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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">To improve debug information for variable length arrays (https://bugs.llvm.org/show_bug.cgi?id=30553) I have proposed some changes to Metadata IR's DISubrange node. To get some
wider visibility on this, I'm sharing this as an RFC with llvm-dev.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">The proposed change is to extend the 'count' field to also accept a Metadata node, rather than only accepting a signed integer. For example:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> !10 = !DILocalVariable(name: "count", scope: !8, file: !6, line: 42, type: !9)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> !11 = !DISubrange(count !10, lowerBound: 0)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> ^^^ references the local variable<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">This allows specifying the size of an array by referencing the DIE of a local variable that holds the size expression. Another reason to extend the count field is to specify the
length with a DIExpression. For instance, we need this to specify the length of an SVE vector as a DWARF expression that is based on a (runtime defined) DWARF pseudo register.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">Some preliminary feedback was that we may want to use something similar for lowerBound as well. Also Adrian suggested (https://reviews.llvm.org/D41697#966736) that it may be useful
in general to reference Metadata nodes in DIExpressions if we ever need to combine local variables with DWARF expressions. Something like that might be needed to e.g. reference a value from an offset of a pointer when using an array descriptor, although I
could use some suggestions on what DWARF code needs to be generated for this general case.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">The whole patch-series can be found here:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> https://reviews.llvm.org/D41695<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> https://reviews.llvm.org/D41696<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> https://reviews.llvm.org/D41697<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"> https://reviews.llvm.org/D41698<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier",serif">Sander<o:p></o:p></span></p>
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