[llvm-dev] Changes to the Developer Policy / IR Backwards Compatibility

[Moritz Angermann via llvm-dev]

Hi,

I’ve noted some change in wording on the current (4.0) developer policy[1],
compared to the 3.9 developer policy[2]. Specifically the part about IR
Backwards Compatibility.

The change from 3.9 to 4.0 is the following:

  - The textual format is not backwards compatible. We don’t change it too often, but there are no specific promises.
  - Additions and changes to the IR should be reflected in test/Bitcode/compatibility.ll.
- - The bitcode format produced by a X.Y release will be readable by all following X.Z releases and the (X+1).0 release.
+ - The current LLVM version supports loading any bitcode since version 3.0.
  - After each X.Y release, compatibility.ll must be copied to compatibility-X.Y.ll. The corresponding bitcode file should be assembled using the X.Y build and committed as compatibility-X.Y.ll.bc.
  - 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.

This raises the following question for me: does this imply that bitcode generated by 4.X won’t necessarily be as
stable as it was for the 3.X series, specifically that the backwards compatability guarantee will be scraped in
LLVM 4.0+?

Cheers,
 Moritz

--
[1]: http://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility
[2]: http://llvm.org/releases/3.9.0/docs/DeveloperPolicy.html#ir-backwards-compatibility