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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I want to get some latest opinions on “Fixed Point” data type support in LLVM from the community.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">From previous mailing list discussions, it seems like Chris<span style="color:#1F497D">
</span>[1, 2] and “other people” [2] prefer of using existing LLVM integer type and builtins to support “Fixed point” data types. I have questions regarding this approach:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph" style="text-indent:-.25in">1.<span style="font-size:7.0pt;font-family:"Times New Roman",serif">
</span>Using exactly the existing LLVM integer type:<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in">a.<span style="font-size:7.0pt;font-family:"Times New Roman",serif"> </span>How to distinguish fixed point data and regular integer data types during lowering and register allocation
(if there is operations that operates on different type of registers than GPRs)? (Any functional, performance concerns here?)<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in">b. For parameter passing, linkage support, should we use “attribute” to support it?<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in">c. Will there be any transformations/optimizations (e.g. SROA) that operates on integer types but may not be suitable to Fixed point types?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in"><o:p> </o:p></p>
<p class="MsoListParagraph" style="text-indent:-.25in">2.<span style="font-size:7.0pt;font-family:"Times New Roman",serif">
</span>Extending the existing LLVM integer type to have sub fields like integer, fraction data parts:<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in">a.<span style="font-size:7.0pt;font-family:"Times New Roman",serif">
</span>It seems like LLVM assumes "We can't have the same bitwidth for different int types". This will have a problem with this approach here. How should we solve it?<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in">b.<span style="font-size:7.0pt;font-family:"Times New Roman",serif">
</span>How to handle fixed point constants with this approach? Also how to handle APInt for this approach?<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in">c.<span style="font-size:7.0pt;font-family:"Times New Roman",serif">
</span>How this can affect optimizations, linkage, lowering and register allocation?
<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:1.0in;text-indent:-.25in"><o:p> </o:p></p>
<p class="MsoNormal"> 3. We are also considering introducing new LLVM data types. We think it will cause missing optimization opportunities (e.g. all optimizations that reply on APInt will stop working for the new data type). Is this a valid concern?
Are there any other issues that we will run into as well?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">[1] <a href="http://lists.llvm.org/pipermail/llvm-dev/2010-November/036645.html">
http://lists.llvm.org/pipermail/llvm-dev/2010-November/036645.html</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">[2] <a href="https://groups.google.com/forum/#!topic/llvm-dev/-bm8HzBQN94">
https://groups.google.com/forum/#!topic/llvm-dev/-bm8HzBQN94</a><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks a lot for any comments, suggestions. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b><span lang="EN-CA" style="color:#1F497D">Best,<o:p></o:p></span></b></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span style="font-size:8.0pt;font-family:"Akkurat Pro";color:#1F497D">Ye Henry Tian<o:p></o:p></span></b></p>
<p class="MsoNormal"><b><span style="font-size:8.0pt;font-family:"Akkurat Pro";color:#1F497D"><o:p> </o:p></span></b></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Akkurat Pro Light";color:#7B7B7B">Programming Technologies Lab<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Akkurat Pro Light";color:#7B7B7B">Huawei Canada Research Center<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Akkurat Pro Light";color:#7B7B7B">19 Allstate Parkway, 3<sup>rd</sup> floor<o:p></o:p></span></p>
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