[llvm] [DeveloperPolicy] Add guidelines for adding/enabling passes (PR #158591)

[Nikita Popov via llvm-commits]

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@@ -1185,6 +1185,50 @@ Suggested disclaimer for the project README and the main project web page:
    necessarily a reflection of the completeness or stability of the code, it
    does indicate that the project is not yet endorsed as a component of LLVM.
 
+Adding or enabling a new LLVM pass
+----------------------------------
+
+The guidelines here are primarily targeted at the enablement of new major
+passes in the target-independent optimization pipeline. Small additions, or
+backend-specific passes, require a lesser degree of care.
+
+When adding a new pass, the goal should be to enable it as part of the default
+optimization pipeline as early as possible and then continue development
+incrementally. The recommended workflow is:
+
+1. Implement a basic version of the pass and add it to the pass pipeline behind
+   a flag that is disabled by default.
+2. Enable the pass by default. Separating this step allows easily disabling the
+   pass if issues are encountered, without having to revert the entire
+   implementation.
----------------
nikic wrote:

I've added an extra sentence "The initial version should focus on handling simple cases correctly and efficiently" here. Is that clearer? Happy to take suggestions on how to improve the wording here.

https://github.com/llvm/llvm-project/pull/158591