[bolt] Fix MSVC builds

We need to explicitly mark DWARFUnitInfo as non-copyable since MSVC's
STL has a `noexcept(false)` move constructor for `unordered_map`; see
the added comment for more details.

An alternative might be using SmallVector instead of std::vector, since
that never tries to copy elements [1]. That would result in a bunch of
API changes though, so I figured a smaller targeted fix was better.

[1] https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h

Reviewed By: ayermolo, maksfb

Differential Revision: https://reviews.llvm.org/D154924

1 files changed, 14 insertions, 0 deletions

diff --git a/bolt/include/bolt/Core/DIEBuilder.h b/bolt/include/bolt/Core/DIEBuilder.h
index e2da876..2eac4ce 100644
--- a/bolt/include/bolt/Core/DIEBuilder.h
+++ b/bolt/include/bolt/Core/DIEBuilder.h
@@ -58,6 +58,20 @@ public:
     // A map of DIE offsets in original DWARF section to DIE ID.
     // Whih is used to access DieInfoVector.
     std::unordered_map<uint64_t, uint32_t> DIEIDMap;
+
+    // Some STL implementations don't have a noexcept move constructor for
+    // unordered_map (e.g. https://github.com/microsoft/STL/issues/165 explains
+    // why the Microsoft STL doesn't). In that case, the default move
+    // constructor generated for DWARFUnitInfo isn't noexcept either, and thus
+    // resizing a vector of DWARFUnitInfo will copy elements instead of moving
+    // them (https://en.cppreference.com/w/cpp/utility/move_if_noexcept).
+    // DWARFUnitInfo isn't copyable though, since the DieInfoVector member is a
+    // vector of unique_ptrs and unique_ptr isn't copyable, so using a vector of
+    // DWARFUnitInfo causes build errors. Explicitly marking DWARFUnitInfo as
+    // non-copyable forces vector resizes to move instead and fixes the issue.
+    DWARFUnitInfo() = default;
+    DWARFUnitInfo(const DWARFUnitInfo &) = delete;
+    DWARFUnitInfo(DWARFUnitInfo &&) = default;
   };
 
   enum class ProcessingType { DWARF4TUs, DWARF5TUs, CUs };