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Re: Conflicts between C standard and 754-2008



On 2011-01-06 15:05:35 +0200, Hossam A. H. Fahmy wrote:
Yes, commutativity is the swapping of the terms around the operator but in
the examples:


 x=100,  y=1e+30,  z=1e+30
  z-y+x=100

  z+x-y=0

and without any parenthesis I assumed that the evaluation will always
proceed from left to right and I swapped the second and third term. So we
are getting:
(z-y)+x = 100
and
(z+x)-y=0

which is as expected in 754. However, C says that (-y)+x is the same as
x+(-y) so an optimizing compiler can assume that the two statements z-y+x
and z+x-y (just commutativity on the 2nd and 3rd term) are equivalent which
they are not under 754.

No, you're also using the associativity here, because a+b+c is
interpreted unambiguously as (a+b)+c. And from (a+b)+c to (a+c)+b,
you're not using the commutativity only, but a succession of
commutativity and associativity transformations.

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Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.net/>
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