[Bug c++/55513] New: Incorrect snprintf folding when building with -std=c++0x

[Bug c++/55513] New: Incorrect snprintf folding when building with -std=c++0x

[gnobal at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

             Bug #: 55513
           Summary: Incorrect snprintf folding when building with
                    -std=c++0x
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gnobal@hotmail.com


Using the following program:

  #include <cstdio>
  #include <iostream>

  int main() {
    char str[10];
    const int strLength = snprintf(str, sizeof(str), "Hello");
    std::cout << strLength << ':' << str << std::endl;
    return 0;
  }

When built with the following command:
g++ test_snprintf.cc -O0 -g3 -std=c++0x

The output of running it is:
5: 
instead of:
5: Hello

The program will work fine if either of the following changes are made
1. -std=c++0x is removed from the build command line.
2. The const is removed from the definition of the variable strLength

Here's a disassembly of main() up until the call to ostream::operator<<, when
built with -std=c++0x:
   0x0000000000400a6c <+0>:     push   %rbp
   0x0000000000400a6d <+1>:     mov    %rsp,%rbp
   0x0000000000400a70 <+4>:     sub    $0x30,%rsp
   0x0000000000400a74 <+8>:     mov    %fs:0x28,%rax
   0x0000000000400a7d <+17>:    mov    %rax,-0x8(%rbp)
   0x0000000000400a81 <+21>:    xor    %eax,%eax
   0x0000000000400a83 <+23>:    movl   $0x5,-0x24(%rbp)
   0x0000000000400a8a <+30>:    mov    $0x5,%esi
   0x0000000000400a8f <+35>:    mov    $0x6010a0,%edi
   0x0000000000400a94 <+40>:    callq  0x4008a0 <_ZNSolsEi@plt>
   0x0000000000400a99 <+45>:    mov    $0x3a,%esi
   0x0000000000400a9e <+50>:    mov    %rax,%rdi
   0x0000000000400aa1 <+53>:    callq  0x4008e0
<_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_c@plt>

Here's a disassembly of main() up until the call to ostream::operator<<, when
built without -std=c++0x _or_ when the const is removed (same result):
   0x0000000000400a6c <+0>:     push   %rbp
   0x0000000000400a6d <+1>:     mov    %rsp,%rbp
   0x0000000000400a70 <+4>:     sub    $0x30,%rsp
   0x0000000000400a74 <+8>:     mov    %fs:0x28,%rax
   0x0000000000400a7d <+17>:    mov    %rax,-0x8(%rbp)
   0x0000000000400a81 <+21>:    xor    %eax,%eax
   0x0000000000400a83 <+23>:    lea    -0x20(%rbp),%rax
   0x0000000000400a87 <+27>:    movl   $0x6c6c6548,(%rax)
   0x0000000000400a8d <+33>:    movw   $0x6f,0x4(%rax)
   0x0000000000400a93 <+39>:    movl   $0x5,-0x24(%rbp)
   0x0000000000400a9a <+46>:    mov    -0x24(%rbp),%eax
   0x0000000000400a9d <+49>:    mov    %eax,%esi
   0x0000000000400a9f <+51>:    mov    $0x6010a0,%edi
   0x0000000000400aa4 <+56>:    callq  0x4008a0 <_ZNSolsEi@plt>
   0x0000000000400aa9 <+61>:    mov    $0x3a,%esi
   0x0000000000400aae <+66>:    mov    %rax,%rdi
   0x0000000000400ab1 <+69>:    callq  0x4008e0
<_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_c@plt>


OS Version: Ubuntu 12.10

uname -a output:
Linux desktop 3.5.0-18-generic #29-Ubuntu SMP Fri Oct 19 10:26:51 UTC 2012
x86_64 x86_64 x86_64 GNU/Linux

gcc --version output:
gcc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2

[Bug c++/55513] Incorrect snprintf folding when building with -std=c++0x

[gnobal at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

--- Comment #1 from Amit Schreiber <gnobal at hotmail dot com> 2012-11-28 10:05:16 UTC ---
Created attachment 28819
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28819
The program

[Bug c++/55513] [4.7 Regression] Incorrect snprintf folding when building with -std=c++0x

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-11-28
      Known to work|                            |4.6.3, 4.8.0
            Summary|Incorrect snprintf folding  |[4.7 Regression] Incorrect
                   |when building with          |snprintf folding when
                   |-std=c++0x                  |building with -std=c++0x
     Ever Confirmed|0                           |1
      Known to fail|                            |4.7.1, 4.7.2

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-11-28 10:18:57 UTC ---
Confirmed. -fno-builtin fixes it too.

