<div dir="ltr">Thank you very much, David and Paul, for your inputs. <div><br></div><div>-Jini.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Nov 5, 2019 at 11:39 PM Robinson, Paul <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal">If you’re really interested, I’d say the first step would be to do some analysis on existing .debug_info sections and see if you can demonstrate a size win. I should think some would be easy to show: DW_AT_byte_size on DW_TAG_pointer_type
would be the same quite a lot (always, for most but not all targets), for example. I’d think that DW_AT_decl_file would be a candidate only for file#1. But these are just intuitive guesses; data is what you want. The question then is, how much extra complexity
does it add to the DIE management.<u></u><u></u></p>
<p class="MsoNormal">--paulr<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org" target="_blank">llvm-dev-bounces@lists.llvm.org</a>> <b>On Behalf Of
</b>David Blaikie via llvm-dev<br>
<b>Sent:</b> Tuesday, November 5, 2019 11:24 AM<br>
<b>To:</b> Jini Susan George <<a href="mailto:jini.susan.george@gmail.com" target="_blank">jini.susan.george@gmail.com</a>><br>
<b>Cc:</b> <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [llvm-dev] DWARFv5 DW_FORM_implicit_const support in LLVM<u></u><u></u></p>
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<p class="MsoNormal" style="margin-bottom:12pt">Nah, doesn't look like LLVM generates implicit_const for any compilation output (the output support is only used in unit tests at the moment/in the patch you mentioned - probably shouldn't've bothered with that
(the dumping support is separate & seems fine to have in-tree))<br>
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LLVM uses FORM_flag_present for cases where adding the attribute at all is only done when the 'true' value is needed (eg: we don't put "DW_AT_declaration false" on non-declaration, the attribute isn't used on non-declarations at all). Arguably, LLVM could avoid
using flag_present when a DIE might otherwise share an abbreviation eg: DW_AT_artificial is done as flag_present, but that means an artificial and non-artificial DW_TAG_subprogram use entirely different abbreviations.<br>
<br>
I think it'd be hard to justify using implicit_const in most cases - because the value changes between different uses & that would then need different abbreviations which would end up more expensive (more bytes) than if the alternative inline-value forms had
been used.<br>
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But I guess if someone comes up with a heuristic about when to use implicit_const & data to support it being an improvement in DWARF size, I'm not completely opposed to the idea.<br>
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<p class="MsoNormal">On Mon, Nov 4, 2019 at 11:54 PM Jini Susan George via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">Hello folks, <u></u><u></u></p>
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<p class="MsoNormal">I was interested in the support we have for the attribute form DW_FORM_implicit_const (DWARFv5 feature) in clang/LLVM. And I had some doubts wrt this. I noticed that support for this was put in here in 2017:
<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal"><a href="https://rev.ng/gitlab/revng-bar-2019/llvm/commit/d9df13befcbc702e239b650dd1f55778d72b8571" target="_blank"><span style="font-family:"Times New Roman",serif;color:black">https://rev.ng/gitlab/revng-bar-2019/llvm/commit/d9df13befcbc702e239b650dd1f55778d72b8571</span></a><u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">From what I could make out, the support for generating DW_FORM_implicit_const is there, but I could not make out the DWARF attributes for which this form is being generated currently. Are there any attributes for which this form is generated
currently ? <u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">Gcc typically generates this form for a bunch of attributes like: DW_AT_decl_file, DW_AT_decl_column, DW_AT_accessibility, DW_AT_defaulted, DW_AT_byte_size, etc<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">Any pointers are deeply appreciated.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Thanks,<u></u><u></u></p>
<p class="MsoNormal">Jini.<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
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