[docs] Add a note on iteration of unordered containers to coding standards

Summary: Beware of non-determinism due to ordering of pointers

Reviewers: dblaikie, dexonsmith

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37525

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312667 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/CodingStandards.rst b/docs/CodingStandards.rst
index c4e25e4..0de2fc6 100644 (file)
--- a/docs/CodingStandards.rst
+++ b/docs/CodingStandards.rst
@@ -811,6 +811,21 @@ As a rule of thumb, use ``auto &`` unless you need to copy the result, and use
   for (const auto *Ptr : Container) { observe(*Ptr); }
   for (auto *Ptr : Container) { Ptr->change(); }
 
+Beware of non-determinism due to ordering of pointers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In general, there is no relative ordering among pointers. As a result,
+when unordered containers like sets and maps are used with pointer keys
+the iteration order is undefined. Hence, iterating such containers may
+result in non-deterministic code generation. While the generated code
+might not necessarily be "wrong code", this non-determinism might result
+in unexpected runtime crashes or simply hard to reproduce bugs on the
+customer side making it harder to debug and fix.
+
+As a rule of thumb, in case an ordered result is expected, remember to
+sort an unordered container before iteration. Or use ordered containers
+like vector/MapVector/SetVector if you want to iterate pointer keys.
+
 Style Issues
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