<div dir="ltr">Yeah - Jim and I were kicking around ideas for how it could be done this morning, and came to much the same conclusion.<div><br></div><div>I've got some bugs to clear out of my queue first, but when I get back to this I'll send out a proposal.</div>
<div><br></div><div>- Lang.</div><div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Thu, May 8, 2014 at 11:58 AM, Kaylor, Andrew <span dir="ltr"><<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Tiering of optimization levels is obviously a valid use case. I think it would be possible to work support for that back into MCJIT, but with multiple module
support relinking would be a bit of a nightmare. Off the cuff, I’d say you’d need some kind of stub or offset table system to handle it.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Personally, I really like the way that WebKit handles that by not letting MCJIT control things it doesn’t understand.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">-Andy<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
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<div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0in 0in 0in">
<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""> Jim Grosbach [mailto:<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>]
<br>
<b>Sent:</b> Wednesday, May 07, 2014 5:38 PM<br>
<b>To:</b> Kaylor, Andrew<br>
<b>Cc:</b> Lang Hames; <a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank">llvm-commits@cs.uiuc.edu</a></span></p><div><div class="h5"><br>
<b>Subject:</b> Re: [llvm] r208257 - [RuntimeDyld] Make RuntimeDyldImpl::resolveExternalSymbols preserve the<u></u><u></u></div></div><p></p>
</div>
</div><div><div class="h5">
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">FWIW, support for recompilation of modules was a conscious intention of the design originally in order to support tiering up levels of optimization. The implementation since then has obviously diverged from that a bit, but it’s still a
relevant design goal.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">You make a good point on the section permissions. Supporting recompilation is more than just relocation handling and would require nontrivial juggling in a memory manager to do effectively.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">-Jim<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<div>
<p class="MsoNormal">On May 7, 2014, at 4:43 PM, Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>> wrote:<u></u><u></u></p>
</div>
<p class="MsoNormal"><br>
<br>
<u></u><u></u></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I’m not sure I’d agree that MCJIT is supposed to support recompilation of a module. I believe we’ve explicitly said that you can’t modify a module after it
has been added to MCJIT, so why would you recompile it? In any event, if you do recompile a module, the relocation information will be recreated. </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">But I take it that you’re actually addressing some case where a module is moved after it has been compiled and put into an executable state. I don’t understand
this case either. Why would you move a module after it has been made executable?</span><u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span><u></u><u></u></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Also, I would expect that you’ll run into problems with memory managers that set the permissions for code sections to RX after a module has been finalized.
In those cases, if you try to reapply relocations (presumably to a module that didn’t move) it will cause a protection fault. Conversely, if you allow a module to be moved after it has been finalized, don’t you run into issues with the client having to keep
track of whether or not the module is in an executable state?</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">-Andy</span><u></u><u></u></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span><u></u><u></u></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> </span></span><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Lang
Hames [<a href="mailto:lhames@gmail.com" target="_blank">mailto:lhames@gmail.com</a>]<span> </span><br>
<b>Sent:</b><span> </span>Wednesday, May 07, 2014 4:09 PM<br>
<b>To:</b><span> </span>Kaylor, Andrew<br>
<b>Cc:</b><span> </span><a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank">llvm-commits@cs.uiuc.edu</a><br>
<b>Subject:</b><span> </span>Re: [llvm] r208257 - [RuntimeDyld] Make RuntimeDyldImpl::resolveExternalSymbols preserve the</span><u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
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<div>
<div>
<p class="MsoNormal">Hi Andy,<u></u><u></u></p>
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<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
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<div>
<p class="MsoNormal">MCJIT is supposed to support recompilation of a module, and linking multiple modules (thanks for making the latter work! :). So we need to keep external relocations around in case the module they point to is recompiled.<u></u><u></u></p>
