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I think it's better to pass amount in bytes here, as specified by
user (alignas takes bytes, not bits).<br>
<div class="moz-cite-prefix">On 09/13/2016 09:00 PM, Robinson, Paul
wrote:<br>
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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 moz-do-not-send="true"
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 [<a class="moz-txt-link-freetext" href="mailto:dblaikie@gmail.com">mailto:dblaikie@gmail.com</a>]
<br>
<b>Sent:</b> Monday, September 12, 2016 6:11 PM<br>
<b>To:</b> Robinson, Paul;
<a class="moz-txt-link-abbreviated" href="mailto:reviews+D24426+public+6ee6274d38fdf0d6@reviews.llvm.org">reviews+D24426+public+6ee6274d38fdf0d6@reviews.llvm.org</a>;
<a class="moz-txt-link-abbreviated" href="mailto:vleschuk@accesssoftek.com">vleschuk@accesssoftek.com</a>; <a class="moz-txt-link-abbreviated" href="mailto:echristo@gmail.com">echristo@gmail.com</a>;
<a class="moz-txt-link-abbreviated" href="mailto:aprantl@apple.com">aprantl@apple.com</a>; <a class="moz-txt-link-abbreviated" href="mailto:mehdi.amini@apple.com">mehdi.amini@apple.com</a><br>
<b>Cc:</b> cfe-commits (<a class="moz-txt-link-abbreviated" href="mailto:cfe-commits@lists.llvm.org">cfe-commits@lists.llvm.org</a>)<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"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">On Mon, Sep 12, 2016 at 6:01 PM
Robinson, Paul <<a moz-do-not-send="true"
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
moz-do-not-send="true"
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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<div>
<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
moz-do-not-send="true"
href="mailto:cfe-commits-bounces@lists.llvm.org"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:cfe-commits-bounces@lists.llvm.org">cfe-commits-bounces@lists.llvm.org</a></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
moz-do-not-send="true"
href="mailto:reviews%2BD24426%2Bpublic%2B6ee6274d38fdf0d6@reviews.llvm.org"
target="_blank">
<a class="moz-txt-link-abbreviated" href="mailto:reviews+D24426+public+6ee6274d38fdf0d6@reviews.llvm.org">reviews+D24426+public+6ee6274d38fdf0d6@reviews.llvm.org</a></a>; <a
moz-do-not-send="true"
href="mailto:vleschuk@accesssoftek.com"
target="_blank">
<a class="moz-txt-link-abbreviated" href="mailto:vleschuk@accesssoftek.com">vleschuk@accesssoftek.com</a></a>; <a
moz-do-not-send="true"
href="mailto:echristo@gmail.com"
target="_blank">
<a class="moz-txt-link-abbreviated" href="mailto:echristo@gmail.com">echristo@gmail.com</a></a>; <a
moz-do-not-send="true"
href="mailto:aprantl@apple.com"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:aprantl@apple.com">aprantl@apple.com</a></a>;
<a moz-do-not-send="true"
href="mailto:mehdi.amini@apple.com"
target="_blank">mehdi.amini@apple.com</a></span><br>
<span class="gmailmsg"><b>Cc:</b> <a
moz-do-not-send="true"
href="mailto:cfe-commits@lists.llvm.org"
target="_blank">
<a class="moz-txt-link-abbreviated" href="mailto:cfe-commits@lists.llvm.org">cfe-commits@lists.llvm.org</a></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"> <o:p></o:p></p>
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<p class="MsoNormal"
style="mso-margin-top-alt:auto;margin-bottom:12.0pt"> <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 moz-do-not-send="true"
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 moz-do-not-send="true"
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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<p class="MsoNormal"
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<br>
<a moz-do-not-send="true"
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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<pre class="moz-signature" cols="72">--
Best Regards,
Victor</pre>
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