PR 59406 note that FNV hash functions are incorrect

	PR libstdc++/59406
	* include/bits/functional_hash.h: Add comment noting difference from
	FNV-1a.
	* include/tr1/functional_hash.h: Likewise.
	* libsupc++/hash_bytes.cc: Likewise.

From-SVN: r242454

libstdc++-v3/ChangeLog

 2016-11-15  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/59406
+	* include/bits/functional_hash.h: Add comment noting difference from
+	FNV-1a.
+	* include/tr1/functional_hash.h: Likewise.
+	* libsupc++/hash_bytes.cc: Likewise.
+
 	* include/bits/basic_string.h: Reuse _If_sv alias template for new
 	constructor.
 

libstdc++-v3/include/bits/functional_hash.h

@@ -200,6 +200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return hash(&__val, sizeof(__val), __hash); }
   };
 
+  // A hash function similar to FNV-1a (see PR59406 for how it differs).
   struct _Fnv_hash_impl
   {
     static size_t

libstdc++-v3/include/tr1/functional_hash.h

@@ -83,6 +83,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Fowler / Noll / Vo (FNV) Hash (type FNV-1a)
   // (Used by the next specializations of std::tr1::hash.)
 
+  // N.B. These functions should work on unsigned char, otherwise they do not
+  // correctly implement the FNV-1a algorithm (see PR59406).
+  // The existing behaviour is retained for backwards compatibility.
+
   /// Dummy generic implementation (for sizeof(size_t) != 4, 8).
   template<size_t>
     struct _Fnv_hash_base

libstdc++-v3/libsupc++/hash_bytes.cc

@@ -112,6 +112,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }
 
   // Implementation of FNV hash for 32-bit size_t.
+  // N.B. This function should work on unsigned char, otherwise it does not
+  // correctly implement the FNV-1a algorithm (see PR59406).
+  // The existing behaviour is retained for backwards compatibility.
   size_t
   _Fnv_hash_bytes(const void* ptr, size_t len, size_t hash)
   {
@@ -157,6 +160,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }
 
   // Implementation of FNV hash for 64-bit size_t.
+  // N.B. This function should work on unsigned char, otherwise it does not
+  // correctly implement the FNV-1a algorithm (see PR59406).
+  // The existing behaviour is retained for backwards compatibility.
   size_t
   _Fnv_hash_bytes(const void* ptr, size_t len, size_t hash)
   {