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Re: check integer overflow with GCC 4.5.2

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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)

-- 
pete

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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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