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 <jwak...@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. diff --git a/libstdc++-v3/include/bits/functional_hash.h b/libstdc++-v3/include/bits/functional_hash.h index dc09683..cee1ea8 100644 --- 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 --- 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 --- 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) {