<div dir="ltr">Hi folks,<div><br></div><div>some more information on this feature - as a reminder I started one month ago to work on an expensive check that would verify that pass return status is correctly reported by passes, i.e. no pass return « IR not modified » while actually modifying it.</div><div>It took ~20 pass fixes to achieve that goal, as many passes were not respectful of that contract, but as of 3667d87a33d3c8d4072a41fd84bb880c59347dc0, <a href="https://reviews.llvm.org/D80916" rel="noreferrer" target="_blank">https://reviews.<span class="gmail-il">llvm</span>.org/D80916</a> has been merged in master and the check is active, which should prevent further regression on that topic.</div><div><br></div><div>Thanks a lot to @foad, @jdoerfert, @fhahn, @calixte (and others I'm sorry to forgot) for their help during the reviews.</div><table class="gmail-phui-status-list-view"><tbody><tr class="gmail-"><td class="gmail-phui-status-item-target"><br></td><td class="gmail-phui-status-item-note"></td></tr></tbody></table></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Jun 12, 2020 at 11:24 PM Mehdi AMINI <<a href="mailto:joker.eph@gmail.com">joker.eph@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jun 11, 2020 at 8:42 AM Serge Guelton via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi folks,<br>
<br>
Per the documentation[0], whenever an LLVM pass doesn't modify the IR it's run on, it<br>
should return `false`--it's okay to return `true` if no change happen, just less<br>
optimal. In the New PM area, this is generally translated into a `PreservedAnalyses::all()`.<br>
<br>
<a href="https://reviews.llvm.org/D80916" rel="noreferrer" target="_blank">https://reviews.llvm.org/D80916</a> provides an `EXPENSIVE_CHECK` that computes a<br>
hash of the IR before and after the pass, and checks that any change is<br>
correctly reported. The hash is currently incomplete (on purpose, let's start<br>
small), but it turns out a dozen of passes do not satisfy that<br>
requirement.<br>
<br>
This could lead to various category of bugs (dangling references, inconsistent<br>
state, etc). This affects both New and Legacy PM, as passes tend to wrap functions<br>
that report their status.<br>
<br>
I wrote a bunch of patches for all failure detected by this check, as I cannot land the<br>
check now, it would break the buildbots :-) Any help to review the remaining<br>
ones [1] is appreciated.<br>
<br>
Once that check lands and we're relatively confident on the quality of the<br>
return status, some more optimizations could be triggered, like<br>
<a href="https://reviews.llvm.org/D80707" rel="noreferrer" target="_blank">https://reviews.llvm.org/D80707</a>.<br></blockquote><div><br></div><div>Awesome feature! I am really fond of these pieces of infrastructure that can defend against human mistakes and save countless hours of debugging when subtle issues arise.</div><div><br></div><div>Thanks Serge,</div><div><br></div><div>-- </div><div>Mehdi</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
<br>
[0] <a href="https://llvm.org/docs/WritingAnLLVMPass.html#the-runonmodule-method" rel="noreferrer" target="_blank">https://llvm.org/docs/WritingAnLLVMPass.html#the-runonmodule-method</a><br>
[1] <a href="https://reviews.llvm.org/D81230" rel="noreferrer" target="_blank">https://reviews.llvm.org/D81230</a><br>
<a href="https://reviews.llvm.org/D81236" rel="noreferrer" target="_blank">https://reviews.llvm.org/D81236</a><br>
<a href="https://reviews.llvm.org/D81256" rel="noreferrer" target="_blank">https://reviews.llvm.org/D81256</a><br>
<a href="https://reviews.llvm.org/D81238" rel="noreferrer" target="_blank">https://reviews.llvm.org/D81238</a><br>
<a href="https://reviews.llvm.org/D81225" rel="noreferrer" target="_blank">https://reviews.llvm.org/D81225</a><br>
<br>
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