On 13/06/16 21:49 +0200, François Dumont wrote:
Hi
I eventually would like to propose the attached patch.
In tr1 I made sure we use a special past-the-end iterator that
makes usage of lower_bound result without check safe.
I'm confused ... isn't that already done?
_S_n_primes is defined as:
enum { _S_n_primes = sizeof(unsigned long) != 8 ? 256 : 256 + 48 };
The table of primes is:
extern const unsigned long __prime_list[] = // 256 + 1 or 256 + 48 + 1
Which means that _S_n_primes is already one less, so that the "end"
returned by lower_bound is already dereferenceable. That's what the
comment in the table suggests too:
// Sentinel, so we don't have to test the result of lower_bound,
// or, on 64-bit machines, rest of the table.
#if __SIZEOF_LONG__ != 8
4294967291ul
So ...
diff --git a/libstdc++-v3/include/tr1/hashtable_policy.h
b/libstdc++-v3/include/tr1/hashtable_policy.h
index 4ee6d45..24d1a59 100644
--- a/libstdc++-v3/include/tr1/hashtable_policy.h
+++ b/libstdc++-v3/include/tr1/hashtable_policy.h
@@ -420,8 +420,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Prime_rehash_policy::
_M_next_bkt(std::size_t __n) const
{
- const unsigned long* __p = std::lower_bound(__prime_list, __prime_list
- + _S_n_primes, __n);
+ // Past-the-end iterator is made dereferenceable to avoid check on
+ // lower_bound result.
+ const unsigned long* __p
+ = std::lower_bound(__prime_list, __prime_list + _S_n_primes - 1, __n);
Is this redundant? Unless I'm misunderstanding something, _S_n_primes
already handles this.
The other changes in tr1/hashtable_policy.h are nice simplifications.
diff --git a/libstdc++-v3/src/c++11/hashtable_c++0x.cc
b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
index a5e6520..7cbd364 100644
--- a/libstdc++-v3/src/c++11/hashtable_c++0x.cc
+++ b/libstdc++-v3/src/c++11/hashtable_c++0x.cc
@@ -46,22 +46,36 @@ namespace __detail
{
// Optimize lookups involving the first elements of __prime_list.
// (useful to speed-up, eg, constructors)
- static const unsigned char __fast_bkt[12]
- = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11 };
+ static const unsigned char __fast_bkt[13]
+ = { 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11, 13, 13 };
- if (__n <= 11)
+ if (__n <= 12)
{
_M_next_resize =
__builtin_ceil(__fast_bkt[__n] * (long double)_M_max_load_factor);
return __fast_bkt[__n];
}
+ // Number of primes without sentinel.
constexpr auto __n_primes
= sizeof(__prime_list) / sizeof(unsigned long) - 1;
+ // past-the-end iterator is made dereferenceable.
+ constexpr auto __prime_list_end = __prime_list + __n_primes - 1;
I don't think this comment clarifies things very well.
Because of the sentinel and because __n_primes doesn't include the
sentinel, (__prime_list + __n_primes) is already dereferenceable
anyway, so the comment doesn't explain why there's *another* -1 here.
const unsigned long* __next_bkt =
- std::lower_bound(__prime_list + 5, __prime_list + __n_primes, __n);
+ std::lower_bound(__prime_list + 6, __prime_list_end, __n);
+
+ if (*__next_bkt == __n && __next_bkt != __prime_list_end)
+ ++__next_bkt;
Can we avoid this check by searching for __n + 1 instead of __n with
the lower_bound call?
If I understand the logic correctly we can do it like this:
// Number of primes without sentinel:
constexpr auto __n_primes
= sizeof(__prime_list) / sizeof(unsigned long) - 1;
// The greatest prime in the table:
constexpr auto __prime_list_end = __prime_list + __n_primes - 1;
const auto __next_bkt =
std::lower_bound(__prime_list + 6, __prime_list_end, __n + 1);
if (__next_bkt == __prime_list_end)
_M_next_resize = size_t(-1); // Reached maximum bucket count.
else
_M_next_resize =
__builtin_ceil(*__next_bkt * (long double)_M_max_load_factor);
return *__next_bkt;
i.e.
- Ignore the sentinel (keeping it only for backward compatibility).
- Search for __n + 1 so we find the *next* bucket count.
- Don't include the largest prime in the search, because even if __n >
largest prime, we're going to use that largest value anyway, so:
- if __n >= second largest prime then lower_bound will return the end
iterator, which points to the largest prime.
Does this behave correctly?
I'd really like to see it tested for the boundary conditions, i.e.
verify that _Prime_rehash_policy::_M_next_bkt behaves as expected when
passed prime numbers, and when passed N-1, N and N+1 where N is the
largest prime in the table.
+ if (__next_bkt == __prime_list_end)
+ // Set next resize to the max value so that we never try to rehash again
+ // as we already reach the biggest possible bucket number.
+ // Note that it might result in max_load_factor not being respected.
+ _M_next_resize = std::size_t(-1);
+ else
_M_next_resize =
__builtin_ceil(*__next_bkt * (long double)_M_max_load_factor);
+
return *__next_bkt;
}
N.B. this chunk of the patch doesn't apply due to whitespace
differences, what are you diffing against?
In your patch these two lines are already indented:
_M_next_resize =
__builtin_ceil(*__next_bkt * (long double)_M_max_load_factor);
But in the current code on trunk they are not.
diff --git
a/libstdc++-v3/testsuite/23_containers/unordered_set/hash_policy/rehash.cc
b/libstdc++-v3/testsuite/23_containers/unordered_set/hash_policy/rehash.cc
index 2dac583..9658131 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_set/hash_policy/rehash.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/hash_policy/rehash.cc
@@ -34,8 +34,8 @@ void test()
us.insert(i);
if (bkt_count != us.bucket_count())
{
- // Container has been rehashed, lets check that it won't be rehash again
- // if we remove and restore the last 2 inserted elements:
+ // Container has been rehashed, lets check that it won't be rehash
+ // again if we remove and restore the last 2 inserted elements:
rehashed = true;
bkt_count = us.bucket_count();
VERIFY( us.erase(i) == 1 );
This chunk of the patch doesn't apply either. The indentation already
looks correct on trunk.
