[llvm-dev] returns_twice / noreturn

[Hal Finkel via llvm-dev]

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;
> }
>
> Produces the following LLVM IR:
>
> ; Function Attrs: nounwind uwtable
> define i32 @f(i32 %n) local_unnamed_addr #0 {
> entry:
>   %call = call i32 (...) @fork() #3
>   %cmp = icmp eq i32 %call, 0
>   br i1 %cmp, label %if.end, label %if.then
>
> if.then:                           ; preds = %entry
>   call void @join(i32 %call) #4
>   unreachable
>
> if.end:                            ; preds = %entry
>   %inc = add nsw i32 %n, 1
>   ret i32 %inc
> }
>
> ; Function Attrs: returns_twice
> declare i32 @fork(...) local_unnamed_addr #1
>
> ; Function Attrs: noreturn
> declare void @join(i32) local_unnamed_addr #2
>
> Where the n++ has been moved after the if, is that legal?

Why wouldn't it be?

>
> Also, technically, f could also returns_twice or noreturn (depending 
> on the return values of fork).
>
> So my question is: do they have semantic or they are only "clues" for 
> heuristic purposes?

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.

  -Hal

>
> -- 
> *Alexandre Isoard*
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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