[PATCH] PR libstdc++/87135 Rehash only when necessary (LWG2156)

[Jonathan Wakely]

On 13/09/18 07:49 +0200, François Dumont wrote:
>All changes limited to hashtable_c++0x.cc file.
>
>_Prime_rehash_policy::_M_next_bkt now really does what its comment was 
>declaring that is to say:
>Â  // Return a prime no smaller than n.
>
>_Prime_rehash_policy::_M_need_rehash rehash only when _M_next_size is 
>exceeded, not only when it is reach.
>
>Â Â Â  PR libstdc++/87135
>Â Â Â  * src/c++11/hashtable_c++0x.cc:
>Â Â Â  (_Prime_rehash_policy::_M_next_bkt): Return a prime no smaller than
>Â Â Â  requested size, but not necessarily greater.
>Â Â Â  (_Prime_rehash_policy::_M_need_rehash): Rehash only if target size is
>Â Â Â  strictly greater than next resize threshold.
>Â Â Â  * testsuite/23_containers/unordered_map/modifiers/reserve.cc: 
>Adapt test
>Â Â Â  to validate that there is no rehash as long as number of insertion is
>Â Â Â  lower or equal to the reserved number of elements.
>
>unordered_map tests successful, ok to commit once all other tests 
>completed ?
>
>François

>diff --git a/libstdc++-v3/src/c++11/hashtable_c++0x.cc b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
>index a776a8506fe..ec6031b3f5b 100644
>--- a/libstdc++-v3/src/c++11/hashtable_c++0x.cc
>+++ b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
>@@ -46,10 +46,10 @@ namespace __detail
>   {
>     // Optimize lookups involving the first elements of __prime_list.
>     // (useful to speed-up, eg, constructors)
>-    static const unsigned char __fast_bkt[13]
>-      = { 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11, 13, 13 };
>+    static const unsigned char __fast_bkt[]
>+      = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11, 13, 13 };
> 
>-    if (__n <= 12)
>+    if (__n < sizeof(__fast_bkt) / sizeof(unsigned char))

sizeof(unsigned char) is defined to be 1, always.

I think this should be just sizeof(__fast_bkt), or if you're trying to
guard against the type of __fast_bkt changing, then use
sizeof(__fast_bkt) / sizeof(__fast_bkt[0]))

OK for trunk with either of those, instead of sizeof(unsigned char).

Thanks.