[PATCH] D120666: [docs] Add note about interaction between clang plugins and -clear-ast-before-backend

[Arthur Eubanks via Phabricator via cfe-commits]

aeubanks updated this revision to Diff 411851.
aeubanks added a comment.

reword


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D120666/new/

https://reviews.llvm.org/D120666

Files:
  clang/docs/ClangPlugins.rst


Index: clang/docs/ClangPlugins.rst
===================================================================
--- clang/docs/ClangPlugins.rst
+++ clang/docs/ClangPlugins.rst
@@ -200,3 +200,17 @@
   PluginASTAction::ActionType getActionType() override {
     return AddAfterMainAction;
   }
+
+Interaction with ``-clear-ast-before-backend``
+----------------------------------------------
+
+To reduce peak memory usage of the compiler, plugins are recommended to run
+*before* the main action, which is usually code generation. This is because
+having any plugins that run after the codegen action automatically turns off
+``-clear-ast-before-backend``.  ``-clear-ast-before-backend`` reduces peak
+memory by clearing the Clang AST after generating IR and before running IR
+optimizations. Use ``CmdlineBeforeMainAction`` or ``AddBeforeMainAction`` as
+``getActionType`` to run plugins while still benefitting from
+``-clear-ast-before-backend``. Plugins must make sure not to modify the AST,
+otherwise they should run after the main action.
+


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