[Bug c/59753] New: Missing -Woverflow warning with signed constant conversion between T_MAX+1 and UT_MAX

[Bug c/59753] New: Missing -Woverflow warning with signed constant conversion between T_MAX+1 and UT_MAX

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

            Bug ID: 59753
           Summary: Missing -Woverflow warning with signed constant
                    conversion between T_MAX+1 and UT_MAX
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net

When writing "int d = some_signed_constant;", one expects a -Woverflow warning
if some_signed_constant > INT_MAX, but one actually gets a warning only if
some_signed_constant > UINT_MAX. Similarly for other types, such as char and
short.

For instance, with the following program, one expects 6 warnings, but one only
gets 2.

#include <stdio.h>
int main (void)
{
  short a = 32768;
  short b = 65535;
  short c = 65536;
  int d = 2147483648;
  int e = 4294967295;
  int f = 4294967296;
  printf ("%d %d %d %d %d %d\n", a, b, c, d, e, f);
  return 0;
}

$ gcc-snapshot tst.c -o tst
tst.c: In function 'main':
tst.c:6:3: warning: overflow in implicit constant conversion [-Woverflow]
   short c = 65536;
   ^
tst.c:9:3: warning: overflow in implicit constant conversion [-Woverflow]
   int f = 4294967296;
   ^

This occurs with:
gcc (Debian 20131201-1) 4.9.0 20131201 (experimental) [trunk revision 205573]
and older versions (4.7 and 4.8 at least).

[Bug c/59753] Missing -Woverflow warning with signed constant conversion between T_MAX+1 and UT_MAX

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |WORKSFORME

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
-Wconversion catches all of these.

[Bug c/59753] Missing -Woverflow warning with signed constant conversion between T_MAX+1 and UT_MAX

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

Vincent Lefèvre <vincent-gcc at vinc17 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|WORKSFORME                  |---

--- Comment #2 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
-Wconversion is used to warn about possibly-suspicious valid code such as:

  unsigned ui = -1;

(see the gcc man page), not for overflows. At least there is an inconsistency
in the use of warnings. There is no reason why 65535 and 65536 should be
treated differently when assigned to a short: neither value is representable in
a short.
>From gcc-bugs-return-442634-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Feb 05 11:42:12 2014
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Subject: [Bug tree-optimization/60042] vectorizer still does too many dependence tests for himeno:jacobi
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`042

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |23855

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
For analysis of the DRs in outer loops we need to fix PR23855.

[Bug c/59753] Missing -Woverflow warning with signed constant conversion between T_MAX+1 and UT_MAX

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
OK, if you use -Wpedantic, you'll get all the overflow warnings even without
-Wconversion.

[Bug c/59753] -Woverflow warning inconsistency with signed constant conversion between T_MAX+1 and UT_MAX vs larger than UT_MAX

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

Vincent Lefèvre <vincent-gcc at vinc17 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|WORKSFORME                  |---
            Summary|Missing -Woverflow warning  |-Woverflow warning
                   |with signed constant        |inconsistency with signed
                   |conversion between T_MAX+1  |constant conversion between
                   |and UT_MAX                  |T_MAX+1 and UT_MAX vs
                   |                            |larger than UT_MAX

--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
There's still an inconsistency without -Wpedantic, which is the point of this
bug. I've changed the bug title to:

  -Woverflow warning inconsistency with signed constant conversion between
T_MAX+1 and UT_MAX vs larger than UT_MAX

If this inconsistency is intentional, then it should be documented. The current
behavior doesn't match the documentation:

    -Wno-overflow
        Do not warn about compile-time overflow in constant expressions.

I fail to see why there should be a warning for

  short c = 65536;

but not for

  short b = 65535;

BTW, there's actually no overflow in the constant expressions themselves, just
in the assignments.
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Subject: [Bug ada/60078] acats c761007 fails on ARM
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Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

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                 CC|                            |ebotcazou at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
Is that on real hardware or simulator?  This test is one of the infamous tests
that require very solid concurrency support from the environment.

[Bug c/59753] -Woverflow warning inconsistency with signed constant conversion between T_MAX+1 and UT_MAX vs larger than UT_MAX

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Vincent Lefèvre from comment #4)
> There's still an inconsistency without -Wpedantic, which is the point of
> this bug. I've changed the bug title to:
> 
>   -Woverflow warning inconsistency with signed constant conversion between
> T_MAX+1 and UT_MAX vs larger than UT_MAX

I seem to remember this inconsistency is due to the exact definition of
overflow given in the standard. But I agree that this is confusing for the rest
of humanity who don't know the standard by heart. Also, the warnings would be
useful by default since they are likely mistakes.

> If this inconsistency is intentional, then it should be documented. The
> current behavior doesn't match the documentation:

Agreed.
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Subject: [Bug middle-end/59737] [4.9 Regression] ice from optimize_inline_calls
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Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with r206042.

[Bug c/59753] -Woverflow warning inconsistency with signed constant conversion between T_MAX+1 and UT_MAX vs larger than UT_MAX

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59753

--- Comment #6 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Manuel López-Ibáñez from comment #5)
> (In reply to Vincent Lefèvre from comment #4)
> > There's still an inconsistency without -Wpedantic, which is the point of
> > this bug. I've changed the bug title to:
> > 
> >   -Woverflow warning inconsistency with signed constant conversion between
> > T_MAX+1 and UT_MAX vs larger than UT_MAX
> 
> I seem to remember this inconsistency is due to the exact definition of
> overflow given in the standard.

Which definition?

As I understand the standard, there's no overflow in the integer constant:
65535 and 65536 are both of type int on the machine in question (C99,
6.4.4.1p5).

Then, in the assignment, there's a conversion of the value to type short (C99,
6.5.16.1p2). Concerning this conversion, since short is a signed integer type
and neither 65535 nor 65536 is representable in the type short, the behavior is
implementation-defined (C99, 6.3.1.3p3). I still don't see why the
inconsistency would be due to the definition of overflow.
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Subject: [Bug ada/60078] acats c761007 fails on ARM
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--- Comment #2 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
it's a real hardware (Altera CyloneV SoC Eva-Board)
with dual core ARMv7
running linux and eglibc