[ThinLTO/LTO] Don't link in unneeded metadata

Summary:
Third patch split out from http://reviews.llvm.org/D14752.

Only map in needed DISubroutine metadata (imported or otherwise linked
in functions and other DISubroutine referenced by inlined instructions).
This is supported for ThinLTO, LTO and llvm-link --only-needed, with
associated tests for each one.

Depends on D14838.

Reviewers: dexonsmith, joker.eph

Subscribers: davidxl, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D14843

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256003 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 0 deletions

diff --git a/include/llvm/Transforms/Utils/ValueMapper.h b/include/llvm/Transforms/Utils/ValueMapper.h
index 7b1382854ba..469022f34c5 100644
--- a/include/llvm/Transforms/Utils/ValueMapper.h
+++ b/include/llvm/Transforms/Utils/ValueMapper.h
@@ -61,6 +61,11 @@ namespace llvm {
     virtual Metadata *mapTemporaryMetadata(Metadata *MD) { return nullptr; }
     virtual void replaceTemporaryMetadata(const Metadata *OrigMD,
                                           Metadata *NewMD) {}
+
+    /// The client should implement this method if some metadata need
+    /// not be mapped, for example DISubprogram metadata for functions not
+    /// linked into the destination module.
+    virtual bool isMetadataNeeded(Metadata *MD) { return true; }
   };
 
   /// RemapFlags - These are flags that the value mapping APIs allow.