(original) (raw)

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?

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?

\--

Alexandre Isoard

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