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NNTP-Posting-Date: Wed, 23 Nov 2011 23:47:46 -0600
Message-ID: <4ECDDA82.4C71@mindspring.com>
Date: Thu, 24 Nov 2011 00:47:46 -0500
From: pete <pfiland@mindspring.com>
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Subject: Re: check integer overflow with GCC 4.5.2
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:13837

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