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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: the language (and the role of Undefined Behavior therein) |
| Date | 2015-08-20 17:48 -0700 |
| Organization | None to speak of |
| Message-ID | <lnsi7d1n7c.fsf@kst-u.example.com> (permalink) |
| References | (16 earlier) <430b14cb-c335-4e54-94c5-d1076a2142fe@googlegroups.com> <55D51DE6.6070606@verizon.net> <75e8b3e2-4171-40bf-9e2d-c3c132272dec@googlegroups.com> <ln1tex3gd3.fsf@kst-u.example.com> <1f86d431-4ab2-447e-830b-5878781225fc@googlegroups.com> |
supercat@casperkitty.com writes:
> On Thursday, August 20, 2015 at 1:35:04 PM UTC-5, Keith Thompson wrote:
>> C99 and C89/C90 say the same thing; there's been no substantial change
>> in the meaning of "undefined behavior". Or am I missing something?
>
> I believe C11 added language to the effect that an action which invokes UB
> at any point during execution renders the entire execution Undefined, even
> the portion before UB was invoked. I don't think any earlier versions of the
> Standard included anything like that.
I'm skeptical that such a change was made, but I'm more than willing to
be convinced. Can you cite the added wording in C11 that says this?
If this is new in C11, you should be able to inhibit such optimizations
with "-std=c99" or equivalent.
[...]
>> What exactly happened 10 years ago?
>
> By the mid 1980s, mainstream compilers for mainstream platforms had
> pretty well established some de facto boundaries for the behaviors of
> some actions where the standard imposed no requirements. Thus, by the
> mid 1990s, programmers faced with the choice of writing nasty code to
> avoid overflow, or less-nasty code to limit the effects, would
> generally perceive no reason to favor the former unless their code
> would need to run on some really obscure hardware.
In the mind 1980s, there was no C standard.
> I don't know the exact historical events, but I suspect a catalyst for
> change was the fact that C++ started imposing requirements on
> Undefined Behavior: if a template expansion would invoke certain
> actions where the C++ Standard had previously imposed no requirements,
> compilers were required to silently reject it and try another **even
> if the platform's documentation would have otherwise guaranteed a
> useful behavior**.
That sounds similar to SFINAE
https://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error
but that applies only when a template expansion would cause a
compile-time error; undefined behavior is not relevant.
[...]
> Many compilers offered one of two guarantees:
>
> 1. Overflows trap.
> 2. Overflows simply yield a number (maybe or maybe not representable as int)
>
> Each guarantee will be useful to some programs, and for the Standard
> to mandate any behavior would invalidate code which relied upon a
> contrary guarantee.
>
> If no code could ever rely upon anything when overflow occurred, that
> would have been a major defect in the language, but instead there
> emerged a simple rule which 99% of compilers tried to follow:
> implementation documentation should specify whether overflows never
> trap, always trap, or sometimes trap, and should indicate what can or
> cannot be guaranteed about the computed values if overflows don't
> trap. Further, if hardware design offers a clear bias toward a
> particular overflow behavior, compilers should use that one unless
> requested otherwise.
>
> Until that rule got threw out the window around 2009, the fact that
> 99% of compilers followed it even when not compelled to by the
> Standard meant that it didn't really matter whether the Standard
> included such a rule or not.
I'm still skeptical that anything in particular happened in 2009, but
I'll gladly believe you if you can offer some citation.
Certainly optimizers have simply gotten more clever over the years.
I'm not aware of any sudden change of the kind you describe.
--
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"
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Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-18 16:19 -0700
Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-18 21:15 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 10:11 -0700
Re: the language (and the role of Undefined Behavior therein) Ike Naar <ike@iceland.freeshell.org> - 2015-08-19 19:58 +0000
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 14:09 -0700
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-19 16:10 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 16:01 -0700
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-19 20:23 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 10:35 -0700
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-20 12:32 -0700
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 13:31 -0700
Re: the language (and the role of Undefined Behavior therein) donalhambra@gmail.com - 2015-08-20 14:23 -0700
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 16:13 -0700
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-20 17:48 -0700
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-21 01:32 -0700
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-21 18:34 -0700
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-21 22:11 -0700
Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-22 09:41 -0400
Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-22 16:59 +0000
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-22 23:30 -0400
Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 03:36 +0000
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 00:25 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 22:57 -0700
Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 19:49 +0000
Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 14:43 -0700
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 16:27 -0700
Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 21:15 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 20:18 -0700
Re: the language (and the role of Undefined Behavior therein) raltbos@xs4all.nl (Richard Bos) - 2015-08-22 14:37 +0000
Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-22 19:12 -0700
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-22 23:38 -0400
Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-23 20:01 -0700
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-23 21:56 -0700
Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-24 17:25 -0700
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-24 18:21 -0700
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 22:22 -0400
Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-25 14:31 +0000
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-25 09:22 -0700
Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-25 18:55 +0000
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-25 12:33 -0700
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-25 16:23 -0700
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 00:31 -0400
Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-24 10:32 -0700
Re: the language (and the role of Undefined Behavior therein) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-22 23:54 -0700
Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 03:45 +0000
Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 08:11 -0700
Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 18:46 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 19:04 -0700
Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 23:40 -0400
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 22:48 -0700
Re: the language (and the role of Undefined Behavior therein) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-24 00:38 -0700
Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-18 21:22 -0400
Re: the language (and the role of Undefined Behavior therein) Ike Naar <ike@iceland.freeshell.org> - 2015-08-20 21:44 +0000
Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 16:48 -0700
Re: the language (and the role of Undefined Behavior therein) Philip Lantz <prl@canterey.us> - 2015-08-20 23:41 -0700
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