[Bug c++/55513] [4.7 Regression] Incorrect snprintf folding when building with -std=c++0x

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.7.3
      Known to fail|                            |4.7.0

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> 2012-11-28 12:35:16 UTC ---
Seems to be simply folded away.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org
            Summary|[4.7 Regression] Incorrect  |[4.7/4.8 Regression]
                   |snprintf folding when       |Incorrect snprintf folding
                   |building with -std=c++0x    |when building with
                   |                            |-std=c++0x

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-28 14:25:44 UTC ---
With const int strLength = snprintf(str, 10, "Hello"); instead it fails also on
the trunk, with sizeof(str) it has been "fixed" by my
http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=192141
changes.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-28 15:15:11 UTC ---
In cp_parser_constant_expression when we call cp_parser_assignment_expression
on the initializer, it returns
((char *) __builtin_memcpy ((char *) &str, (const char *) "Hello", 6);, 5)
As this is C++11, we call potential_rvalue_constant_expression which returns
true (the call in the COMPOUND_EXPR is builtin, and all its arguments are
pontential_rvalue_constant_expression arguments).  I bet somewhere later on we
see that it is const and ignore the TREE_SIDE_EFFECTS initializer.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-28 15:51:21 UTC ---
Reduced testcase:
void
f1 ()
{
  char s[10];
  const int t = __builtin_snprintf (s, 10, "Hello");
  __builtin_printf ("%d %s\n", t, s);
}

void
f2 ()
{
  char s[10];
  const int t = (__builtin_memcpy (s, "Hello", 6), 5);
  __builtin_printf ("%d %s\n", t, s);
}

int
main ()
{
  f1 ();
  f2 ();
}

Even f2 fails to call memcpy, so it isn't related to folding the snprintf into
what it folds to, but about handling COMPOUND_EXPRs with side-effects on the
LHS where the only side-effects are builtin calls.  If the LHS of the
COMPOUND_EXPR is a non-builtin, potential_rvalue_constant_expression returns
false and it is handled correctly, but even if it is say __builtin_exit (0) or
similar, it is optimized away.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org
         AssignedTo|unassigned at gcc dot       |aldyh at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #7 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-12-05 17:08:48 UTC ---
I'll take a look.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
      Known to work|4.8.0                       |
      Known to fail|                            |4.8.0

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

--- Comment #8 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-12-06 21:06:29 UTC ---
After Jason's patch here (trunk@177066):

        PR c++/49813
        * semantics.c (potential_constant_expression_1): Allow any builtin.
        (morally_constexpr_builtin_function_p): Remove.

It seems we return true for any built-in in potential_constant_expression_1. 
This means that check_initializer->store_init_value->maybe_constant_init will
chop off the built-in side of a COMPOUND_EXPR when setting a DECL_INITIAL,
transforming:

(COMPOUND_EXPR (CALL_EXPR built-in) INTEGER_CST)

into

INTEGER_CST

Since potential_constant_expression_1 returns true for any built-in, we end up
removing even a __builtin_exit from code like this:

const int t = (__builtin_exit(0), 777);

Jason, is this by design? Should we not limit the conditional here further?:

                   /* Allow any built-in function; if the expansion
                      isn't constant, we'll deal with that then.  */
                   && !is_builtin_fn (fun))

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

--- Comment #9 from Jason Merrill <jason at gcc dot gnu.org> 2012-12-07 05:21:01 UTC ---
(In reply to comment #8)
>                    /* Allow any built-in function; if the expansion
>                       isn't constant, we'll deal with that then.  */
>                    && !is_builtin_fn (fun))

The problem seems to be in the dealing with it "then";
cxx_eval_builtin_function_call needs to actually check whether folding the call
produces a constant or not.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

--- Comment #10 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-12-08 01:20:33 UTC ---
Author: aldyh
Date: Sat Dec  8 01:19:13 2012
New Revision: 194316

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=194316
Log:
    PR c++/55513
    * semantics.c (cxx_eval_builtin_function_call): Set non_constant_p
    after folding.

Added:
    trunk/gcc/testsuite/g++.dg/pr55513.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #11 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-12-08 01:24:30 UTC ---
Fixed in trunk.

[Bug c++/55513] [4.7/4.8 Regression] Incorrect snprintf folding when building with -std=c++0x

[pluto at agmk dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55513

--- Comment #12 from Pawel Sikora <pluto at agmk dot net> 2012-12-12 09:57:32 UTC ---
(In reply to comment #11)
> Fixed in trunk.

no backport to 4.7 branch?