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<div>
<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal">Cheers,<u></u><u></u></p>
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<div>
<p class="MsoNormal">Lang.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"> <u></u><u></u></p>
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<div>
<p class="MsoNormal">On Wed, May 7, 2014 at 3:56 PM, Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank"><span style="color:purple">andrew.kaylor@intel.com</span></a>> wrote:<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">The discarding of processed relocations was an intentional choice. The relocation map can take up a pretty fair amount of memory, particularly in the case where you have a large number of modules that reference one another. Keeping the
relocation map around will cause memory bloat issues for some clients.<br>
<br>
My thinking was that once a module was finalized you can't remap its sections anymore. Do you have a use case where this is an unacceptable limitation?<br>
<br>
-Andy<u></u><u></u></p>
</div>
<div>
<div>
<p class="MsoNormal"><br>
-----Original Message-----<br>
From:<span> </span><a href="mailto:llvm-commits-bounces@cs.uiuc.edu" target="_blank"><span style="color:purple">llvm-commits-bounces@cs.uiuc.edu</span></a><span> </span>[mailto:<a href="mailto:llvm-commits-bounces@cs.uiuc.edu" target="_blank"><span style="color:purple">llvm-commits-bounces@cs.uiuc.edu</span></a>]
On Behalf Of Lang Hames<br>
Sent: Wednesday, May 07, 2014 3:34 PM<br>
To:<span> </span><a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank"><span style="color:purple">llvm-commits@cs.uiuc.edu</span></a><br>
Subject: [llvm] r208257 - [RuntimeDyld] Make RuntimeDyldImpl::resolveExternalSymbols preserve the<br>
<br>
Author: lhames<br>
Date: Wed May 7 17:34:08 2014<br>
New Revision: 208257<br>
<br>
URL:<span> </span><a href="http://llvm.org/viewvc/llvm-project?rev=208257&view=rev" target="_blank"><span style="color:purple">http://llvm.org/viewvc/llvm-project?rev=208257&view=rev</span></a><br>
Log:<br>
[RuntimeDyld] Make RuntimeDyldImpl::resolveExternalSymbols preserve the relocation entries it applies.<br>
<br>
Prior to this patch, RuntimeDyldImpl::resolveExternalSymbols discarded relocations for external symbols once they had been applied. This causes issues if the client calls MCJIT::finalizeLoadedModules more than once, and updates the location of any symbols in
between (e.g. by calling MCJIT::mapSectionAddress).<br>
<br>
No test case yet: None of our in-tree memory managers support moving sections around. I'll have to hack up a dummy memory manager before I can write a unit test.<br>
<br>
Fixes <<a>rdar://problem/16764378</a>><br>
<br>
<br>
Modified:<br>
llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp<br>
<br>
Modified: llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp<br>
URL:<span> </span><a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp?rev=208257&r1=208256&r2=208257&view=diff" target="_blank"><span style="color:purple">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp?rev=208257&r1=208256&r2=208257&view=diff</span></a><br>
==============================================================================<br>
--- llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp (original)<br>
+++ llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp Wed May<u></u><u></u></p>
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<div>
<p class="MsoNormal">+++ 7 17:34:08 2014<u></u><u></u></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">@@ -620,6 +620,8 @@ void RuntimeDyldImpl::resolveRelocationL<br>
}<br>
<br>
void RuntimeDyldImpl::resolveExternalSymbols() {<br>
+ StringMap<RelocationList> ProcessedSymbols;<br>
+<br>
while (!ExternalSymbolRelocations.empty()) {<br>
StringMap<RelocationList>::iterator i = ExternalSymbolRelocations.begin();<br>
<br>
@@ -665,8 +667,20 @@ void RuntimeDyldImpl::resolveExternalSym<br>
resolveRelocationList(Relocs, Addr);<br>
}<br>
<br>
+ ProcessedSymbols[i->first()] = i->second;<br>
ExternalSymbolRelocations.erase(i);<br>
}<br>
+<br>
+ // Restore the relocation entries that were consumed in the loop above:<br>
+ //<br>
+ // FIXME: Replace the following loop with:<br>
+ // std::swap(ProcessedSymbols, ExternalSymbolRelocations)<br>
+ // once StringMap has copy and move construction.<br>
+ for (StringMap<RelocationList>::iterator I = ProcessedSymbols.begin(),<br>
+ E = ProcessedSymbols.end();<br>
+ I != E; ++I) {<br>
+ ExternalSymbolRelocations[I->first()] = I->second; }<br>
}<u></u><u></u></p>
</div>
<div>
<div>
<p class="MsoNormal"> //===----------------------------------------------------------------------===//<br>
<br>
<br>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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</div></div></div>
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</blockquote></div><br></div>