Re: warning: right shift count >= width of type

[Chris Jefferson <caj@cs.york.ac.uk>]

Dave Korn wrote:

>>-----Original Message-----
>>From: Dale Johannesen 
>>Sent: 29 November 2004 16:31
>>    
>>
>
>  
>
>>On Nov 29, 2004, at 8:18 AM, Dave Korn wrote:
>>    
>>
>>>  Afternoon all.  Here's something that's piqued my curiosity; it's 
>>>probably
>>>owing to some language-lawyerly issue, but it isn't obvious to me.  
>>>This is
>>>on gcc-3.3.3, (cygwin variant, but that's probably not relevant):
>>>
>>>-------------------------<snip!>-------------------------
>>>dk@mace /test/shift-test> cat foo.c
>>>
>>>unsigned int bar (unsigned int baz)
>>>{
>>>unsigned int quux;
>>>
>>>        quux = baz >> 32;
>>>        return quux;
>>>}
>>>  Why isn't the shift operation optimised away and replaced with 
>>>const_int
>>>0?
>>>      
>>>
>>Because that's not what it means.  Shifts by >= word size are 
>>undefined 
>>behavior
>>and will give different results depending on optimization 
>>level and on 
>>whether
>>the shift count is constant or variable.  Don't do that.  (If 
>>you think 
>>it ought to be 0,
>>reflect that most popular CPUs have only 5 bit shift counts, and 
>>consider what the
>>code for x >> y would have to look like.)
>>    
>>
>
>
>  Absolutely so; my curiosity was piqued when I noticed that my
>cross-compiler was generating illegal assembler code with an out-of-range
>operand value that the assembler couldn't fit into the relevant opcode
>bitfield.
>
>  So my question is really "Given that it's undefined, which means that
>whatever the compiler does is correct, and given that there's already code
>in there to detect the situation and issue a warning, which probably means
>that it would be very easy at such a point to replace the offending RTL with
>(const_int 0), is there any specific reason why not to?" 
>
One reason (of course) not to would be if you are bit-shifting by a 
variable. Adding a check at runtime to see if the value of this variable 
is <32 and if not setting the variable to zero would be frowned upon by 
many people, particuarily because the kind of people who undertake 
bitshifting are doing it for performance-related reasons...

Chris