This is the mail archive of the libstdc++@gcc.gnu.org mailing list for the libstdc++ project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: PR 71181 Avoid rehash after reserve


On 14/06/16 22:34 +0200, François Dumont wrote:
On 14/06/2016 13:22, Jonathan Wakely wrote:
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?

Indeed but my intention was to make sentinel values useless so that we can remove them one day.

I don't like current code because when you just look at lower_bound call you can wonder why returned value is not tested. You need to consider how __prime_list has been defined. When you add '- 1' in the call to lower_bound you don't need to look too far to understand it.


_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.

Yes it does for now but not if __prime_list is a the pure list of prime numbers.

OK. And as I said below, lower_bound(primes, primes + nprimes - 1, n)
still works because anything greater than the second-to-last prime
should be treated as the last one anyway.

Would this comment make it clearer?

 // Don't include the last prime in the search, so that anything
 // higher than the second-to-last prime returns a past-the-end
 // iterator that can be dereferenced to get the last prime.
 const unsigned long* __p
   = std::lower_bound(__prime_list, __prime_list + _S_n_primes - 1, __n)



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.

The comment is doing as if there was no sentinel.

OK. I think a similar comment as suggested above could help, by being
more verbose about what's happening.

 // Don't include the last prime in the search, so that anything
 // higher than the second-to-last prime returns a past-the-end
 // iterator that can be dereferenced to get the last prime.



   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?

Yes, that's another option, I will give it a try.

I did some comparisons and this version seems to execute fewer
instructions in some simple tests, according to cachegrind.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]