Re: Miscompilation of remainder expressions

[Michael Veksler <mveksler at tx dot technion dot ac dot il>]

Ian Lance Taylor wrote:
> Joe Buck <Joe.Buck@synopsys.COM> writes:
>
>   
> > How will the kernel know whether the overflow in the divide instruction
> > is because the user's source code has a '%' and not a '/'?  We generate
> > the exact same instruction for i / minus_one(), after all, and in that
> > case the trap really should be there.
> >     
>
> We don't need to generate a trap for INT_MIN / -1.  That is undefined
> signed overflow.  We can legitimately set the quotient register to
> INT_MIN while setting the remainder register to zero.  (Hmmm, what
> should we do if -ftrapv is set?  Probably generate a different code
> sequence in the compiler.)
>   
Simply let the kernel/libc set the overflow flag in this case, and let 
the compiler
append an INTO instruction right after the idivl.
> We do want to generate a trap for x / 0, of course.
>
> Ian
>
>   


--
Michael Veksler
http:///tx.technion.ac.il/~mveksler