[PATCH] PR 59406 note that FNV hash functions are incorrect
Jonathan Wakely
jwakely@redhat.com
Tue Nov 15 20:17:00 GMT 2016
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.
-------------- next part --------------
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.
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)
{
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