Make folly::FixedString::hash constexpr

Reviewed By: Mizuchi

Differential Revision: D29329759

fbshipit-source-id: 588aa9ce11db2c1aea51dcf96e0de3b50633fa29

folly/FixedString.h

@@ -393,7 +393,8 @@ struct ReverseIterator {
 } // namespace detail
 
 // Defined in folly/hash/Hash.h
-std::uint32_t hsieh_hash32_buf(const void* buf, std::size_t len);
+std::uint32_t hsieh_hash32_buf_constexpr(
+    const unsigned char* buf, std::size_t len);
 
 /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** *
  * \class BasicFixedString
@@ -1026,10 +1027,8 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    */
   static constexpr std::size_t max_size() noexcept { return N; }
 
-  // We would need to reimplement folly::Hash to make this
-  // constexpr. :-(
-  std::uint32_t hash() const noexcept {
-    return folly::hsieh_hash32_buf(data_, size_);
+  constexpr std::uint32_t hash() const noexcept {
+    return folly::hsieh_hash32_buf_constexpr(data_, size_);
   }
 
   /**

folly/hash/Hash.h

@@ -305,12 +305,13 @@ inline uint64_t fnva64(
 
 #define get16bits(d) folly::loadUnaligned<uint16_t>(d)
 
-inline uint32_t hsieh_hash32_buf(const void* buf, size_t len) noexcept {
+inline constexpr uint32_t hsieh_hash32_buf_constexpr(
+    const unsigned char* buf, size_t len) noexcept {
   // forcing signed char, since other platforms can use unsigned
-  const unsigned char* s = reinterpret_cast<const unsigned char*>(buf);
+  const unsigned char* s = buf;
   uint32_t hash = static_cast<uint32_t>(len);
-  uint32_t tmp;
-  size_t rem;
+  uint32_t tmp = 0;
+  size_t rem = 0;
 
   if (len <= 0 || buf == nullptr) {
     return 0;
@@ -360,6 +361,11 @@ inline uint32_t hsieh_hash32_buf(const void* buf, size_t len) noexcept {
 
 #undef get16bits
 
+inline uint32_t hsieh_hash32_buf(const void* buf, size_t len) noexcept {
+  return hsieh_hash32_buf_constexpr(
+      reinterpret_cast<const unsigned char*>(buf), len);
+}
+
 inline uint32_t hsieh_hash32(const char* s) noexcept {
   return hsieh_hash32_buf(s, std::strlen(s));
 }

folly/lang/Bits.h

@@ -344,7 +344,7 @@ FOLLY_POP_WARNING
  * Read an unaligned value of type T and return it.
  */
 template <class T>
-inline T loadUnaligned(const void* p) {
+inline constexpr T loadUnaligned(const void* p) {
   static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
   static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
   if (kHasUnalignedAccess) {