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Re: Miscompilation of remainder expressions


Andrew Haley wrote:
Ian Lance Taylor writes:
> Gabriel Dos Reis <gdr@cs.tamu.edu> writes:
> > > Ian, do you believe something along the line of
> > > > # > I mean, could not we generate the following for "%": > > # >
> > # > rem a b := > > # > if abs(b) == 1
> > # > return 0
> > # > return <machine-instruction> a b
> > #
> > # On x86 processors that have conditional moves, why not do the equivalent
> > # of
> > #
> > # neg_b = -b;
> > # cmov(last result is negative,neg_b,b)
> > # __machine_rem(a,b)
> > #
> > # Then there's no disruption of the pipeline.
> > > > is workable for the affected targets?
> > Sure, I think the only real issue is where the code should be
> inserted.


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.

That only works if the operation causes a trap. On x86 this is the case, but Andrew Pinski told me on IM that this was not the case for PPC.


David Daney


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