Re: [llvm-dev] [isocpp-parallel] Proposal for new memory_order_consume definition

From: James Y Knight
Date: Mon Feb 29 2016 - 16:12:47 EST

No, you really don't need undefined behavior in the standard in order
to enable bug-finding.

The standard could've (and still could...) make signed integer
overflow "implementation-defined" rather than "undefined". Compilers
would thus be required to have *some documented meaning* for it (e.g.
wrap 2's-complement, wrap 1's-complement, saturate to min/max, trap,
or whatever...), but must not have the current "Anything goes! I can
set your cat on fire if the optimizer feels like it today!" behavior.

Such a change to the standard would not reduce any ability to do error
checking, as compilers that want to be helpful could perfectly-well
define it to trap at runtime when given certain compiler flags, and
perfectly well warn you of your dependence upon unportable
implementation-defined behavior (or, that your program is going to
trap), at build-time.

[Sending again as a plain-text email, since a bunch of mailing lists
apparently hate on multipart messages that even contain a text/html

On Mon, Feb 29, 2016 at 2:38 PM, Lawrence Crowl via llvm-dev
<llvm-dev@xxxxxxxxxxxxxx> wrote:
> On 2/28/16, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> The fact is, undefined compiler behavior is never a good idea. Not for
>> serious projects.
> Actually, undefined behavior is essential for serious projects, but
> not for the reasons mentioned.
> If the language has no undefined behavior, then from the compiler's view,
> there is no such thing as a bad program. All programs will compile and
> enter functional debug (possibly after shipping to customer). On the
> other hand, a language with undefined behavior makes it possible for
> compilers (and their run-time support) to identify a program as wrong.
> The problem with the latest spate of compiler optimizations was not the
> optimization, but the lack of warnings about exploiting undefined behavior.
> --
> Lawrence Crowl
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