Re: Crazy compiler optimization

[Nicholas Mc Guire <der.herr@hofr.at>]

On Wed, 09 Oct 2013, vijay nag wrote:

> Hello GCC,
> 
> I'm facing a wierd compiler optimization problem. Consider the code
> snippet below
> 
> #include <stdio.h>
> 
> int printChar(unsigned long cur_col, unsigned char c)
> {
>   char buf[256];
>   char* bufp = buf;
>   char cnt = sizeof(buf) - 2; /* overflow in implicit type conversion */
>   unsigned long terminal_width = 500;
> 
>   while ((cur_col++ < terminal_width) && cnt) {
>       *bufp++ = c;
>       cnt--;
>   }
> 
>   *bufp++ = '\n';
>   *bufp = 0;
> 
>   printf("%c\n", buf[0]);
>   return 1;
> }
> 
> int main()
> {
>   printChar(80, '-');
>   return 1;
> }
> 
> While compiler optimization should guarantee that the result of
> execution is same at all optimization levels, I'm observing difference
> in the result of execution of the above program when optimized to
> different levels. Although there is fundamental problem with the
> statement "char cnt = sizeof(buf) - 2", GCC seems to be warning(that
> too only when -pedantic flag is used) about overflow error while
> silently discarding any code related to cnt i.e. the check "&& cnt" in
> the if-clause is silently discarded by the compiler at -O2.
> 
> $]gcc -g char.c -o char.c.unoptimized -m32 -O0 -Wall -Wextra -pedantic
> char.c: In function ?printChar?:
> char.c:8: warning: overflow in implicit constant conversion
>
This compiler optimization dependency is visible with quite a few code examples
that violate the C standard.
Integer overflow/underflow results in undefined behavior - you are in the
wild lands basically - you should not expect C-standard violations to result
in "reliable undefined" code.

See C99 Annex J.2 for details of undefined behaviors.

thx!
hofrat