Merge pull request #1840 from DARMA-tasking/1839-ub-left-shifting-neg…

…ative-signed-values

#1839: use unsigned types for left shifts in BitPacker

src/vt/utils/bits/bits_packer.h

@@ -46,9 +46,27 @@
 
 #include <cassert>
 #include <cstdlib>
+#include <cstring>
 
 namespace vt { namespace utils {
 
+// Non-constexpr C++14 version of bit_cast
+// See: https://en.cppreference.com/w/cpp/numeric/bit_cast
+template<typename Dst, typename Src>
+std::enable_if_t<
+  sizeof(Dst) == sizeof(Src)
+  && std::is_trivially_copyable<Dst>::value
+  && std::is_trivially_copyable<Src>::value
+  && std::is_trivially_constructible<Dst>::value,
+  Dst
+>
+bit_cast(const Src &src) {
+  Dst dst;
+  std::memcpy(&dst, &src, sizeof(Dst));
+  return dst;
+}
+
+
 struct BitPacker {
   using FieldType         = int64_t;
   using FieldUnsignedType = uint64_t;

src/vt/utils/bits/bits_packer.impl.h

@@ -115,9 +115,16 @@ template <typename BitType, typename BitField>
 BitPacker::setFieldDynamic(
   FieldType start, FieldType len, BitField& field, BitType const& segment
 ) {
+  // make_unsigned UB before C++20 for non-integral or bool types (ill-formed after C++20)
+  static_assert(std::is_integral<BitField>::value && !std::is_same<BitField, bool>::value,
+                "BitField must be non-bool integral type");
+  // sizeof(UnsignedBitField) is guaranteed to be the same as sizeof(BitField)
+  using UnsignedBitField = std::make_unsigned_t<BitField>;
   auto const nbits = (sizeof(BitField) * 8) - len;
   field = field & ~(gen_bit_mask(len) << start);
-  field = field | (((static_cast<BitField>(segment) << nbits) >> nbits) << start);
+  auto seg = bit_cast<UnsignedBitField>(static_cast<BitField>(segment));
+  UnsignedBitField bits = ((seg << nbits) >> nbits) << start;
+  field = field | bit_cast<BitField>( bits );
 }
 
 template <FieldType start, FieldType len, typename BitField>