<div dir="ltr">Dear Tijskens,<div><br></div><div style>I was intrigued by your observations  and tried to reproduce them but I failed. Actually, I feel like Matthias that measuring such short minimalistic code section is really tough.</div>
<div style><br></div><div style>Would you be able  to share your benchmark code and the way you compile it such that I can have a more thorough look?</div><div style><br></div><div style>Best</div><div style><br></div><div style>
Sandro</div><div style><br></div><div class="gmail_extra"><br><br><div class="gmail_quote">2014-03-04 18:57 GMT+01:00 Tijskens Engelbert <span dir="ltr"><<a href="mailto:Engelbert.Tijskens@uantwerpen.be" target="_blank">Engelbert.Tijskens@uantwerpen.be</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">



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Dear all,
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<div>I am trying to figure out how to use std::vector<float> efficiently in combination with Vc. (to have dynamic arrays and performance)</div>
<div><br>
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<div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    std::<span style="color:rgb(0,97,65)">vector</span><<span style="color:rgb(147,26,104)">float</span>> x(1024);</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    <span style="color:#931a68">
for</span>( <span style="color:#931a68">int</span> i=0; i<ne; ++i ) {//initialize</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        x[i]=1.0;</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    }</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">// scalar loop using std::vector</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    <span style="color:rgb(147,26,104)">for</span>(
<span style="color:rgb(147,26,104)">int</span> i=0; i<ne; ++i ) {</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        x[i] -= 1.0;</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    }</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">// vector loop using std::vector</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    <span style="color:rgb(147,26,104)">
for</span>( <span style="color:rgb(147,26,104)">int</span> i=0; i<ne; i+=Vc::<span style="color:rgb(0,97,65)">float_v</span>::<span style="color:rgb(3,38,204)">Size</span> )</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    {</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        Vc::<span style="color:#006141">float_v</span> vx( &x[i] );</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        vx -= 1.0;</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        vx.store( &x[i] );</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    }</div>
<div style="margin:0px;font-size:11px;font-family:Monaco"><span style="color:rgb(78,144,114)">// vector loop using Vc::Memory instead of std::vector</span></div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    Vc::<span style="text-decoration:underline;color:#006141">Memory</span><Vc::<span style="color:#006141">float_v</span>,ne> Vx;</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    <span style="color:#931a68">
for</span>( <span style="color:#931a68">int</span> i=0; i<ne; ++i ) {//initialize</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        Vx[i] = 1.0;</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    }</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    Vc::<span style="color:#006141">float_v</span> one(1.);</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">    ET_TIME_THIS</div>
<div style="margin:0px;font-size:11px;font-family:Monaco;color:rgb(57,51,255)">
<span style="color:#000000">    ( </span>"<span style="text-decoration:underline">Vc</span>::Memory<Vc::float_v,<span style="text-decoration:underline">ne</span>>  vector"<span style="color:#000000">,</span></div>

<div style="margin:0px;font-size:11px;font-family:Monaco">        <span style="color:#931a68">
for</span>( <span style="color:#931a68">int</span> i=0; i<nv; ++i ) {</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">            Vx.<span style="text-decoration:underline">vector</span>(i) -= one;</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">        }</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">When i time these loops i get the following results</div>
</div>
<div style="margin:0px;font-size:11px;font-family:Monaco">
<div style="margin:0px;font-family:Menlo">scalar loop using std::vector : 2162 cycles/repetition, 9.4e-07 seconds/repetition, 1   x speedup, 2.3  GHz, 100 repetitions.</div>
<div style="margin:0px;font-family:Menlo">vector loop using std::vector :  357 cycles/repetition, 1.6e-07 seconds/repetition, 6.04x speedup, 2.24 GHz, 100 repetitions.</div>
<div style="margin:0px;font-family:Menlo">vector loop using Vc::Memory  :  288 cycles/repetition, 1.2e-07 seconds/repetition, 7.49x speedup, 2.4  GHz, 100 repetitions.</div>
<div><br>
</div>
<div><br>
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<div>is there a way to improve the vector loop using std::vector? By the way if i write the second loop as</div>
<div>
<div style="margin:0px">// vector loop using std::vector</div>
<div style="margin:0px">    <span style="color:rgb(147,26,104)">for</span>( <span style="color:rgb(147,26,104)">int</span> i=0; i<ne; i+=Vc::<span style="color:rgb(0,97,65)">float_v</span>::<span style="color:rgb(3,38,204)">Size</span> )</div>

<div style="margin:0px">    {</div>
<div style="margin:0px">        Vc::<span style="color:rgb(0,97,65)">float_v</span> vx( &x[i] );</div>
<div style="margin:0px">        Vx.<span style="text-decoration:underline">vector</span>(i) -= one;</div>
<div style="margin:0px">        vx.store( &x[i] );</div>
<div style="margin:0px">    }</div>
</div>
<div style="margin:0px">things get even worse, the speedup being only 4.2x roughly.</div>
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<br></blockquote></div><br></div><br clear="all"><div><br></div>-- <br><div dir="ltr">Dr. Sandro Wenzel<br><div>PH / SFT</div><div>CERN <br><br></div></div></div>