[llvm-dev] returns_twice / noreturn

[Yichao Yu via llvm-dev]

On Fri, Nov 3, 2017 at 8:54 PM, Alexandre Isoard via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On Fri, Nov 3, 2017 at 5:39 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>> On 11/03/2017 07:20 PM, Alexandre Isoard via llvm-dev wrote:
>>
>> Hello,
>>
>> I am not sure about the semantic (if any) of returns_twice and noreturn
>> attributes.
>>
>> int fork() __attribute__((returns_twice));
>> void join(int) __attribute__((noreturn));
>>
>> int f(int n) {
>>   int t = fork();
>>   n++;
>>   if (t != 0)
>>     join(t);
>>   return n;
>> }
>>
>> Where the n++ has been moved after the if, is that legal?
>>
>>
>> Why wouldn't it be?
>
>
> Because fork() could return 0, then n get incremented (first time), we go
> into join(t) which do not return... but jump back into fork() which returns
> again, but 1 this time, then n get incremented (second time), and we return
> n+2.

This is a valid transformation and that's why to get the effect you
want in C/C++, the variable must be marked volatile.

>
> While if we move the n++ outside of that "region", we change that semantic?
> Basically, returns_twice and noreturn have SSA-reaching side-effects.
>
>>
>>
>> They have semantics. returns_twice, however, really means, "it may return
>> more than once". noreturn is interpreted as the name implies. Thus the
>> unreachable after the call.
>
>
> That means we can encode a loop this way? :-)
>
>>
>>
>>  -Hal
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>
>
>
>
> --
> Alexandre Isoard
>
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