Re: Can gcc.dg/torture/pr67828.c be an infinite loop?

[David Brown <david.brown@hesbynett.no>]

On 24/09/2021 10:59, Aldy Hernandez via Gcc wrote:
> 
> 
> On 9/24/21 10:08 AM, Richard Biener wrote:
>> On Fri, Sep 24, 2021 at 10:04 AM Aldy Hernandez via Gcc
>> <gcc@gcc.gnu.org> wrote:
>>>
>>> Is this correct, or did I miss something?
>>
>> Yes, 'c' will wrap to negative SHORT_MIN and terminate the loop via
>> the c>=0 test.
> 
> Huh, so SHORT_MAX + 1 = SHORT_MIN?  I thought that was an overflow, and
> therefore undefined.
> 

C and C++ don't do arithmetic on "short" (or "char").  They are
immediately promoted to "int" (or "unsigned int", as appropriate).  So
if short is smaller than int, the code behaviour is well defined (as
Richard described below).  If short is the same size as int (such as on
the 16-bit mspgcc port of gcc), however, then SHORT_MAX + 1 /would/ be
an overflow and the compiler can assume it does not happen - thus giving
you an infinite loop.

With more common 32-bit int and 16-bit short, the loop should execute
32768 times.

(At least, that is my understanding.)

> 
>>
>> Mind c++ is really (short)(((int)c)++) and signed integer truncation
>> is implementation
>> defined.
>>
>> Richard.
>>
>>> Aldy
>>>
>>
> 
>