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Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
- From: Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>
- To: "Al Grant" <AlGrant at myrealbox dot com>
- Cc: nathan at gcc dot gnu dot org, algrant at acm dot org, gcc-bugs at gcc dot gnu dot org, gcc-prs at gcc dot gnu dot org, nobody at gcc dot gnu dot org, gcc-gnats at gcc dot gnu dot org
- Date: 12 Jul 2002 19:01:43 +0200
- Subject: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
- References: <1026492621.58075ff8AlGrant@myrealbox.com>
"Al Grant" <AlGrant@myrealbox.com> writes:
> 3.3.7 (something else in ISO, maybe 6.3.7)
>
> The result of E1 << E2 is E1 left-shifted E2 bit
> positions; vacated bits are filled with zeros.
>
> For signed types it says nothing more.
>
> Now if signed left-shift is defined at all, in terms of the
> representation, I don't see there's any lack of definition in
> "0x00000080 left-shifted 24 bit positions", it is clearly 0x80000000
> (of the same type). So it's defined unless the standard says
> otherwise, which only C99 seems to.
Defect Report #081 seems to be of interest here
(http://wwwold.dkuug.dk/JTC1/SC22/WG14/www/docs/dr_081.html). It
basically states that the behaviour is implementation defined (for any
signed left shift, not just this case).
--
Falk