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Re: check integer overflow with GCC 4.5.2

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On Wed, 23 Nov 2011 22:29:10 -0800, sailer <xiangdong.ji@gmail.com> wrote:

> On Nov 24, 1:47 pm, pete <pfil...@mindspring.com> wrote:
>> sailer 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
>>
>> How about maybe:
>>
>> #include <limits.h>
>>
>> if (b == 0 || INT_MAX / b >= a)
>
> Thanks for replying, I tried but no luck, call to ovf is still removed
> by GCC.

Perhaps this is because ovf still had undefined behavior in the case
where you would want overflow to be set. If you use Pete's suggestion
*before* doing the multiplication (and avoid performing the multiplication
when it indicates overflow), I suspect it will work.

Philip

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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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