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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Please see
</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">inline.</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p></o:p></span></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:11.0pt;font-family:"Calibri",sans-serif;color:windowtext">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"> Philip Reames [mailto:listmail@philipreames.com]
<br>
<b>Sent:</b> Tuesday, July 14, 2015 10:35 PM<br>
<b>To:</b> Andrew Trick; Swaroop Sridhar<br>
<b>Cc:</b> Juergen Ributzka; LLVM Dev; Lang Hames; Phil Pizlo; Sanjoy Das; Russell Hadley<br>
<b>Subject:</b> Re: [RFC] New StackMap format proposal (StackMap v2)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">[…]</span><o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
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<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
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<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l3 level1 lfo2"><![if !supportLists]><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Offset of the start of the call instruction if the call instruction occurs within a code-range that allows full interruption (that is, all instructions in
a range are considered safe-points)</span><o:p></o:p></p>
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</blockquote>
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<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">If the JIT requires knowledge of call's encoding, it should probably be emitting the call instruction itself within the patchable space reserved by LLVM. <o:p></o:p></p>
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<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-right:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal">Note that the patchpoint may include a mov in addition to call, so the patchpoint address is not the same as the call address.<o:p></o:p></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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</blockquote>
<p class="MsoNormal">Just to be clear, the problem with (2) has nothing to do with StackMaps. It has to do with the fact our entire backend confuses pointers and non-pointers. To have option (2), I believe we'd need to effectively have a stackmap for every
instruction. That's a lot more work than anyone has serious proposed to date. :)<br>
<br>
<span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">Yes. Fully interruptible code will effectively need a StackMap at every instruction.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">Many compilers targeting CLR runtime generate fully-interruptible code ranges for performance critical code (ex: hot loops where it is very expensive to place a safepoint-poll
to yield control to the GC). CLR GCTables use certain compact representation techniques so that the image size stays small.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">Anyway, I did not mean to suggest that LLVM should support for fully-interruptible code – at least not in the short term.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">[…]<o:p></o:p></span></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Therefore, it is helpful to have Call-Site size specified in
</span><span style="font-family:"Courier New",serif">StackMapRecord</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I agree with Andy, that the call-site size should include all bytes between the start of the call instruction and the start of the next instruction.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I suggested this because we want to support a dynamic callback that determines the patchpoint size (given the set of register arguments), and it would be nice to record that decision within the stack map. This is important information for
any code that is responsible for patching because it must patch over all reserved bytes.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The alternative would be for LLVM to emit the call at the end and record just the size of the call instruction encoding. That seems like a silly, x86-specific waste of stackmap space though. The JIT can either do the encoding and keep track
of the info, or a small nop+move_immediate decoder can figure it out for all reasonable cases.<o:p></o:p></p>
</div>
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</blockquote>
<p class="MsoNormal">I would argue in favor of recording the number of bytes recorded for patching. I think that gives us everything we need in practice. Any non-destructive or concurrent patch mechanism is going to need extremely fine grained control anyways.
Trying to do that partly within LLVM and partly without just seems like a mistake.
<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">To clarify my understanding of your proposal,
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">If we have a statepoint intrinsic, whose call instruction encodes to 5 bytes. Will the call-site size recorded in the StackMap be:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo5"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red"><span style="mso-list:Ignore">(a)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">5 bytes, if the num-patch-bytes argument to the statepoint is 0<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo5"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red"><span style="mso-list:Ignore">(b)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:red">10 bytes, if the num-patch-bytes argument to the statepoint is 10?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Swaroop.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></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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