[PATCH] D27546: [ASTReader] Sort RawComments before merging

[Bruno Cardoso Lopes via Phabricator via cfe-commits]

bruno created this revision.
bruno added reviewers: manmanren, akyrtzi, rsmith.
bruno added a subscriber: cfe-commits.

`RawComments` are sorted by comparing underlying `SourceLocation`'s. This is done by comparing `FileID` and `Offset`; when the `FileID` is the same it means
the locations are within the same TU and the `Offset` is used, etc.

FileID, from the source code: "A mostly-opaque identifier, where 0 is "invalid", >0 is this module, and <-1 is something loaded from another
module.". That said, when de-serializing SourceLocations, FileID's from RawComments loaded from other modules get negative IDs where previously they
were positive. This makes imported RawComments unsorted, leading to a wrong merge with other comments from the current TU. Fix that by sorting RawComments
properly after de-serialization and before merge.

This fixes an assertion in `ASTContext::getRawCommentForDeclNoCache`, which fires only in a debug build of clang. There's seems to be no reliable way to test this.
Additionally, I tried to use `llvm::array_pod_sort`, but that didn't seem to cope well with `BeforeThanCompare<RawComment>(SourceMgr)` or lambdas, and a `SourceMgr` is needed to perform the sort.


https://reviews.llvm.org/D27546

Files:
  lib/Serialization/ASTReader.cpp


Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -8481,6 +8481,10 @@
       }
     }
   NextCursor:
+    // De-serialized SourceLocations get negative FileIDs for other modules,
+    // potentially invalidating the original order. Sort it again.
+    std::sort(Comments.begin(), Comments.end(),
+              BeforeThanCompare<RawComment>(SourceMgr));
     Context.Comments.addDeserializedComments(Comments);
   }
 }


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