[Bug c++/12963] New: Wrong and misleading warning encourages writing non-portable code

[Bug c++/12963] New: Wrong and misleading warning encourages writing non-portable code

[bagnara at cs dot unipr dot it]

For the C++ function

unsigned char
foo(unsigned char c) {
  if (c <= 255)
    return 255;
  else
    return c;
}

g++ 3.4 20031102 (and also 3.3.2, but NOT 3.3.1) generates,
without any -W option, the following warning:

g++ -c bug.cc
bug.cc: In function `unsigned char foo(unsigned char)':
bug.cc:3: warning: comparison is always true due to limited range of data type

I believe this warning should not be issued, since unsigned chars may
be more than 8 bits wide.  The fact that in the particular implementation
of ISO C++ I am using right now unsigned chars are 8 bits wide should be
irrelevant (I certainly do not want to write unportable code to avoid that warning).

-- 
           Summary: Wrong and misleading warning encourages writing non-
                    portable code
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bagnara at cs dot unipr dot it
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-08 21:02 -------
The warning is correct for this implementation and since you are compiling with this 
implementation, you can recieve warnings (yes it would be better if it was enabled by a -
W* but I think it is good warning no matter what).  To disable the warning use -w (notice 
the lower case w).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[bagnara at cs dot unipr dot it]

------- Additional Comments From bagnara at cs dot unipr dot it  2003-11-08 22:44 -------
(In reply to comment #1)
> The warning is correct for this implementation and since you are compiling
with this 
> implementation, you can recieve warnings (yes it would be better if it was
enabled by a -
> W* but I think it is good warning no matter what).  To disable the warning use
-w (notice 
> the lower case w).

If I understand correctly, -w inhibits all warning messages, which is
certainly not what I call for.  The fact that, when producing code for
platforms where CHAR_BIT == 8, the test is redundant is a matter for
the optimizer (who can omit the test), not something the user should
be warned about.  In general, I believe target-dependent warnings
should default to disabled.  By the way, are there other target-dependent
warnings in GCC?  I mean that this is the first time I meet one (unless
I am missing something).

Now try with

void*
foo(void* c) {
  if (c <= (void*) 0xFFFFFFFF)
    return 0;
  else
    return c;
}

and you will see that (notice -W -Wall)

g++ -W -Wall -c t.cc -O3 -fomit-frame-pointer -S

does not produce any warning.  Yet, the produced code is,

.globl _Z3fooPv
	.type	_Z3fooPv, @function
_Z3fooPv:
.LFB3:
	xorl	%eax, %eax
	ret

as expected.
Would you advocate a warning also in this case?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-15 08:55 -------
There are a large number of target-dependent (and sizeof dependent) warnings in GCC. GCC also 
warns about "integer overflow in expression" and fp.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-21 15:58 -------
Since GCC is warning based on the target you are using, I am closing as will not fix.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[bagnara at cs dot unipr dot it]

------- Additional Comments From bagnara at cs dot unipr dot it  2003-11-21 21:13 -------
(In reply to comment #4)
> Since GCC is warning based on the target you are using, I am closing as will
not fix.

In my opinion, this is a mistake: people wishing to write 100% portable code
should not be forced to switch all warnings off in order to get rid
of this useless warning (yes, useless, because there is nothing sensible the
sensible programmer can do about it).
Warnings such as this one should be issued only when specifically requested.
At the very least, it should be possible to selectively switch off this
kind of warnings.
But closing the bug report as you are doing (without even addressing my
question "why then there is no such a warning for void*, not even with
-W -Wall?") will not make the problem go away.  The problem is there.
I don't think this anxiety to close bug reports before the issue has
been studied and discussed in depth is going to help.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[bangerth at dealii dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-11-21 21:52:20
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2003-11-21 21:52 -------
I agree with Robert on this point: we shouldn't get this warning
_when not giving any flag_. IMHO, the warning is quite valid and tells
the programmer about something that he might not have considered, but
in case he has it is pointless to issue a warning that is issued
even without any of the -W... flags and without the possibility to
selectively switch it off.

