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Subject: Re: [ub] Objectives and tasks for SG12
From: Jens Maurer (Jens.Maurer_at_[hidden])
Date: 2013-05-30 14:36:39


On 05/29/2013 10:36 PM, Nevin Liber wrote:
> On 29 May 2013 14:35, Jens Maurer <Jens.Maurer_at_[hidden] <mailto:Jens.Maurer_at_[hidden]>> wrote:
>
>
> (1) Is a compiler diagnostic acceptable? Yes.
> (2) Is a run-time abort acceptable? Yes.
> (3) Is an unspecified result value acceptable? Yes.
> (4) Is it acceptable that your compiler changes the behavior
> of unrelated code that follows the overflow? That's very surprising.
>
> Giving compilers latitude to choose among 1-3 (depending on the
> target audience) is fine, but, in my opinion, prohibiting option 4
> would be an improvement.
>
>
> The counter argument is usually that (4) has a run time cost in that
> the overflow must now be detected instead of just assumed that it
> cannot happen. This effectively penalizes correct programs.

This statement seems to be inaccurate for the majority of current
hardware. Signed integer overflow will just "work" on the
hardware level and give you some result, i.e. implement (3).
No extra checking is needed.

This is exactly the reason why I think "signed integer overflow"
is a good example for the discussion: Current hardware exhibits
only a limited set of behavior, yet the C++ standard does not
reflect that, but gives permission to the compiler to do anything
it wants.

Can we quantify what we give up if we model current hardware
behavior more closely?

> What does "behavior of unrelated code" even mean once we've invoked
> undefined behavior?

Well, I thought the goal of SG12 was to discuss whether the current
definition of "undefined behavior" should be retained for some (which?)
cases, or whether something could be done to restrain the set of valid
executions. For example, we already have the concept of "unspecified
behavior", e.g. the sequencing of evaluation of function arguments is
unspecified. This means the implementation is restricted to choose
among a set of possible behaviors, and not exhibit arbitrary behavior.

Jens


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