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<p>Hi Martin,</p>
I think this is another example of why we might consider having such
function level attributes.. yes.<br>
<br>
Chad<br>
<br>
<br>
<div class="moz-cite-prefix">On 4/4/2018 11:42 AM, Martin J.
O'Riordan via llvm-dev wrote:<br>
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<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">Sorry,
my reply “to all” left out LLVM-Dev<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="EN-US">From:</span></b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="EN-US"> Martin J. O'Riordan
[<a class="moz-txt-link-freetext" href="mailto:MartinO@theheart.ie">mailto:MartinO@theheart.ie</a>] <br>
<b>Sent:</b> 04 April 2018 16:41<br>
<b>To:</b> 'David Blaikie' <a class="moz-txt-link-rfc2396E" href="mailto:dblaikie@gmail.com"><dblaikie@gmail.com></a>;
'<a class="moz-txt-link-abbreviated" href="mailto:mcrosier@codeaurora.org">mcrosier@codeaurora.org</a>'
<a class="moz-txt-link-rfc2396E" href="mailto:mcrosier@codeaurora.org"><mcrosier@codeaurora.org></a>; 'Chandler Carruth'
<a class="moz-txt-link-rfc2396E" href="mailto:chandlerc@gmail.com"><chandlerc@gmail.com></a>; 'Eric Christopher'
<a class="moz-txt-link-rfc2396E" href="mailto:echristo@gmail.com"><echristo@gmail.com></a><br>
<b>Subject:</b> RE: [llvm-dev] [RFC] Adding function
attributes to represent codegen optimization level<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">Would
implementing GCC’s ‘</span><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">__attribute__((optimize(...)))</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
help?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">I
can’t find any good documentation for this attribute, but it
seems that GCC supports this in two forms:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">__attribute__((optimize(2)))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">meaning
optimise equivalent to ‘</span><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">-O2</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’,
and:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">__attribute_((optimize("fblah",
"O1")))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">meaning
optimise with the provided discrete options.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">
MartinO<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="EN-US">From:</span></b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="EN-US"> llvm-dev [<a
href="mailto:llvm-dev-bounces@lists.llvm.org"
moz-do-not-send="true">mailto:llvm-dev-bounces@lists.llvm.org</a>]
<b>On Behalf Of </b>David Blaikie via llvm-dev<br>
<b>Sent:</b> 04 April 2018 15:41<br>
<b>To:</b> <a href="mailto:mcrosier@codeaurora.org"
moz-do-not-send="true">mcrosier@codeaurora.org</a>;
Chandler Carruth <<a href="mailto:chandlerc@gmail.com"
moz-do-not-send="true">chandlerc@gmail.com</a>>; Eric
Christopher <<a href="mailto:echristo@gmail.com"
moz-do-not-send="true">echristo@gmail.com</a>><br>
<b>Cc:</b> <a href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [llvm-dev] [RFC] Adding function
attributes to represent codegen optimization level<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">On Tue, Apr 3, 2018 at 12:47 PM via
llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:<o:p></o:p></p>
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<p class="MsoNormal">All,<br>
A recent commit, D43040/r324557, changed the behavior of
the gold plugin<br>
when compiling with LTO. The change now causes the
codegen optimization<br>
level to default to CodeGenOpt::Default (i.e., -O2)
rather than use the<br>
LTO optimization level. The argument was made that the
LTO optimization<br>
level should control the amount of cross-module
optimizations done by<br>
LTO, but it should not control the codegen optimization
level; that<br>
should be based off of the optimization level used
during the initial<br>
compilation phase (i.e., bitcode generation).<o:p></o:p></p>
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<p class="MsoNormal"><br>
At least as I recall the discussion around
optnone/optsize in the pass was that these were in some
way special semantically distinct properties (optnone
being "good for debugging" (or good for debugging
compilers - what's the baseline behavior before
optimizations are applied), optsize being "make this fit
into something it wouldn't otherwise fit into") but that
the gradiations of -ON didn't fit into this kind of
model & wouldn't ever be implemented as function
attributes.<br>
<br>
CC'd Chandler & Eric who I think had opinions/were
involved in those previous discussions.<br>
<o:p></o:p></p>
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<p class="MsoNormal"><br>
Assuming the argument is reasonable (it make sense to
me), I was hoping<br>
to solicit feedback on how to proceed. The suggestion
in D43040/r324557<br>
was to add function attributes to represent the
compile-time<br>
optimization level (which also seems reasonable to me).<br>
<br>
As a first step, I've put together two patches: 1) an
llvm patch that<br>
adds the function attributes to the LLVM IR and 2) a
clang patch that<br>
attaches these attributes to each function based on the
codegen<br>
optimization level. I then use the function level
attributes to<br>
"reconstruct" to codegen optimization level used with
LTO.<br>
<br>
Please understand this is very much a WIP and just a
very small step<br>
towards a final solution.<br>
<br>
Here are the patches for reference:<br>
Clang: D45226<br>
LLVM: D45225<br>
<br>
Regards,<br>
Chad<br>
<br>
<br>
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