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<p class="MsoNormal">There isn’t any target-independent code to help form VLIW instruction words, so you’ll have to write code for your specific target to do that. Since it’s target-specific code, you can do whatever transforms are appropriate, including rewriting
instruction opcodes.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">LLVM supports the notion of a “bundle”, which might be helpful. It’s a set of multiple instructions which is treated as one instruction by various backend passes. This allows representing the different parts of a VLIW instruction word
as separate instructions, but allows passes that don’t understand your target-specific bundles to treat it as one big instruction.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In-tree, you can look at the Hexagon backend to see how this works in practice.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Eli<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>Ronny BarTov via llvm-dev<br>
<b>Sent:</b> Sunday, January 19, 2020 1:38 AM<br>
<b>To:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> [EXT] [llvm-dev] extending LLVM<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Hello,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">in vliw architectures, the behavior of a machine command is fixed in whatever vliw it is included.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">In some cases, however, the appearance of some other specific commands within the same vliw, affects the behavior of the original command,
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">or define a new way of operation such that the entire behavior of the vliw can no longer be considered as the cumulative effect of the individual, separate, commands.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">examples for the above might be:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">- allowing more than a single CC (condition code) changing command to be present in the same vliw. a possible definition for such a case<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> might be to set the new CC to the logical OR of all the new values from the commands in the vliw that write to the CC.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">- forcing specific execution units.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> suppose both I1 and I2 commands can be assigned to functional units<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> FU1 or FU2. we might insist that whenever I1 and I2 are in the same vliw, then I1 must go to FU1 while I2 must go to FU2.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">- two instructions can normally have a long encoding, but might use a combined, shorter encoding that would make them fit together inside a single vliw.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Is there a systematic way to handle such cases in LLVM ?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Thx<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">ronny<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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