[Bug tree-optimization/85700] Spurious -Wstringop-truncation warning with strncat

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85700

Martin Sebor  changed:

   What|Removed |Added

 Status|ASSIGNED|RESOLVED
 Resolution|--- |FIXED

--- Comment #5 from Martin Sebor  ---
Patch committed to the trunk of GCC 9 in r262110.  Patrick, please reopen the
bug if it's important to you that it be backported to GCC 8 (e.g., if it's
preventing you or your users from enabling the new option).

[Bug tree-optimization/85700] Spurious -Wstringop-truncation warning with strncat

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85700

--- Comment #4 from Martin Sebor  ---
Author: msebor
Date: Mon Jun 25 20:33:28 2018
New Revision: 262110

URL: https://gcc.gnu.org/viewcvs?rev=262110=gcc=rev
Log:
PR tree-optimization/85700 - Spurious -Wstringop-truncation warning with
strncat

gcc/ChangeLog:

PR tree-optimization/85700
* gimple-fold.c (gimple_fold_builtin_strncat): Adjust comment.
* tree-ssa-strlen.c (is_strlen_related_p): Handle integer subtraction.
(maybe_diag_stxncpy_trunc): Distinguish strncat from strncpy.

gcc/testsuite/ChangeLog:

PR tree-optimization/85700
* gcc.dg/Wstringop-truncation-4.c: New test.


Added:
trunk/gcc/testsuite/gcc.dg/Wstringop-truncation-4.c
Modified:
trunk/gcc/ChangeLog
trunk/gcc/gimple-fold.c
trunk/gcc/testsuite/ChangeLog
trunk/gcc/tree-ssa-strlen.c

[Bug tree-optimization/85700] Spurious -Wstringop-truncation warning with strncat

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85700

Martin Sebor  changed:

   What|Removed |Added

   Keywords||patch

--- Comment #3 from Martin Sebor  ---
Patch: https://gcc.gnu.org/ml/gcc-patches/2018-05/msg01189.html

[Bug tree-optimization/85700] Spurious -Wstringop-truncation warning with strncat

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85700

Martin Sebor  changed:

   What|Removed |Added

 Status|NEW |ASSIGNED
   Assignee|unassigned at gcc dot gnu.org  |msebor at gcc dot 
gnu.org

--- Comment #2 from Martin Sebor  ---
Let me look into it.

[Bug tree-optimization/85700] Spurious -Wstringop-truncation warning with strncat

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85700

Martin Sebor  changed:

   What|Removed |Added

   Keywords||diagnostic
 Status|UNCONFIRMED |NEW
   Last reconfirmed||2018-05-08
 CC||msebor at gcc dot gnu.org
  Component|c++ |tree-optimization
 Ever confirmed|0   |1

--- Comment #1 from Martin Sebor  ---
Confirmed.  Strncat is handled in maybe_diag_stxncpy_trunc() the same way as
strncpy and the approach doesn't seem right for the former.  The following test
case shows the inconsistency.  (The difference between C and C++ is due to the
C  defining strncat as a macro which suppresses the warning due to
-Wno-system-headers; in C++, strncat is defined as a function so
-Wno-system-headers has no effect on calls to it.)

$ cat u.c && gcc -O2 -S -Wall -ftrack-macro-expansion=0 /build/tmp/u.c
#define strncat __builtin_strncat
#define strlen __builtin_strlen

char a[4], b[4];

void f1 (void)
{
  strncat (a, "1", sizeof a - strlen (a) - 1);   // no warning (good)
}

void f2 (void)
{
  strncat (a, "12", sizeof a - strlen (a) - 1);  // bogus -Wstringop-truncation
}

void fx (void)
{
  strncat (a, b, sizeof a - strlen (a) - 1); // bogus -Wstringop-truncation
}

/build/tmp/u.c: In function ‘f2’:
/build/tmp/u.c:13:3: warning: ‘__builtin_strncat’ output may be truncated
copying between 0 and 3 bytes from a string of length 2 [-Wstringop-truncation]
   strncat (a, "12", sizeof a - strlen (a) - 1);  // bogus
-Wstringop-truncation
   ^~~~
/build/tmp/u.c: In function ‘fx’:
/build/tmp/u.c:18:3: warning: ‘__builtin_strncat’ output may be truncated
copying between 0 and 3 bytes from a string of length 3 [-Wstringop-truncation]
   strncat (a, b, sizeof a - strlen (a) - 1); // bogus
-Wstringop-truncation
   ^