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<div class="moz-cite-prefix">On 11/03/2017 07:20 PM, Alexandre
Isoard via llvm-dev wrote:<br>
</div>
<blockquote
cite="mid:CANLM5Ley341MH72-38KK6xZMYOOxEutOSdu8zWnt+=qF0tVH1A@mail.gmail.com"
type="cite">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<div dir="ltr">Hello,
<div><br>
</div>
<div>I am not sure about the semantic (if any) of returns_twice
and noreturn attributes.</div>
<div><br>
</div>
<div><font face="monospace, monospace">int fork()
__attribute__((returns_twice));</font></div>
<div><font face="monospace, monospace">void join(int)
__attribute__((noreturn));</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="monospace, monospace">int f(int n) {</font></div>
<div><font face="monospace, monospace"> int t = fork();</font></div>
<div><font face="monospace, monospace"> n++;</font></div>
<div><font face="monospace, monospace"> if (t != 0)</font></div>
<div><font face="monospace, monospace"> join(t);</font></div>
<div><font face="monospace, monospace"> return n;</font></div>
<div><font face="monospace, monospace">}</font></div>
<div><br>
</div>
<div>Produces the following LLVM IR:</div>
<div><br>
</div>
<div>
<div><font face="monospace, monospace">; Function Attrs:
nounwind uwtable</font></div>
<div><font face="monospace, monospace">define i32 @f(i32 %n)
local_unnamed_addr #0 {</font></div>
<div><font face="monospace, monospace">entry:</font></div>
<div><font face="monospace, monospace"> %call = call i32
(...) @fork() #3</font></div>
<div><font face="monospace, monospace"> %cmp = icmp eq i32
%call, 0</font></div>
<div><font face="monospace, monospace"> br i1 %cmp, label
%if.end, label %if.then</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="monospace, monospace">if.then:
; preds = %entry</font></div>
<div><font face="monospace, monospace"> call void @join(i32
%call) #4</font></div>
<div><font face="monospace, monospace"> unreachable</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="monospace, monospace">if.end:
; preds = %entry</font></div>
<div><font face="monospace, monospace"> %inc = add nsw i32
%n, 1</font></div>
<div><font face="monospace, monospace"> ret i32 %inc</font></div>
<div><font face="monospace, monospace">}</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="monospace, monospace">; Function Attrs:
returns_twice</font></div>
<div><font face="monospace, monospace">declare i32 @fork(...)
local_unnamed_addr #1</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="monospace, monospace">; Function Attrs:
noreturn</font></div>
<div><font face="monospace, monospace">declare void @join(i32)
local_unnamed_addr #2</font></div>
<div><br>
</div>
<div>Where the n++ has been moved after the if, is that legal?</div>
</div>
</div>
</blockquote>
<br>
Why wouldn't it be?<br>
<br>
<blockquote
cite="mid:CANLM5Ley341MH72-38KK6xZMYOOxEutOSdu8zWnt+=qF0tVH1A@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>
<div><br>
</div>
<div>Also, technically, f could also returns_twice or noreturn
(depending on the return values of fork).</div>
<div><br>
</div>
<div>So my question is: do they have semantic or they are only
"clues" for heuristic purposes?</div>
</div>
</div>
</blockquote>
<br>
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.<br>
<br>
-Hal<br>
<br>
<blockquote
cite="mid:CANLM5Ley341MH72-38KK6xZMYOOxEutOSdu8zWnt+=qF0tVH1A@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>
<div><br>
</div>
-- <br>
<div class="gmail_signature">
<div dir="ltr"><b>Alexandre Isoard</b><br>
</div>
</div>
</div>
</div>
<br>
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</pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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