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
