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

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 4bba6a4..d82aaa4 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,11 @@
 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.
 

diff --git a/libstdc++-v3/include/bits/functional_hash.h b/libstdc++-v3/include/bits/functional_hash.h
index dc09683..cee1ea8 100644 (file)
--- a/libstdc++-v3/include/bits/functional_hash.h
+++ b/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

diff --git a/libstdc++-v3/include/tr1/functional_hash.h b/libstdc++-v3/include/tr1/functional_hash.h
index 4edc49a..8148e4d 100644 (file)
--- a/libstdc++-v3/include/tr1/functional_hash.h
+++ b/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

diff --git a/libstdc++-v3/libsupc++/hash_bytes.cc b/libstdc++-v3/libsupc++/hash_bytes.cc
index 1042de6..7d76c34 100644 (file)
--- a/libstdc++-v3/libsupc++/hash_bytes.cc
+++ b/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)
   {