Commit 6c4dfb8a authored by Lucian Grijincu's avatar Lucian Grijincu Committed by Facebook Github Bot 4

folly: ubsan: cast to unsigned to avoid negative-left-shift

Summary:
The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
  bits are filled with zeros.

  If E1 has a signed type and non-negative value, and E1 × 2E2 is
  representable in the result type, then that is the resulting value;
  otherwise, the behavior is undefined.

The code assumed the signed left shift worked like an unsigned left
shift, so make that explicit.

Reviewed By: meyering

Differential Revision: D3280325

fbshipit-source-id: 178b95ee36b7a1126a79bb825c2ad2ffa6fa9464
parent c03fd9b7
...@@ -71,7 +71,8 @@ uint64_t decodeVarint(Range<T*>& data); ...@@ -71,7 +71,8 @@ uint64_t decodeVarint(Range<T*>& data);
inline uint64_t encodeZigZag(int64_t val) { inline uint64_t encodeZigZag(int64_t val) {
// Bit-twiddling magic stolen from the Google protocol buffer document; // Bit-twiddling magic stolen from the Google protocol buffer document;
// val >> 63 is an arithmetic shift because val is signed // val >> 63 is an arithmetic shift because val is signed
return static_cast<uint64_t>((val << 1) ^ (val >> 63)); auto uval = static_cast<uint64_t>(val);
return static_cast<uint64_t>((uval << 1) ^ (val >> 63));
} }
inline int64_t decodeZigZag(uint64_t val) { inline int64_t decodeZigZag(uint64_t val) {
......
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