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Re: Miscompilation of remainder expressions
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.