<html><body><div style="font-size: 12pt; font-family: arial, helvetica, sans-serif; color: rgb(0, 0, 0);"><div>> Would a patch to fix the tail merging
<div>> issue be of interest? As composed now I have it not allowing merging</div>
<div>? where a pre- or post-symbol differs but I can also imagine enhancements</div>
<div>> to update uses of the symbol.</div>
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<div>Indeed having the symbol uses updated would be appreciated. In our downstream side I've also did preventing tail duplication, but that did have very negative effect in multiple scores.</div>
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<blockquote style="margin: 0 0 0 .8em; border-left: 1px #ccc solid; padding-left: 1em;"><hr id="MESSAGE_DATA_MARKER"><strong>From: </strong>Reid <llvm-dev@lists.llvm.org><br><strong>To: </strong>David <dag@obbligato.org><br><strong>Cc: </strong>llvm-dev <llvm-dev@lists.llvm.org><br><strong>Date: </strong>Thursday, 8 July 2021 4:55 AM CEST<br><strong>Subject: </strong>Re: [llvm-dev] Address of instruction in codegen<br><br>
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<div class="gmail_attr" dir="ltr">On Wed, Jul 7, 2021 at 7:40 PM David Greene <<a href="mailto:dag@obbligato.org" target="_blank" rel="noopener noreferrer">dag@obbligato.org</a>> wrote:</div>
<blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left: 1px solid #cccccc; padding-left: 1ex;">Reid Kleckner <<a href="mailto:rnk@google.com" target="_blank" rel="noopener noreferrer">rnk@google.com</a>> writes:<br><br>> A word of caution: depending on where you are in codegen, there are late<br>> passes that can delete or duplicate MachineInstrs (tail duplication comes<br>> to mind). You need to be reasonably confident that the annotated<br>> instruction won't be affected by such passes, or you will get errors from<br>> the assembler about an undefined label or duplicate label definitions.<br><br>I actually ran into that. :) Would a patch to fix the tail merging<br>issue be of interest? As composed now I have it not allowing merging<br>where a pre- or post-symbol differs but I can also imagine enhancements<br>to update uses of the symbol.</blockquote>
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<div>You know, maybe it would be OK. Initially I was going to say, one of the existing applications is debug info to mark heap allocation call sites, and we don't want debug info to affect codegen. However, if such a call site were to be duplicated, we'd end up with assembler errors from MC, so it sort of already doesn't work. Maybe there is a better way to handle this application.</div>
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<div>The other applications are marking setjmp calls for control flow guard and something in speculative load hardening, both security features where powering down control flow optimizations would be fine.</div>
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