Re: Miscompilation of remainder expressions

[Joe Buck <Joe dot Buck at synopsys dot COM>]

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.

I suppose that the trap handler could try to analyze the code following
the divide instruction; if the quotient result is never used and the
divisor is -1, it could replace the remainder result with zero and return.
But that would be rather hairy, if it is even feasible.  Alternatively,
the divide instruction could be marked somehow, but I have no idea how.