@@ -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
@@ -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
@@ -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)
{
The PR points out that our FNV hash functions don't correctly implement the FNV-1a function. Since the code is only kept for backwards compatibility we probably don't want to change the results, so this just adds comments to point out the issue. 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. Tested powerpc64le-linux, committed to trunk. commit 138a2ac8bc4bcbef28815ef89758f13e93fa433b Author: Jonathan Wakely <jwakely@redhat.com> Date: Tue Nov 15 20:00:54 2016 +0000 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.