[Bug c++/23608] New: -Wsign-compare and const propagation

[Bug c++/23608] New: -Wsign-compare and const propagation

[br1 at internet dot com dot uy]

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Compiling

#define FIVE 5

int main()
{
    int i = 5;
    int const ic = 5;

    i < 5u;
    ic < 5u;
    FIVE < 5u; // line 10
}


with -Wsign-compare produces the output

pp.cpp:8: warning: comparison between signed and unsigned integer expressions
pp.cpp:9: warning: comparison between signed and unsigned integer expressions


MSVC only warns on line 8.  G++'s current behaviour is unfortunate because it 
favors #defines over consts.

Regards,
Bruno Martínez

-- 
           Summary: -Wsign-compare and const propagation
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: br1 at internet dot com dot uy
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608

[Bug c++/23608] -Wsign-compare and const propagation

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-29 02:34 -------
ic by the C++ standard does not need to propagate its value into ic < 5u so I think the warning is 
warranted.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608

[Bug c++/23608] -Wsign-compare and const propagation

[br1 at internet dot com dot uy]

------- Additional Comments From br1 at internet dot com dot uy  2005-08-29 22:24 -------
Could you point me to the section in the standard?  I thought warning were a 
quality of implementation matter.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608

Re: [Bug c++/23608] -Wsign-compare and const propagation

[Andrew Pinski]

On Aug 29, 2005, at 6:24 PM, br1 at internet dot com dot uy wrote:

>
> ------- Additional Comments From br1 at internet dot com dot uy  
> 2005-08-29 22:24 -------
> Could you point me to the section in the standard?  I thought warning 
> were a
> quality of implementation matter.

No you misunderstood me.  What I was trying to say the const propagation
does not need to happen.  Since that is the case the warning is still 
valid.

-- Pinski

[Bug c++/23608] -Wsign-compare and const propagation

[pinskia at physics dot uc dot edu]

------- Additional Comments From pinskia at physics dot uc dot edu  2005-08-29 23:01 -------
Subject: Re:  -Wsign-compare and const propagation


On Aug 29, 2005, at 6:24 PM, br1 at internet dot com dot uy wrote:

>
> ------- Additional Comments From br1 at internet dot com dot uy  
> 2005-08-29 22:24 -------
> Could you point me to the section in the standard?  I thought warning 
> were a
> quality of implementation matter.

No you misunderstood me.  What I was trying to say the const propagation
does not need to happen.  Since that is the case the warning is still 
valid.

-- Pinski



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608

[Bug c++/23608] -Wsign-compare and const propagation

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-08-29 23:10 -------
What Andrew is trying to say is that in C, even variables that are marked  
'const' can be modified. Thus, the compiler can't (naively, i.e. without  
using flow analysis) determine that 'i' or 'ic' have the value 5, and  
therefore warn that there is a comparison between signed and unsigned.  
However, in the last line, the compiler knows the value that is being   
compared to 5u and sees that nothing bad can happen here. It therefore  
does not warn.  
 
In C++, the situation is different, since const variables can't be changed. 
The compiler could therefore be able to determine that the second comparison 
does not warrant a warning. This notwithstanding, the compiler doesn't do  
this right now. 
 
As annoying as this is, I don't consider this a bug. It may be classified 
as an enhancement, though, if someone wants to make this warning contigent 
upon the availability of forward propagation of values. General consensus 
has been, however, not to do this. 
 
W. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Priority|P2                          |P3
   Last reconfirmed|0000-00-00 00:00:00         |2005-08-29 23:10:37
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608

[Bug c++/23608] -Wsign-compare and const propagation

[br1 at internet dot com dot uy]

------- Additional Comments From br1 at internet dot com dot uy  2005-08-29 23:23 -------
I don't know much about const in C, just C++.  I just wanted to have this 
request entered in your database, as a possible enhancement, of course.

Regards,
Bruno

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608

[Bug c++/23608] -Wsign-compare and const propagation

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2005-08-30 00:09 -------
Subject: Re:  -Wsign-compare and const propagation

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

| As annoying as this is, I don't consider this a bug. It may be classified 
| as an enhancement, though, if someone wants to make this warning contigent 
| upon the availability of forward propagation of values. General consensus 
| has been, however, not to do this. 

I agree with Wolfgang's clear analysis.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23608