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Date: Wed, 09 Oct 2013 10:02:00 -0000
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Subject: Re: Crazy compiler optimization
From: vijay nag <vijunag@gmail.com>
To: Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
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On Wed, Oct 9, 2013 at 3:24 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> 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.

Alright that is perfectly valid behaviour. Why does compiler consider
it to be a unsigned type at optimization level zero ? i.e. I see a
wrap around after
-128 to 128 ?