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check integer overflow with GCC 4.5.2

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  check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-23 20:25 -0800
    Re: check integer overflow with GCC 4.5.2 pete <pfiland@mindspring.com> - 2011-11-24 00:47 -0500
      Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-23 22:29 -0800
        Re: check integer overflow with GCC 4.5.2 "Philip Lantz" <prl@canterey.us> - 2011-11-24 01:23 -0800
          Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:14 -0800
            Re: check integer overflow with GCC 4.5.2 James Kuyper <jameskuyper@verizon.net> - 2011-11-28 07:04 -0500
            Re: check integer overflow with GCC 4.5.2 Noob <root@127.0.0.1> - 2011-11-29 14:12 +0100
              Re: check integer overflow with GCC 4.5.2 Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-29 16:16 +0200
              Re: check integer overflow with GCC 4.5.2 88888 Dihedral <dihedral88888@googlemail.com> - 2011-11-29 06:28 -0800
    Re: check integer overflow with GCC 4.5.2 Robert Wessel <robertwessel2@yahoo.com> - 2011-11-24 00:28 -0600
    Re: check integer overflow with GCC 4.5.2 Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-24 09:46 +0100
      Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-24 01:08 -0800
        Re: check integer overflow with GCC 4.5.2 "Philip Lantz" <prl@canterey.us> - 2011-11-24 01:26 -0800
          Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:09 -0800
      Re: check integer overflow with GCC 4.5.2 "io_x" <a@b.c.invalid> - 2011-11-24 14:19 +0100
    Re: check integer overflow with GCC 4.5.2 gwowen <gwowen@gmail.com> - 2011-11-24 09:10 -0800
    Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:21 -0800
      Re: check integer overflow with GCC 4.5.2 Keith Thompson <kst-u@mib.org> - 2011-11-28 16:31 -0800
        Re: check integer overflow with GCC 4.5.2 Ian Collins <ian-news@hotmail.com> - 2011-11-29 13:53 +1300
    Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:34 -0800
      Re: check integer overflow with GCC 4.5.2 pete <pfiland@mindspring.com> - 2011-11-28 18:59 -0500
        Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-29 19:44 -0800
          Re: check integer overflow with GCC 4.5.2 James Kuyper <jameskuyper@verizon.net> - 2011-11-30 11:08 -0500
            Re: check integer overflow with GCC 4.5.2 Keith Thompson <kst-u@mib.org> - 2011-11-30 09:01 -0800
    Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-29 19:46 -0800

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#14015

sailer wrote:

>  is it safe enough?

No.

>          int words;

>         ovf(words, sizeof(int), &bytes, &overflow);

(words) should be initialized 
before its value is passed to ovf().

-- 
pete

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#14060

On Nov 29, 7:59 am, pete <pfil...@mindspring.com> wrote:
> sailer wrote:
> >  is it safe enough?
>
> No.
>
> >          int words;
> >         ovf(words, sizeof(int), &bytes, &overflow);
>
> (words) should be initialized
> before its value is passed to ovf().
>
> --
> pete

It's a demo code fragment, I checked the asm code to figure out
whether the overflow detection works,
and the type of bytes should be 'unsigned int', it was a typo error
and I tried to post a correction but
failed.

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#14079

On 11/29/2011 10:44 PM, sailer wrote:
> On Nov 29, 7:59 am, pete <pfil...@mindspring.com> wrote:
>> sailer wrote:
>>>  is it safe enough?
>>
>> No.
>>
>>>          int words;
>>>         ovf(words, sizeof(int), &bytes, &overflow);
>>
>> (words) should be initialized
>> before its value is passed to ovf().
>>
>> --
>> pete
> 
> It's a demo code fragment, I checked the asm code to figure out
> whether the overflow detection works,

Let that be a lesson: even "demo code fragments" should have defined
behavior; if they don't, your test becomes meaningless.

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#14087

James Kuyper <jameskuyper@verizon.net> writes:
> On 11/29/2011 10:44 PM, sailer wrote:
>> On Nov 29, 7:59 am, pete <pfil...@mindspring.com> wrote:
>>> sailer wrote:
>>>>  is it safe enough?
>>>
>>> No.
>>>
>>>>          int words;
>>>>         ovf(words, sizeof(int), &bytes, &overflow);
>>>
>>> (words) should be initialized
>>> before its value is passed to ovf().
>>>
>>> --
>>> pete
>> 
>> It's a demo code fragment, I checked the asm code to figure out
>> whether the overflow detection works,
>
> Let that be a lesson: even "demo code fragments" should have defined
> behavior; if they don't, your test becomes meaningless.

Right.  If your code has undefined behavior, an optimizing compiler is
permitted to transform it in ways that depend on the assumption that the
behavior is defined.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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#14061

On Nov 24, 12:25 pm, sailer <xiangdong...@gmail.com> wrote:
> Hi,
>
> I have the following code which was used to check whether  the result
> of (a * b)  is larger than the maximum integer value.
>
> ovf(int a, int b, int *r, int *v)
> {
>         *r = a * b;
>         if (b == 0 || a == *r / b)
>                 *v = 0;
>         else
>                 *v = 1;
>
> }
>
> int main()
> {
>     int words;
>     int bytes;
>     int overflow;
>
>     //... whatever
>     ovf(words, sizeof(int), &bytes, &overflow);
>     if (overflow)
>        printf("overflow");
>
> }
>
> It doesn't work with GCC 4.5.2 if enabling -O2 or -O3, asm code
> generated by GCC is as follows, call to ovf is eliminated, seems that
> GCC evaluates (a == *r/b) to TRUE at compile time, is there anyway to
> revise ovf to let it satisfy GCC? thanks in advance.
>
> main:
>         pushl   %ebp    #
>         xorl    %eax, %eax      #
>         movl    %esp, %ebp      #,
>         popl    %ebp    #
>         ret
>         .size   main, .-main
>         .ident  "GCC: (GNU) 4.5.2"
>         .section        .note.GNU-stack,"",@progbits

BTW. Thanks all for helping solving the problem, and sorry that I also
posted the same question to comp.lang.c++ without the knowledge of
cross-post.

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