Fix compile with GCC 6

[dim]

Summary

Compiling with GCC 6 fails with "error: constructor required before non-static data member for ‘llvm::ValueEnumerator::MDRange::First’ has been parsed". The patch Replace MDRange's declaration of default constructor with a definition.

[rivanvx]

I'm using

$ g++ --version
g++ (GCC) 6.0.0 20160331 (Red Hat 6.0.0-0.19)
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

and

$ g++ --version
g++ (Debian 6-20160313-1) 6.0.0 20160313 (experimental) [trunk revision 234167]
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Both fail.

GCC 4.9 on Debian, 5 on Fedora 23 work fine. Can't really judge if this is a compiler bug or not, my knowledge of arcane C++11 wizardry is apprentice-level.

(Edit: correct information about GCC 6 on Debian.)

[dexonsmith]

Thanks for tracking down a fix. LGTM.

[rivanvx]

Is GCC 6 being stricter than older versions? Is this behavior standard-compliant?

[rivanvx]

GCC 6.1 (GCC 6 final release in GCC's funky release numbering scheme) does not have this issue anymore: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70528

[dexonsmith]

Thanks for the update!

[dim]

Actually, the gcc maintainers reverted this change again, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70528 and https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=235171

So can we please apply this anyway? It is a minor change for such a fix.

[dim]

Note, this problem also occurs with gcc release versions 5.4.0 and 6.2.0.

[rivanvx]

Fine with me.

llvm/trunk/lib/Bitcode/Writer/ValueEnumerator.h

private:
   /// Range of metadata IDs, as a half-open range.
   struct MDRange {
     unsigned First = 0;
     unsigned Last = 0;
 
     /// Number of strings in the prefix of the metadata range.
     unsigned NumStrings = 0;
 
-    MDRange() = default;
+    MDRange() {}
     explicit MDRange(unsigned First) : First(First) {}
   };
   SmallDenseMap<unsigned, MDRange, 1> FunctionMDInfo;
 
   bool ShouldPreserveUseListOrder;
 
   typedef DenseMap<AttributeSet, unsigned> AttributeGroupMapType;
   AttributeGroupMapType AttributeGroupMap;