       if (strcmp (res, t[2]) != 0)
-- 
2.7.4

Re: Make more use of idx_t

[Paul Eggert]

Those all look good to me.

Re: Make more use of idx_t

[Bruno Haible]

Paul Eggert wrote:
> Those all look good to me.

OK, I pushed them.

Bruno

Re: Make more use of idx_t

[Paul Eggert]

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On 12/6/20 3:32 AM, Bruno Haible wrote:
> Paul Eggert wrote:
>> Those all look good to me.
> 
> OK, I pushed them.

When I looked into this a bit more when hacking on canonicalize-lgpl, I 
realized I spoke too soon. First, there's some cruft left over from when 
idx_t could be unsigned. More important, defining IDX_WIDTH to be 
PTRDIFF_WIDTH - 1 is confusing as this makes idx_t different from all other 
signed types in that IDX_WIDTH does not count the sign bit. I can see why it 
doesn't count the sign bit (the sign bit is always zero) but I can also see 
why it should (shift counts are limited by the *_WIDTH macros). If we're 
going to have a macro that stands for PTRDIFF_WIDTH - 1 it'd probably be 
better to use a different naming convention, to avoid that confusion. 
However, nobody's using any such macro now and possibly we'll never need it 
so in the meantime I merely substituted a comment for the #define in the 
attached patch.

Hope this is OK; further comments welcome of course.

[-- Attachment #2: 0001-idx-simplify-IDX_MAX-remove-IDX_WIDTH.patch --]
[-- Type: text/x-patch, Size: 3725 bytes --]

From 03d1588fca2bffe92b08bb4f4ba69273f4843431 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 16 Dec 2020 23:52:24 -0800
Subject: [PATCH] idx: simplify IDX_MAX, remove IDX_WIDTH
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* lib/idx.h (IDX_MAX): Simplify by removing obsolete reference
to UNSIGNED_IDX_T.
(IDX_WIDTH): Remove, since it’s not used and its value
arguably should be PTRDIFF_WIDTH anyway.
---
 ChangeLog |  8 ++++++++
 lib/idx.h | 33 ++++++++++++++-------------------
 2 files changed, 22 insertions(+), 19 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index d50539adc..445e2c074 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2020-12-17  Paul Eggert  <eggert@cs.ucla.edu>
+
+	idx: simplify IDX_MAX, remove IDX_WIDTH
+	* lib/idx.h (IDX_MAX): Simplify by removing obsolete reference
+	to UNSIGNED_IDX_T.
+	(IDX_WIDTH): Remove, since it’s not used and its value
+	arguably should be PTRDIFF_WIDTH anyway.
+
 2020-12-16  Bruno Haible  <bruno@clisp.org>
 
 	posix_spawn_file_actions_addfchdir-tests: Rename test.
diff --git a/lib/idx.h b/lib/idx.h
index 0095467dc..ad7d31a2b 100644
--- a/lib/idx.h
+++ b/lib/idx.h
@@ -21,11 +21,12 @@
 /* Get ptrdiff_t.  */
 #include <stddef.h>
 
-/* Get PTRDIFF_WIDTH.  */
+/* Get PTRDIFF_MAX.  */
 #include <stdint.h>
 
 /* The type 'idx_t' holds an (array) index or an (object) size.
-   Its implementation is a signed integer type, capable of holding the values
+   Its implementation promotes to a signed integer type,
+   which can hold the values
      0..2^63-1 (on 64-bit platforms) or
      0..2^31-1 (on 32-bit platforms).
 
@@ -87,32 +88,26 @@
        or to the C standard.  Several programming languages (Ada, Haskell,
        Common Lisp, Pascal) already have range types.  Such range types may
        help producing good code and good warnings.  The type 'idx_t' could
-       then be typedef'ed to a (signed!) range type.  */
+       then be typedef'ed to a range type that is signed after promotion.  */
 
-#if 0
-/* In the future, idx_t could be typedef'ed to a signed range type.  */
-/* Note: The clang "extended integer types", supported in Clang 11 or newer
+/* In the future, idx_t could be typedef'ed to a signed range type.
+   The clang "extended integer types", supported in Clang 11 or newer
    <https://clang.llvm.org/docs/LanguageExtensions.html#extended-integer-types>,
    are a special case of range types.  However, these types don't support binary
    operators with plain integer types (e.g. expressions such as x > 1).
    Therefore, they don't behave like signed types (and not like unsigned types
    either).  So, we cannot use them here.  */
-typedef <some_range_type> idx_t;
-#else
-/* Use the signed type 'ptrdiff_t' by default.  */
+
+/* Use the signed type 'ptrdiff_t'.  */
 /* Note: ISO C does not mandate that 'size_t' and 'ptrdiff_t' have the same
-   size, but it is so an all platforms we have seen since 1990.  */
+   size, but it is so on all platforms we have seen since 1990.  */
 typedef ptrdiff_t idx_t;
-#endif
 
 /* IDX_MAX is the maximum value of an idx_t.  */
-#if defined UNSIGNED_IDX_T
-# define IDX_MAX SIZE_MAX
-#else
-# define IDX_MAX PTRDIFF_MAX
-#endif
-
-/* IDX_WIDTH is the number of bits in an idx_t (31 or 63).  */
-#define IDX_WIDTH (PTRDIFF_WIDTH - 1)
+#define IDX_MAX PTRDIFF_MAX
+
+/* So far no need has been found for an IDX_WIDTH macro.
+   Perhaps there should be another macro IDX_VALUE_BITS that does not
+   count the sign bit and is therefore one less than PTRDIFF_WIDTH.  */
 
 #endif /* _IDX_H */
-- 
2.27.0

Re: Make more use of idx_t

[Bruno Haible]

Paul Eggert wrote:
> If we're 
> going to have a macro that stands for PTRDIFF_WIDTH - 1 it'd probably be 
> better to use a different naming convention, to avoid that confusion.

Makes sense. Yes, IDX_VALUE_BITS would be a better name than IDX_WIDTH.

Bruno