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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Is this code considered to transgress the signed overflow purism?
Date: Sun, 30 Apr 2017 14:25:04 -0700
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Tim Rentsch <txr@alumni.caltech.edu> writes:
> supercat@casperkitty.com writes:
>> On Friday, April 28, 2017 at 3:19:17 AM UTC-5, David Brown wrote:
>>> The context here is C programming unless explicitly stated otherwise,
>>> and in that context left-shifting a negative value is undefined.
>>
>> Did C89 become off-topic?  In C89, left-shifting a negative value was
>> defined in a way [...]
>
> As he is wont to do, supercat is talking out of school here.
> Before the C99 standard, a left shift of a negative value
> is not defined behavior but implementation-defined behavior.
> Quoting the official response in DR 81:
>
>     Subclause 6.3 states that the binary operator < , among
>     others, has implementation-defined aspects for signed types.
>     Therefore, the answer to ``What does it mean to left shift a
>     signed value?'' is that it is implementation-defined.

It would be helpful if you'd Cite the DR you're referring to:
    http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_081.html

(Note that the DR incorrectly refers to "<" rather than "<<",
presumably a typo.)

I'm not sure that supercat's interpretation is entirely unreasonable
given what the C90 standard actually says.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"