[llvm] da036b4 - [Docs][NewPM] Add note about required passes

[Arthur Eubanks via llvm-commits]

Author: Arthur Eubanks
Date: 2020-09-28T21:45:14-07:00
New Revision: da036b4514702f3a7c1d2981ff11b3067bad4329

URL: https://github.com/llvm/llvm-project/commit/da036b4514702f3a7c1d2981ff11b3067bad4329
DIFF: https://github.com/llvm/llvm-project/commit/da036b4514702f3a7c1d2981ff11b3067bad4329.diff

LOG: [Docs][NewPM] Add note about required passes

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D88342

Added: 
    

Modified: 
    llvm/docs/WritingAnLLVMNewPMPass.rst

Removed: 
    


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diff  --git a/llvm/docs/WritingAnLLVMNewPMPass.rst b/llvm/docs/WritingAnLLVMNewPMPass.rst
index a876ec4ceb00..f992c850c193 100644
--- a/llvm/docs/WritingAnLLVMNewPMPass.rst
+++ b/llvm/docs/WritingAnLLVMNewPMPass.rst
@@ -207,3 +207,32 @@ test at ``llvm/test/Transforms/HelloNew/helloworld.ll``. See
 
   $ ninja -C build check-llvm
   # runs our new test alongside all other llvm lit tests
+
+FAQs
+====
+
+Required passes
+---------------
+
+A pass that defines a static ``isRequired()`` method that returns true is a required pass. For example:
+
+.. code-block:: c++
+
+  class HelloWorldPass : public PassInfoMixin<HelloWorldPass> {
+  public:
+    PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+
+    static bool isRequired() { return true; }
+  };
+
+A required pass is a pass that may not be skipped. An example of a required
+pass is ``AlwaysInlinerPass``, which must always be run to preserve
+``alwaysinline`` semantics. Pass managers are required since they may contain
+other required passes.
+
+An example of how a pass can be skipped is the ``optnone`` function
+attribute, which specifies that optimizations should not be run on the
+function. Required passes will still be run on ``optnone`` functions.
+
+For more implementation details, see
+``PassInstrumentation::runBeforePass()``.