Let's keep this open until we reach some kind of consensus whether it
is possible to implement this or not.

W.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2003-11-21 23:15 -------
Subject: Re:  Wrong and misleading warning encourages writing non-portable code

"bangerth at dealii dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| Let's keep this open until we reach some kind of consensus whether it
| is possible to implement this or not.

I would suggest "enhancement request".

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[jsm at polyomino dot org dot uk]

------- Additional Comments From jsm at polyomino dot org dot uk  2003-11-21 23:52 -------
Subject: Re:  Wrong and misleading warning encourages writing
 non-portable code

On Fri, 21 Nov 2003, bangerth at dealii dot org wrote:

> I agree with Robert on this point: we shouldn't get this warning
> _when not giving any flag_. IMHO, the warning is quite valid and tells

As I previously pointed out
<http://gcc.gnu.org/ml/gcc-patches/2003-03/msg01954.html>.  Given this
sort of portability issue, a separate option (maybe in -Wextra, but
probably not in -Wall), such as -Wcomparison-fixed, is appropriate for
these warnings and those already in -Wextra I mentioned in that message.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[bagnara at cs dot unipr dot it]

------- Additional Comments From bagnara at cs dot unipr dot it  2004-09-20 13:12 -------
To my claim that the warning "comparison is always false due to limited range of
data type" should be disabled by default, I would like to add the fact that GCC
is highly inconsistent in this respect.  In fact, as the attached file v.cc
shows, the warning is given only for char and short whereas it is not given for
int, long and long long.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-09-20 16:29 -------
Besides the consistency issue, which is a bug and should be reported elsewhere, 
I do not understand how this encourage writing non-portable code. On the 
contrary, it does *help* it.

Say that you have a program which assumes that char is 16bits, and you port to 
an architecture which has chars with 8bits. Instead of silently failing, you 
will get warnings like this one which would help you catching problems.

I do not think you provided a good rationale of why we should disable such a 
warning. Could you please describe your real-life problem about portability and 
this warning?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |giovannibajo at gcc dot gnu
                   |                            |dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[jsm at polyomino dot org dot uk]

------- Additional Comments From jsm at polyomino dot org dot uk  2004-09-20 23:08 -------
Subject: Re:  Wrong and misleading warning encourages writing
 non-portable code

On Mon, 20 Sep 2004, giovannibajo at libero dot it wrote:

> Besides the consistency issue, which is a bug and should be reported elsewhere, 
> I do not understand how this encourage writing non-portable code. On the 
> contrary, it does *help* it.

A program has some data in a type such as char or uid_t which needs to be 
stored in an externally defined structure, or transmitted by an externally 
defined protocol.  This external definition imposes a limit of (say 255) 
on the value held in (say) unsigned char.  A portable program checks that 
the data is within the defined range before storing or transmitting it.  
Some checks are redundant on a particular system, so receive these 
warnings; if the warnings are to be silenced, that means removing checks 
that are not dead code on other systems.  I have encountered such problems 
(warnings that cannot effectively be silenced) in real code, both with 
system typedefs such as uid_t and user typedefs that may be configured to 
be a type such as unsigned char or a wider type.  My comment #8 still 
applies: this does not even belong in -Wall.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[abramobagnara at tin dot it]

------- Additional Comments From abramobagnara at tin dot it  2004-09-21 07:24 -------
(In reply to comment #1)
> The warning is correct for this implementation and since you are compiling
with this 
> implementation, you can recieve warnings (yes it would be better if it was
enabled by a -
> W* but I think it is good warning no matter what).  To disable the warning use
-w (notice 
> the lower case w).

I hope that there is a general consensus about the fact that the presence of
expected warnings not avoidable on compilation log is a bad thing.

