<div dir="ltr">Yeah, I'm with you here. I don't think there's any particular debug info gain to having debug info on materializing a constant either.<div><br></div><div>Vedant: Ultimately I think I'm in favor of this plan.</div><div><br></div><div>Thanks!</div><div><br></div><div>-eric<br><br><div class="gmail_quote"><div dir="ltr">On Wed, Jun 20, 2018 at 4:32 PM Matthias Braun via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space"><div>FWIW: Debug information on constants feels odd to me. They are just values not something that is executed so conceptually I would not expect them to "happen" at a specific time/place in the program. That said most numbers are copied into registers or stored into memory and that is of course an interesting action. So in the original example I would hope to see debug info on whatever instructions are used to fill the array with values.</div><div><br></div><div>That said I'm not familiar with the inner workings of dwarf or other debugger formats, so it may very well be reasonable to backfill the information in a late pass to avoid having assembler instructions without debug info as some people proposed.</div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br></div><div>- Matthias</div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br><blockquote type="cite"><div>On Jun 20, 2018, at 11:09 AM, via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="m_6241682479114180037Apple-interchange-newline"><div><div class="m_6241682479114180037WordSection1" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">DwarfDebug::beginInstruction sets the location to line 0 because otherwise the location is implicitly the same location as the last instruction of the physically preceding block. That location is often completely unrelated to what's at the top of the new block. Line 0 isn't great, but at least it's not a complete lie.<u></u><u></u></span></div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">--paulr<u></u><u></u></span></div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><a name="m_6241682479114180037__MailEndCompose"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></a></div><div style="border-style:none none none solid;border-left-width:1.5pt;border-left-color:blue;padding:0in 0in 0in 4pt"><div><div style="border-style:solid none none;border-top-width:1pt;border-top-color:rgb(181,196,223);padding:3pt 0in 0in"><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><b><span style="font-size:10pt;font-family:Tahoma,sans-serif">From:</span></b><span style="font-size:10pt;font-family:Tahoma,sans-serif"><span class="m_6241682479114180037Apple-converted-space"> </span><a href="mailto:vsk@apple.com" target="_blank">vsk@apple.com</a> [<a href="mailto:vsk@apple.com" target="_blank">mailto:vsk@apple.com</a>]<span class="m_6241682479114180037Apple-converted-space"> </span><br><b>Sent:</b><span class="m_6241682479114180037Apple-converted-space"> </span>Wednesday, June 20, 2018 1:48 PM<br><b>To:</b><span class="m_6241682479114180037Apple-converted-space"> </span>Reid Kleckner<br><b>Cc:</b><span class="m_6241682479114180037Apple-converted-space"> </span>Robinson, Paul; llvm-dev; Justin Bogner; David Li; David Blaikie<br><b>Subject:</b><span class="m_6241682479114180037Apple-converted-space"> </span>Re: [RFC] Removing debug locations from ConstantSDNodes<u></u><u></u></span></div></div></div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div><blockquote style="margin-top:5pt;margin-bottom:5pt"><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">On Jun 19, 2018, at 6:36 PM, Reid Kleckner <<a href="mailto:rnk@google.com" style="color:purple;text-decoration:underline" target="_blank">rnk@google.com</a>> wrote:<u></u><u></u></div></blockquote><div><blockquote style="margin-top:5pt;margin-bottom:5pt"><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div><div><div><div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">On Tue, Jun 19, 2018 at 5:46 PM Vedant Kumar <<a href="mailto:vsk@apple.com" style="color:purple;text-decoration:underline" target="_blank">vsk@apple.com</a>> wrote:<u></u><u></u></div></div><blockquote style="border-style:none none none solid;border-left-width:1pt;border-left-color:rgb(204,204,204);padding:0in 0in 0in 6pt;margin-left:4.8pt;margin-right:0in"><div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">Someone (Reid?) mentioned that we could try sinking constants to their point of first use as an alternative, and (IIUC) create new nodes with distinct DebugLocs for each use of a constant. I don't recall the details of that alternative clearly. Based on my (likely incorrect) understanding of it, dropping locations from constants outright might be simpler.<u></u><u></u></div></div></div></blockquote><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">Our use case was in fastisel, so things are different. I don't think my solution will help you.<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">In our case, users were complaining about code like this:<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">volatile int do_something;<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">void f() {<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"> ++do_something;<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"> foo(1, 2, 3);<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"> ++do_something;<br>}<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">We'd generate locations like:<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"> .loc line 1<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"> incl do_something(%rip)<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"> movl $1, %ecx<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"> movl $2, %edx</span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"> movl $3, %r8d</span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"> .loc line 2 # line 2 starts here, instead of 3 instructions earlier</span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"> callq foo</span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"> .loc line 3</span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"> incl do_something(%rip)</span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><span style="background-color:white;background-position:initial initial;background-repeat:initial initial"><br><br></span><u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">Our users really wanted the line table entry for line 2 to include the constant materialization code for some VS debugger feature.<u></u><u></u></div></div></div></div></div></blockquote><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">Got it. (For others following along, there's more discussion about this debugger feature in the commit description for r327581.)<br><br><br><br><u></u><u></u></div><div><div><div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">I think if you remove locations from ConstantSDNodes, you might want to add a late pass that propagates source locations backwards onto location-less instructions. This would also avoid some special cases when a basic block starts with an instruction that lacks location information. See CodeViewDebug::beginInstruction and DwarfDebug::beginInstruction for what we do today.<u></u><u></u></div></div></div></div></div></div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">Interesting, I hadn't considered doing this.<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">What happens in the special case where a block starts with a location-less instruction? Ah, I see that CodeViewDebug::beginInstruction does a forward scan to find the first instruction with location. This can fail though: there might not be such an instruction, in which case... I assume we either apply a line-0 location, or nothing at all, similar to Dwarf::beginInstruction?<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">It's a bit unclear to me what the benefits of a late pass that backwards-propagated locations would be. I suppose we'd have fewer forward scans in CodeViewDebug::beginInstruction, so is the benefit a compile-time win (which would offset the cost of a late pass)? Would the final debug info quality would be better in some way I'm just missing?<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif"><u></u> <u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">thanks!<u></u><u></u></div></div><div><div style="margin:0in 0in 0.0001pt;font-size:12pt;font-family:"Times New Roman",serif">vedant<u></u><u></u></div></div></div></div><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">_______________________________________________</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">LLVM Developers mailing list</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important"><a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a></span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important"><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a></span></div></blockquote></div><br></div>_______________________________________________<br>
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