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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">FWIW, since I am not really much of a contributor to LLVM.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">The points Chandler makes for why passes must be allowed to create dead code are excellent. Dead code elimination is a general compiler cleanup<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">pass, do be run once in a while to get rid of code that for whatever reasons is dead. Every pass should not have to police themselves not to produce dead code.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">This strategy (of having independent passes to do well-defined specific things) is pretty much (it seems to me) one of the founding principles of LLVM design,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">and one of its very key strengths. Dead code elimination should not be built into all other passes.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">Hal gets to the heart of the matter (in my opinion) when he writes:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Wingdings"><span style="mso-list:Ignore">Ø<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]> 2. Should unreachable code be allowed to contain nonsense (like instructions that depend on themselves)? To this, I believe the answer is no. We currently permit this, and I think that a lot of the bugs regarding unreachable
code some from this. I've yet to hear a good argument why, for example, JumpThreading must produce self-referential instructions in trivially-dead regions.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">To me this looks like an example of incorrect SSA IR. SSA form is not supposed to allow for the case of a use of a def where the def cannot reach. And my understanding
was that “undef” LLVM IR was created exactly for specifying cases where a def was required for proper SSA form, but where none was available.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">Clearly ( to me at least), an instruction cannot use in its inputs a def that occurs later in the same basic-block (and that includes the def the instruction
itself defines).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">That would either need to be an undef, or a phi that merges an undef and the def that occurs later.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">Years ago (late 90s, early 2000s) the Intel compiler had quite a few problems with SSA form that could be traced to somewhat undisciplined handling of<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">“undefined” defs, PHI removal, and this type of degenerate SSA. We did as is being suggested currently for LLVM, and made such IR illegal, and changed<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">our internal verifiers to catch those. After a short time, this shook loose some remaining bugs with mishandling of SSA form, and improved compiler stability<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">in this regard. I think that is an excellent direction for LLVM infrastructure to go in.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">Kevin B. Smith<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D">Intel Compiler Team
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<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""> llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>Chandler Carruth<br>
<b>Sent:</b> Monday, February 23, 2015 11:05 PM<br>
<b>To:</b> Hal Finkel<br>
<b>Cc:</b> Katya Romanova; LLVM Developers Mailing List; Rafael Espindola<br>
<b>Subject:</b> Re: [LLVMdev] Jump Theading/GVN bug - moving discussion to llvm-dev<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Will try to reply to the larger thread again soon, but a quick reply on a bit of a tangent...<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Feb 23, 2015 at 8:45 PM, Hal Finkel <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"> 2. Should unreachable code be allowed to contain nonsense (like instructions that depend on themselves)? To this, I believe the answer is no. We currently permit this, and I think that a lot of the bugs regarding unreachable code some
from this. I've yet to hear a good argument why, for example, JumpThreading must produce self-referential instructions in trivially-dead regions.<o:p></o:p></p>
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<p class="MsoNormal"><br>
I could easily see tightening the rules on what passes the verifier within unreachable code to try to make it substantially cheaper, easier, and less error prone to handle unreachable code. It seems likely that there are good pragmatic compromises here that
would actually cost nothing. Obvious example is, as you highlight, self-referential non-phi instructions could easily just be rejected, and asserted during construction by the IR builder.<o:p></o:p></p>
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