On 02.08.2020 17:50, Taylor R Campbell wrote:
>> Date: Sun, 2 Aug 2020 17:35:06 +0200
>> From: Kamil Rytarowski <kamil%netbsd.org@localhost>
>>
>> On 02.08.2020 16:44, Taylor R Campbell wrote:
>>>> Date: Sun, 2 Aug 2020 16:04:15 +0200
>>>> From: Kamil Rytarowski <kamil%netbsd.org@localhost>
>>>>
>>>> On 02.08.2020 15:57, Taylor R Campbell wrote:
>>>>> But it sounds like the original motivation is that it triggered
>>>>> -Wvla...which frankly strikes me as a compiler bug since there's
>>>>> obviously no actual VLA created in sizeof; as far as I can tell
>>>>> there's no semantic difference between sizeof(device_t[n]) and
>>>>> sizeof(device_t) * n.
>>>>
>>>> This is not true:
>>>
>>> Which part of what I said are you claiming is not true, and what are
>>> you illustrating with the example program below?
>>
>> Calling it a compiler bug.
>>
>> Clang behaves the same way.
>>
>> $ clang -Wvla test.c
>> test.c:6:37: warning: variable length array used [-Wvla]
>> printf("sizeof = %zu\n", sizeof(int[argc]));
>> ^
>> 1 warning generated.
>>
>> Creating VLA is not needed for using it as an intermediate. In practice
>> in most/all cases it is optimized and actual VLA is not allocated.
>
> This is not a matter of optimization in practice. It's absolutely not
> an `intermediate' at all -- there is no VLA created, period, any more
> than sizeof(malloc(1)) causes any actual call to malloc or sizeof(*p)
> causes any dereference of a pointer.
>
> This makes -Wvla less useful as a tool, because apparently it flags
> code that unquestionably does not have any bad effects of VLAs. This
> happens because -Wvla is dumb -- it just looks for the syntax, not for
> the semantics of creating VLAs. That's why I call it a bug -- it
> raises a false positive that makes it less useful.
>
Compilers warns about VLA using ("variable length array used"), not
creating. There is no distinct compiler warning to discriminate VLA
creating from usage.
Anyway, code just using VLA is not that frequent, avoiding it is rather
simple and VLA can be avoided altogether (at least for non-external).
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