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xo-arena: DArenaHashmap: speedup iteration, relying on bookends

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This commit is contained in:
Roland Conybeare 2026-01-08 18:01:24 -05:00
commit 579a244c8c
1 changed files with 26 additions and 14 deletions
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40
include/xo/arena/DArenaHashMap.hpp
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@ -339,12 +339,15 @@ namespace xo {
using value_type = std::pair<const Key, Value>;
public:
DArenaHashMapIterator(uint8_t * c, uint8_t * e, value_type * p)
: ctrl_{c}, ctrl_end_{e}, pos_{p} {}
DArenaHashMapIterator(uint8_t * c, value_type * p)
: ctrl_{c}, pos_{p} {}
value_type & operator*() const { return *pos_; }
value_type * operator->() const { return pos_; }
/** true iff iterator at sentinel position (not dereferencable state !) **/
bool _at_slot_sentinel() const { return is_sentinel(*ctrl_) && (*ctrl_ != c_iterator_bookend); }
bool operator==(const DArenaHashMapIterator & x) const {
return this->pos_ == x.pos_;
}
@ -357,19 +360,27 @@ namespace xo {
do {
++ctrl_;
++pos_;
} while ((ctrl_ != ctrl_end_) && this->is_sentinel());
/** end condition: iterator ends at last non-wrapped position.
* relyin on bookend sentinel values at known offset from 'wrap' section
*
* ctrl_ ctrl_ + c_group_size
* | |
* v v
* <----------------- control_size(n_slot) ---------------->
* <-stub-> <----------- n_slot ----------> <group> <-stub->
* +--------+-------------------------------+-------+--------+
* | 0xF0 | empty / data / tombstone | wrap | 0xF0 |
* +--------+-------------------------------+-------+--------+
**/
} while (is_sentinel(*ctrl_)
&& (*(ctrl_ + c_group_size) != c_iterator_bookend));
return *this;
}
bool is_sentinel() const {
return DArenaHashMapUtil::is_sentinel(*ctrl_);
}
private:
uint8_t * ctrl_ = nullptr;
uint8_t * ctrl_end_ = nullptr;
value_type * pos_ = nullptr;
};
@ -417,12 +428,15 @@ namespace xo {
bool verify_ok(verify_policy p = verify_policy::throw_only()) const;
iterator begin() {
if (this->empty()) [[unlikely]] {
return this->end();
}
iterator ix(&(store_.control_[c_control_stub]),
&(store_.control_[c_control_stub + store_.capacity()]),
&(store_.slots_[0]));
if (ix.is_sentinel()) {
/* first occupied position in table */
if (ix._at_slot_sentinel()) {
/* advance to first occupied position in table */
++ix;
}
@ -431,7 +445,6 @@ namespace xo {
iterator end() {
iterator ix(&(store_.control_[c_control_stub + store_.capacity()]),
&(store_.control_[c_control_stub + store_.capacity()]),
&(store_.slots_[store_.capacity()]));
return ix;
@ -783,7 +796,6 @@ namespace xo {
if (equal_(slot.first, key)) {
return iterator(&(store_.control_[c_control_stub + slot_ix]),
&(store_.control_[c_control_stub + N]),
&slot);
}