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

[Chris Jefferson <caj at cs dot york dot ac dot 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