[llvm-dev] returns_twice / noreturn

[Alexandre Isoard via llvm-dev]

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.

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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