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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I hadn't thought Clang wanted to be *quite* so knowledgeable about targets, and similarly not so tightly tied to byte-addressable targets. But if both of those
things are actually okay, then it's fine to set the alignment value here to what would be passed through to DWARF.<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> David Blaikie [mailto:dblaikie@gmail.com]
<br>
<b>Sent:</b> Monday, September 12, 2016 6:11 PM<br>
<b>To:</b> Robinson, Paul; reviews+D24426+public+6ee6274d38fdf0d6@reviews.llvm.org; vleschuk@accesssoftek.com; echristo@gmail.com; aprantl@apple.com; mehdi.amini@apple.com<br>
<b>Cc:</b> cfe-commits (cfe-commits@lists.llvm.org)<br>
<b>Subject:</b> Re: [PATCH] D24426: DebugInfo: Pass non-zero alignment to DIBuilder only if aligment was forced<o:p></o:p></span></p>
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<p class="MsoNormal">On Mon, Sep 12, 2016 at 6:01 PM Robinson, Paul <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a name="m_-8606035682448503423__MailEndCompose"><span class="gmailmsg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The text in the committee
draft is different (e.g., the exhortation about non-default alignment is gone), with an example to the effect that a value of 8 means the entity's address is a multiple of 8 (not 2^8). So, alignment is conceived in terms of address bits, whatever those represent
(not always bytes). </span></span></a><o:p></o:p></p>
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<p class="MsoNormal">Not sure I quite follow. OK, so in an octet addressable world (which LLVM is - there have been some attempts to support non-octet addressing, but I don't think any have been near to successful) then DW_AT_alignment is byte alignment (1
means there are no zero bits in the address, 2 means there's 1 trailing zero bit in the address, etc).<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span class="gmailmsg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If Clang is being infested with more target knowledge, okay, but that means
tolerating the weirder targets in these cases. Dividing by CHAR_BITS makes an assumption that isn't necessarily correct.</span></span><o:p></o:p></p>
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<p class="MsoNormal">Clang has the knowledge already - it knows the alignment of the types its allocating, etc. So I'm not sure what infestation you're referring to.<br>
<br>
I've sort of lost track of what we're discussing here.<br>
<br>
Essentially what I'm suggesting is that Clang should put whatever number is going to go in the DWARF, into the metadata. I don't believe the LLVM backends have greater knowledge than the frontend does in this domain - have I missed something there, are there
examples where that could/would be true?<br>
<br>
- David<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span class="gmailmsg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr</span></span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span class="gmailmsg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">P.S. The committee is hoping to get a draft out for public comment Real Soon
Now.</span></span><o:p></o:p></p>
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<p class="MsoNormal">Looking forward to it :) <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span class="gmailmsg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> </span></span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span class="gmailmsg"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b></span><span class="gmailmsg"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
cfe-commits [mailto:<a href="mailto:cfe-commits-bounces@lists.llvm.org" target="_blank">cfe-commits-bounces@lists.llvm.org</a>]
<b>On Behalf Of </b>David Blaikie via cfe-commits</span></span><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""><br>
<span class="gmailmsg"><b>Sent:</b> Monday, September 12, 2016 5:12 PM</span><br>
<span class="gmailmsg"><b>To:</b> <a href="mailto:reviews%2BD24426%2Bpublic%2B6ee6274d38fdf0d6@reviews.llvm.org" target="_blank">
reviews+D24426+public+6ee6274d38fdf0d6@reviews.llvm.org</a>; <a href="mailto:vleschuk@accesssoftek.com" target="_blank">
vleschuk@accesssoftek.com</a>; <a href="mailto:echristo@gmail.com" target="_blank">
echristo@gmail.com</a>; <a href="mailto:aprantl@apple.com" target="_blank">aprantl@apple.com</a>;
<a href="mailto:mehdi.amini@apple.com" target="_blank">mehdi.amini@apple.com</a></span><br>
<span class="gmailmsg"><b>Cc:</b> <a href="mailto:cfe-commits@lists.llvm.org" target="_blank">
cfe-commits@lists.llvm.org</a></span><br>
<span class="gmailmsg"><b>Subject:</b> Re: [PATCH] D24426: DebugInfo: Pass non-zero alignment to DIBuilder only if aligment was forced</span></span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Mon, Sep 12, 2016 at 5:00 PM Paul Robinson <<a href="mailto:paul.robinson@sony.com" target="_blank">paul.robinson@sony.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">probinson added a subscriber: probinson.<br>
<br>
================<br>
Comment at: lib/CodeGen/CGDebugInfo.cpp:3691<br>
@@ -3635,1 +3690,3 @@<br>
+ if (D->hasAttr<AlignedAttr>())<br>
+ AlignInBits = D->getMaxAlignment();<br>
StringRef DeclName, LinkageName;<br>
----------------<br>
dblaikie wrote:<br>
> is max alignment the right thing here? Should it be min alignment?<br>
> (is alignment in bits the desired thing across all of this too? It looked like in the backend patch there was some division by CHAR_BITS, etc?)<br>
I should think bits is the right choice here; seems more the province of the backend to convert it into the appropriate addressable units (commonly but not universally chars).<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><br>
The alternative thinking is that we've a generally sense we want to make more of this type information opaque to LLVM - so I'm somewhat inclined to make the frontend do the work of choosing what to emit and the backend just being as simple as possible.<br>
<br>
Hmm, seems like the DWARF spec details I can find: <a href="http://www.dwarfstd.org/ShowIssue.php?issue=140528.1" target="_blank">http://www.dwarfstd.org/ShowIssue.php?issue=140528.1</a> don't really specify what the value of DW_AT_alignment is, it's sort of
assumed, by the looks of it? I'm assuming it's bytes, the same as the byte_size attribute.<br>
<br>
<br>
<o:p></o:p></p>
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<br>
<a href="https://reviews.llvm.org/D24426" target="_blank">https://reviews.llvm.org/D24426</a><br>
<br>
<o:p></o:p></p>
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