Is the following well defined and allowed?

[Steven Schveighoffer]

On 3/2/18 10:26 AM, Timon Gehr wrote:
> On 02.03.2018 16:05, Steven Schveighoffer wrote:
>> On 3/2/18 10:00 AM, Timon Gehr wrote:
>>> On 02.03.2018 15:39, Steven Schveighoffer wrote:
>>>>
>>>>
>>>>> In this interpetation, -noboundscheck switches DMD to a different 
>>>>> dialect of D. In that dialect, out-of-bounds accesses (and 
>>>>> overlapping copies, apparently) always have UB, in both @system and 
>>>>> @safe code. That defeats the purpose of @safe. Which is why I don't 
>>>>> really care for that dialect.
>>>>
>>>> I agree, I think we should remove the option to disable bounds 
>>>> checks on @safe code, in any way. It's too dangerous. If you want 
>>>> performance that comes without bounds checks, use a trusted escape, 
>>>> or write system code.
>>>
>>> I.e., the -release flag should not remove assertions in @safe code, 
>>> or at the very least it should not turn them into sources of UB.
>>
>> -release flag already operates this way.
> 
> That's not what the spec says:
> 
> https://dlang.org/dmd-linux.html#switch-release
> 
> "Compile release version, which means not emitting run-time checks for 
> contracts and asserts. Array bounds checking is not done for system and 
> trusted functions, and assertion failures are undefined behaviour."

I misunderstood, yes, assertions are turned off in @safe code, but not 
array bounds checking (which is what I thought you meant).

Yes, I think assertions should be kept in @safe code. It's weird to have 
array bounds checks kept, but not assertions (which is how you would do 
equivalent bounds checks in a custom type).

> Note that this is the only way to stop checking assertions. There is no 
> option to just ignore them.

I'm not sure what the purpose of just ignoring assertions and not 
contracts (which are made up of assertions anyway).

The boundscheck switch can control the bounds checking independently.

> 
>> It's the -noboundscheck or -boundscheck=off that causes problems.
>>
> 
> At least the specification advises to use caution:
> 
> "off: Bounds checks are disabled completely (even in @safe code). This 
> option should be used with caution and as a last resort to improve 
> performance. Confirm turning off @safe bounds checks is worthwhile by 
> benchmarking."

I suppose the one reason to use -boundscheck=off is to check if it does 
save any performance, and then go in and manually use trusted escapes 
for those where it counts.

I'd rather have it named, however, -boundscheck=unsafe.

-Steve