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Re: Miscompilation of remainder expressions
On Wed, Jan 17, 2007 at 11:17:36AM -0800, Ian Lance Taylor wrote:
> Joe Buck <Joe.Buck@synopsys.COM> writes:
>
> > On Wed, Jan 17, 2007 at 05:48:34PM +0000, Andrew Haley wrote:
> > > From a performance/convenience angle, the best place to handle this is
> > > either libc or the kernel. Either of these can quite easily fix up
> > > the operands when a trap happens, with zero performance degradation of
> > > existing code. I don't think there's any need for gcc to be altered
> > > to handle this.
> >
> > 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.)
>
> We do want to generate a trap for x / 0, of course.
>
Then you have to fix the code generation for PPC, which never traps.
All (?) 3-register arithmetic instructions have the option to
set an overflow flag that you can check later.
Gabriel