This lower the attention threshold of programmers and contributors about true
problems and reduce the readability of compilation log. I suppose that the
availability of -Werror is derived from this same reasoning.

Taken for granted that, I think that the presence of warning options like
-wWARNING-NAME to disable specific warnings could solve the problem in a per
file fashion.

Better than that the availability of something like
#pragma expected-warning line WARNING-NAME
might remove the warning generated by the following line labeling it as checked,
expected and/or unavoidable.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[jsm at polyomino dot org dot uk]

------- Additional Comments From jsm at polyomino dot org dot uk  2004-09-22 22:24 -------
Subject: Re:  Wrong and misleading warning encourages writing
 non-portable code

On Tue, 21 Sep 2004, abramobagnara at tin dot it wrote:

> Better than that the availability of something like
> #pragma expected-warning line WARNING-NAME
> might remove the warning generated by the following line labeling it as checked,
> expected and/or unavoidable.

That this is desired is generally agreed.  It's just that no-one has yet 
produced a design for it and still had time left to implement it after all 
the arguing over that design.

DJ produced a design <http://gcc.gnu.org/ml/gcc/2004-06/msg01188.html> 
(and thread).  This led to a mechanical patch adding a parameter 0 to 
every call to warning() 
<http://gcc.gnu.org/ml/gcc-patches/2004-07/msg02229.html>; it wasn't 
applied, nor was there any followup.  Previously, Stan Shebs gave a 
proposal <http://gcc.gnu.org/ml/gcc/2003-01/msg01065.html> (and thread).  
There have been various other such discussions.  
<http://gcc.gnu.org/ml/gcc/2000-06/msg00639.html> has a different 
proposal.  <http://gcc.gnu.org/ml/gcc/1998-09/msg00353.html> included, I 
think uniquely, a patch, but there were objections and it was reverted 
within 24 hours.

Implementing such a feature is not hard.  I could probably do a plausible 
implementation - that did not require all calls to diagnostic functions to 
be converted at once, rather allowing the conversion to having individual 
warnings controllable to be a gradual one - in a few days (having 
enable-mapped-location unconditionally on would help though), as could 
many other people.  Producing a consensus that any given approach is 
right, or even that any given approach is not unacceptable, is the hard 
part.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug c++/12963] Wrong and misleading warning encourages writing non-portable code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-20 17:50 -------
*** Bug 20550 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |qrczak at knm dot org dot pl


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug middle-end/12963] Wrong and misleading warning encourages writing non-portable code

[qrczak at knm dot org dot pl]

------- Additional Comments From qrczak at knm dot org dot pl  2005-03-20 19:10 -------
> Better than that the availability of something like
> #pragma expected-warning line WARNING-NAME
> might remove the warning generated by the following line labeling it as checked,
> expected and/or unavoidable.

This would not help in my case because it's a regular type-generic C macro, not
generated code. The line number of the macro definition is not stable (changes
as surrounding code evolves), and it makes no sense to mark all its invocations.

>From my point of view a perfect solution would be making this obvious workaround
working:

int test(int x) {
   if ((long long)x <= 0x123456789ABCLL) return 1;
   else return 0;
}

There should be no warning if the lhs is cast to the wider type. GCC seems to be
inferring that the range of possible values of (long long)x is the range of int.
The runtime comparison is eliminated, which is good; this implies that the
warning should not be tied to the comparison being eliminated.

I'm not judging whether the warning should be emitted at all. Maybe it should be
removed altogether; after all, it's not guaranteed to be redundant on all
platforms. It definitely should not be emitted by default, especially without
any warning options, especially if the cast suggests that the programmer is
aware of it.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug middle-end/12963] Wrong and misleading warning encourages writing non-portable code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-21 00:24 -------
*** Bug 20573 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sthoenna at efn dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963

[Bug middle-end/12963] Wrong and misleading warning encourages writing non-portable code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-06-24 23:10 -------
*** Bug 22178 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eggert at gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963