GNU bug report logs - #11927
shred.c i686-specific warning from gcc-4.7 on fedora 17

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Jim Meyering <jim <at> meyering.net>
Subject: bug#11927: closed (Re: bug#11927: shred.c i686-specific warning
 from gcc-4.7 on fedora 17)
Date: Sun, 15 Jul 2012 09:43:01 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#11927: shred.c i686-specific warning from gcc-4.7 on fedora 17

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 11927 <at> debbugs.gnu.org.

-- 
11927: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11927
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Jim Meyering <jim <at> meyering.net>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 11927-done <at> debbugs.gnu.org
Subject: Re: bug#11927: shred.c i686-specific warning from gcc-4.7 on fedora 17
Date: Sun, 15 Jul 2012 11:36:16 +0200

Paul Eggert wrote:
> On 07/12/2012 02:27 PM, Jim Meyering wrote:
>> I didn't really understand the cause
>
> That one is one of my least favorite warnings
> because sometimes it warns about real bugs but
> often, as in your case, the warning is either
> bogus or so hard to figure out that it might as
> well be bogus.  I see that you and I and others have
> been filing GCC bug reports in this area:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51309
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34515
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52560
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49234
>
> In this particular case the shred.c rewrite seems harmless.

Thanks for the review.
I've just hit it again, so have pushed this:

From b255b9a68a0dcec113a193daa602bdcbda2658db Mon Sep 17 00:00:00 2001
From: Jim Meyering <meyering <at> redhat.com>
Date: Sun, 15 Jul 2012 11:31:18 +0200
Subject: [PATCH] build: shred.c: avoid i686-specific gcc -Wstrict-overflow
 warning

* src/shred.c: Avoid gcc -Wstrict-overflow warning.
Addresses http://bugs.gnu.org/11927
---
 src/shred.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/src/shred.c b/src/shred.c
index 7a28260..322bcf1 100644
--- a/src/shred.c
+++ b/src/shred.c
@@ -485,10 +485,11 @@ dopass (int fd, char const *qname, off_t *sizep, int type,

       offset += soff;

+      bool done = offset == size;
+
       /* Time to print progress? */
-      if (n
-          && ((offset == size && *previous_human_offset)
-              || thresh <= (now = time (NULL))))
+      if (n && ((done && *previous_human_offset)
+                || thresh <= (now = time (NULL))))
         {
           char offset_buf[LONGEST_HUMAN_READABLE + 1];
           char size_buf[LONGEST_HUMAN_READABLE + 1];
@@ -498,8 +499,7 @@ dopass (int fd, char const *qname, off_t *sizep, int type,
             = human_readable (offset, offset_buf,
                               human_floor | human_progress_opts, 1, 1);

-          if (offset == size
-              || !STREQ (previous_human_offset, human_offset))
+          if (done || !STREQ (previous_human_offset, human_offset))
             {
               if (size < 0)
                 error (0, 0, _("%s: pass %lu/%lu (%s)...%s"),
@@ -516,7 +516,7 @@ dopass (int fd, char const *qname, off_t *sizep, int type,
                     = human_readable (size, size_buf,
                                       human_ceiling | human_progress_opts,
                                       1, 1);
-                  if (offset == size)
+                  if (done)
                     human_offset = human_size;
                   error (0, 0, _("%s: pass %lu/%lu (%s)...%s/%s %d%%"),
                          qname, k, n, pass_string, human_offset, human_size,
--
1.7.11.2.194.g7bdb748

[Message part 3 (message/rfc822, inline)]

From: Jim Meyering <jim <at> meyering.net>
To: bug-coreutils <at> gnu.org
Subject: shred.c i686-specific warning from gcc-4.7 on fedora 17
Date: Thu, 12 Jul 2012 23:27:15 +0200

[mostly for the record, and so I don't forget.
 I'm not ready to push this just yet. ]

Building shred with fedora 17 and its stock gcc 4.7 on i686,
I see this warning/error:

    CC     shred.o
  shred.c: In function 'dopass':
  shred.c:501:14: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
  shred.c:504:18: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
  cc1: all warnings being treated as errors
  make[3]: *** [shred.o] Error 1
  make[3]: Leaving directory `/f12/home/meyering/w/co/cu/src'
  make[2]: *** [all] Error 2
  make[2]: Leaving directory `/f12/home/meyering/w/co/cu/src'
  make[1]: *** [all-recursive] Error 1
  make[1]: Leaving directory `/f12/home/meyering/w/co/cu'
  make: *** [all] Error 2

I didn't really understand the cause but ended up
writing the following patch to avoid the problem
while making the code slightly more readable:


diff --git a/src/shred.c b/src/shred.c
index 7a28260..322bcf1 100644
--- a/src/shred.c
+++ b/src/shred.c
@@ -485,10 +485,11 @@ dopass (int fd, char const *qname, off_t *sizep, int type,

       offset += soff;

+      bool done = offset == size;
+
       /* Time to print progress? */
-      if (n
-          && ((offset == size && *previous_human_offset)
-              || thresh <= (now = time (NULL))))
+      if (n && ((done && *previous_human_offset)
+                || thresh <= (now = time (NULL))))
         {
           char offset_buf[LONGEST_HUMAN_READABLE + 1];
           char size_buf[LONGEST_HUMAN_READABLE + 1];
@@ -498,8 +499,7 @@ dopass (int fd, char const *qname, off_t *sizep, int type,
             = human_readable (offset, offset_buf,
                               human_floor | human_progress_opts, 1, 1);

-          if (offset == size
-              || !STREQ (previous_human_offset, human_offset))
+          if (done || !STREQ (previous_human_offset, human_offset))
             {
               if (size < 0)
                 error (0, 0, _("%s: pass %lu/%lu (%s)...%s"),
@@ -516,7 +516,7 @@ dopass (int fd, char const *qname, off_t *sizep, int type,
                     = human_readable (size, size_buf,
                                       human_ceiling | human_progress_opts,
                                       1, 1);
-                  if (offset == size)
+                  if (done)
                     human_offset = human_size;
                   error (0, 0, _("%s: pass %lu/%lu (%s)...%s/%s %d%%"),
                          qname, k, n, pass_string, human_offset, human_size,

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