Document what backwards compatibility we provide for bitcode.

llvm-svn: 213813

diff --git a/llvm/docs/DeveloperPolicy.rst b/llvm/docs/DeveloperPolicy.rst
index 74a8979..508a04f 100644 (file)
--- a/llvm/docs/DeveloperPolicy.rst
+++ b/llvm/docs/DeveloperPolicy.rst
@@ -436,6 +436,29 @@ list, development list, or LLVM bug tracker component. If someone sends you
 a patch privately, encourage them to submit it to the appropriate list first.
 
 
+IR Backwards Compatibility
+--------------------------
+
+When the IR format has to be changed, keep in mind that we try to maintain some
+backwards compatibility. The rules are intended as a balance between convenience
+for llvm users and not imposing a big burden on llvm developers:
+
+* The textual format is not backwards compatible. We don't change it too often,
+  but there are no specific promises.
+
+* The bitcode format produced by a X.Y release will be readable by all following
+  X.Z releases and the (X+1).0 release.
+
+* Newer releases can ignore features from older releases, but they cannot
+  miscompile them. For example, if nsw is ever replaced with something else,
+  dropping it would be a valid way to upgrade the IR.
+
+* Debug metadata is special in that it is currently dropped during upgrades.
+
+* Non-debug metadata is defined to be safe to drop, so a valid way to upgrade
+  it is to drop it. That is not very user friendly and a bit more effort is
+  expected, but no promises are made.
+
 .. _copyright-license-patents:
 
 Copyright, License, and Patents