[PATCH] D88342: [Docs][NewPM] Add note about required passes

[Arthur Eubanks via Phabricator via llvm-commits]

aeubanks created this revision.
aeubanks added reviewers: ychen, asbirlea.
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Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D88342

Files:
  llvm/docs/WritingAnLLVMNewPMPass.rst


Index: llvm/docs/WritingAnLLVMNewPMPass.rst
===================================================================
--- llvm/docs/WritingAnLLVMNewPMPass.rst
+++ llvm/docs/WritingAnLLVMNewPMPass.rst
@@ -207,3 +207,30 @@
 
   $ ninja -C build check-llvm
   # runs our new test alongside all other llvm lit tests
+
+FAQs
+====
+
+Required passes
+---------------
+
+If the pass is a required pass, meaning it must run on the IR to produce
+semantically valid output, add a static ``isRequired()`` method that returns
+true to the pass class.
+
+.. code-block:: c++
+
+  class HelloWorldPass : public PassInfoMixin<HelloWorldPass> {
+  public:
+    PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+
+    static bool isRequired() { return true; }
+  };
+
+This will make it run even in the presence of something that skips
+optimization passes, like the ``optnone`` function attribute.
+
+``AlwaysInlinerPass`` is an example of a required pass in-tree.
+
+For more info on how this works, see
+``PassInstrumentation::runBeforePass()``.


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