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From: "walter.gcc at wjd dot nu" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97628] New: format truncation false positive
 for O1 and mtune=nocona
Date: Thu, 29 Oct 2020 12:42:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97628

            Bug ID: 97628
           Summary: format truncation false positive for O1 and
                    mtune=3Dnocona
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: walter.gcc at wjd dot nu
  Target Milestone: ---

Created attachment 49468
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D49468&action=3Dedit
trimmed down problem from Asterisk say.c

Per the discussion here:
https://gerrit.asterisk.org/c/asterisk/+/14501

I believe there is a bug in gcc 9 where it falsely reports:

  warning: =E2=80=98%d=E2=80=99 directive output may be truncated writing
    between 1 and 11 bytes into a region of size 10
    [-Wformat-truncation=3D]

See the attached example, it only triggers when:

- optimization is 1 or higher
- arch tuning is set to nocona

Example:

$ gcc -O1 -mtune=3Dnocona -S -Wall say2.c
say2.c: In function =E2=80=98ast_say_number_full_zh=E2=80=99:
say2.c:37:32: warning: =E2=80=98%d=E2=80=99 directive output may be truncat=
ed writing between 1
and 11 bytes into a region of size 10 [-Wformat-truncation=3D]
   37 |             snprintf(buf, 10, "%d", num);
      |                                ^~
say2.c:37:31: note: directive argument in the range [-2147483648, 9]
...

But when omitting either -O1 or -mtune=3Dnocona we do not get this warning.

And if you read the code, you would see that at line 37, num cannot be nega=
tive
(because it would've hit a different if-branch).

gcc version is the Ubuntu/Focal one:

    $ gcc --version
    gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0


Cheers,
Walter Doekes
OSSO B.V.=