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<p>Hi Haoran,</p>
<div class="moz-cite-prefix">On 03.10.2020 12:02, Haoran Xu via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CA+jENmLSmtT07UxoXeTMDW=A5HCenYA_Bt5KKwnrUk3paTohYQ@mail.gmail.com">
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<div>Hello,</div>
<div><br>
</div>
<div>Could anyone kindly explain to me why the 'g()' in the
following function cannot have tail call optimization?</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<div
style="color:rgb(0,0,0);background-color:rgb(255,255,254);font-family:"Consolas,
"">
<div><span style="color:rgb(0,0,255)">void</span><span
style="color:rgb(0,0,0)"> f(</span><span
style="color:rgb(0,0,255)">int</span><span
style="color:rgb(0,0,0)">* x);</span></div>
<div><span style="color:rgb(0,0,255)">void</span><span
style="color:rgb(0,0,0)"> g();</span></div>
<div><span style="color:rgb(0,0,255)">void</span><span
style="color:rgb(0,0,0)"> h(</span><span
style="color:rgb(0,0,255)">int</span><span
style="color:rgb(0,0,0)"> v) {</span></div>
<div><span style="color:rgb(0,0,0)"> f(&v);</span></div>
<div><span style="color:rgb(0,0,0)"> g();</span></div>
<div><span style="color:rgb(0,0,0)">}</span></div>
</div>
</div>
</blockquote>
<div>
<div>A while ago I was taught that tail call optimization
cannot apply if local variables needs to be kept alive, but
'g()' doesn't seem to require anything to be alive on the
stack.</div>
<div>I tried to manually add 'tail' to the emitted LLVM IR and
it appears to work. <br>
</div>
<div><br>
</div>
<div>Any idea how I could fix this and let clang automatically
generate tail call?</div>
</div>
</div>
</blockquote>
<p><br>
</p>
<p>In that test case the pointer to "v" could escape through call to
f();<br>
That prevents from doing tail call for g().<br>
If it is already known that f() does not capture its parameter <br>
then it could be declared with attribute noescape :<br>
<br>
<a class="moz-txt-link-freetext" href="https://clang.llvm.org/docs/AttributeReference.html#noescape">https://clang.llvm.org/docs/AttributeReference.html#noescape</a><br>
<br>
In this case tailcall happens:<br>
<br>
cat test.cpp<br>
void f(__attribute__((noescape)) int* x);<br>
void g();<br>
void h(int v) {<br>
f(&v);<br>
g();<br>
}<br>
<br>
clang++ -O3 -S test.cpp<br>
<br>
grep _Z1gv test.s<br>
<br>
jmp _Z1gv # TAILCALL</p>
<p><br>
</p>
<br>
<blockquote type="cite"
cite="mid:CA+jENmLSmtT07UxoXeTMDW=A5HCenYA_Bt5KKwnrUk3paTohYQ@mail.gmail.com">
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<div>
<div>Thanks!</div>
<div><br>
</div>
<div>Best,</div>
<div>Haoran<br>
</div>
</div>
</div>
<br>
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</blockquote>
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