Re: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division

["Al Grant" <AlGrant@myrealbox.com>]

The following reply was made to PR c/7284; it has been noted by GNATS.

From: "Al Grant" <AlGrant@myrealbox.com>
To: nathan@compsci.bristol.ac.uk
Cc: falk.hueffner@student.uni-tuebingen.de,
	nathan@gcc.gnu.org,
	algrant@acm.org,
	gcc-bugs@gcc.gnu.org,
	gcc-prs@gcc.gnu.org,
	nobody@gcc.gnu.org,
	gcc-gnats@gcc.gnu.org
Subject: Re: Re: c/7284: incorrectly simplifies leftshift followed by signed  power-of-2 division
Date: Fri, 12 Jul 2002 17:06:44 +0000

 >you need to read more carefully.
 >KnR 2 A7.8 says the same as C99,
 
 You need to read more carefully.  K&R2 says something quite different from =
 C99.  It says that in the absence of overflow, the operation is equivalen=
 t to a
 multiplication.  It does _not_ say that if the multiplication overflows the=
  result of the shift is undefined, let alone that program behavior is und=
 efined.
 
 >C++ says [5]/5 that if the result is not in the range >of representable va=
 lues,
 >the behaviour is undefined.
 
 But left-shift is an operation on the representation, i.e. the bit pattern.=
   For signed left-shift (in C89 and C++) it is not defined any other way.=
   How is it meaningful to talk about the representability of operations o=
 n the representation, and say that the result of such an operation might =
 be unrepresentable?
 Representability is a property of the integers as numbers.
 
 It might be meaningful to think about the result of such an operation havin=
 g a representation that did not correspond to any value (e.g. was a trap =
 representation) but a non-valued representation is a  totally different c=
 oncept from a non-representable value.  Besides, there are no such intege=
 r representations on the platform for which I reported the bug.