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Re: Miscompilation of remainder expressions


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.

Andrew.


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