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Re: Crazy compiler optimization


On 9 October 2013 10:36, 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--;
>   }


> Basically the crash here is because of elimination of the check in the
> if-clause "&& cnt" which is causing stack overrun and thereby SIGSEGV.
> While standards may say that the behaviour is
> undefined when an unsigned value is stored in a signed value,

Standards do not say that. 254 cannot be presented in a char if char
is a signed type, so it's an overflow, which is undefined behaviour.
Storing an unsigned value that doesn't overflow is OK.

> can a
> language lawyer explain to me why GCC chose to eliminate code
> pertaining to cnt considering it as dead-code ?

cnt is initialized to -2 (after an overflow) and then you decrement it
so it gets more negative.  The "&& cnt" condition will never be false,
because cnt starts non-zero and gets further from zero, so will never
reach zero.


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