<div dir="ltr"><div>Hi Alexey,</div><div><br></div><div>Thanks for the explanation! That's really a tricky case, I understand it now.</div><div><br></div><div>Best,</div><div>Haoran<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Alexey <<a href="mailto:avl.lapshin@gmail.com">avl.lapshin@gmail.com</a>> 于2020年10月3日周六 上午3:35写道：<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
  
    
  
  <div>
    <p>Hi Haoran,</p>
    <div>On 03.10.2020 12:02, Haoran Xu via
      llvm-dev wrote:<br>
    </div>
    <blockquote type="cite">
      
      <div dir="ltr">
        <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>
              <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 href="https://clang.llvm.org/docs/AttributeReference.html#noescape" target="_blank">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">
      <div dir="ltr">
        <div>
          <div>Thanks!</div>
          <div><br>
          </div>
          <div>Best,</div>
          <div>Haoran<br>
          </div>
        </div>
      </div>
      <br>
      <fieldset></fieldset>
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</pre>
    </blockquote>
  </div>

</blockquote></div>