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.
