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docs: update raw_hash_set comments to reflect floating windows #2032

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93 changes: 44 additions & 49 deletions absl/container/internal/raw_hash_set.h
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Original file line number Diff line number Diff line change
Expand Up @@ -76,12 +76,16 @@
//
// The length of this array is computed by `RawHashSetLayout::alloc_size` below.
//
// Control bytes (`ctrl_t`) are bytes (collected into groups of a
// platform-specific size) that define the state of the corresponding slot in
// the slot array. Group manipulation is tightly optimized to be as efficient
// as possible: SSE and friends on x86, clever bit operations on other arches.
// Control bytes (`ctrl_t`) are bytes that define the state of the corresponding
// slot in the slot array. To optimize probe sequence operations, we use
// unaligned memory access to load a "window" of multiple control bytes
// (a platform-specific size, usually 16) starting at the probe index.
// Window manipulation is tightly optimized to be as efficient as possible:
// SSE and friends on x86, clever bit operations on other arches.
//
// Group 1 Group 2 Group 3
// Probe Index 0 Probe Index 2
// +---------------+ +---------------+
// | | | |
// +---------------+---------------+---------------+
// | | | | | | | | | | | | | | | | | | | | | | | | |
// +---------------+---------------+---------------+
Expand Down Expand Up @@ -117,13 +121,13 @@
// `H1(hash(x))` and the capacity, we construct a `probe_seq` that visits every
// group of slots in some interesting order.
//
// We now walk through these indices. At each index, we select the entire group
// starting with that index and extract potential candidates: occupied slots
// with a control byte equal to `H2(hash(x))`. If we find an empty slot in the
// group, we stop and return an error. Each candidate slot `y` is compared with
// `x`; if `x == y`, we are done and return `&y`; otherwise we continue to the
// next probe index. Tombstones effectively behave like full slots that never
// match the value we're looking for.
// We now walk through these indices. At each index, we select a window of
// control bytes starting at that index and extract potential candidates:
// occupied slots with a control byte equal to `H2(hash(x))`. If we find an
// empty slot in the window, we stop and return an error. Each candidate slot
// `y` is compared with `x`; if `x == y`, we are done and return `&y`; otherwise
// we continue to the next probe index. Tombstones effectively behave like full
// slots that never match the value we're looking for.
//
// The `H2` bits ensure when we compare a slot to an object with `==`, we are
// likely to have actually found the object. That is, the chance is low that
Expand Down Expand Up @@ -944,8 +948,7 @@ class CommonFields : public CommonFieldsGenerationInfo {
CommonFields(non_soo_tag_t, const CommonFields& that)
: capacity_(that.capacity_),
size_(that.size_),
heap_or_soo_(that.heap_or_soo_) {
}
heap_or_soo_(that.heap_or_soo_) {}

// Movable
CommonFields(CommonFields&& that) = default;
Expand Down Expand Up @@ -2137,10 +2140,9 @@ class raw_hash_set {
std::is_nothrow_default_constructible<key_equal>::value &&
std::is_nothrow_default_constructible<allocator_type>::value) {}

explicit raw_hash_set(
size_t bucket_count, const hasher& hash = hasher(),
const key_equal& eq = key_equal(),
const allocator_type& alloc = allocator_type())
explicit raw_hash_set(size_t bucket_count, const hasher& hash = hasher(),
const key_equal& eq = key_equal(),
const allocator_type& alloc = allocator_type())
: settings_(CommonFields::CreateDefault<SooEnabled()>(), hash, eq,
alloc) {
if (bucket_count > DefaultCapacity()) {
Expand Down Expand Up @@ -2746,8 +2748,7 @@ class raw_hash_set {

if (src.is_small()) {
if (src.empty()) return;
if (insert_slot(src.single_slot()))
src.erase_meta_only_small();
if (insert_slot(src.single_slot())) src.erase_meta_only_small();
return;
}
for (auto it = src.begin(), e = src.end(); it != e;) {
Expand Down Expand Up @@ -2828,7 +2829,7 @@ class raw_hash_set {
template <class K = key_type>
void prefetch([[maybe_unused]] const key_arg<K>& key) const {
if (capacity() == DefaultCapacity()) return;
// Avoid probing if we won't be able to prefetch the addresses received.
// Avoid probing if we won't be able to prefetch the addresses received.
#ifdef ABSL_HAVE_PREFETCH
prefetch_heap_block();
if (is_small()) return;
Expand All @@ -2840,8 +2841,7 @@ class raw_hash_set {

template <class K = key_type>
ABSL_DEPRECATE_AND_INLINE()
iterator find(const key_arg<K>& key,
size_t) ABSL_ATTRIBUTE_LIFETIME_BOUND {
iterator find(const key_arg<K>& key, size_t) ABSL_ATTRIBUTE_LIFETIME_BOUND {
return find(key);
}
// The API of find() has one extension: the type of the key argument doesn't
Expand All @@ -2856,8 +2856,8 @@ class raw_hash_set {

template <class K = key_type>
ABSL_DEPRECATE_AND_INLINE()
const_iterator find(const key_arg<K>& key,
size_t) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
const_iterator
find(const key_arg<K>& key, size_t) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
return find(key);
}
template <class K = key_type>
Expand Down Expand Up @@ -3269,11 +3269,12 @@ class raw_hash_set {
ABSL_SWISSTABLE_ASSERT(capacity() == 1);
constexpr bool kUseMemcpy =
PolicyTraits::transfer_uses_memcpy() && SooEnabled();
size_t index = GrowSooTableToNextCapacityAndPrepareInsert<
kUseMemcpy ? OptimalMemcpySizeForSooSlotTransfer(sizeof(slot_type)) : 0,
kUseMemcpy>(common(), GetPolicyFunctions(),
HashKey<hasher, K, kIsDefaultHash>{hash_ref(), key},
force_sampling);
size_t index = GrowSooTableToNextCapacityAndPrepareInsert < kUseMemcpy
? OptimalMemcpySizeForSooSlotTransfer(sizeof(slot_type))
: 0,
kUseMemcpy > (common(), GetPolicyFunctions(),
HashKey<hasher, K, kIsDefaultHash>{hash_ref(), key},
force_sampling);
return {iterator_at(index), true};
}

Expand Down Expand Up @@ -3478,16 +3479,10 @@ class raw_hash_set {
// side-effect.
//
// See `CapacityToGrowth()`.
size_t growth_left() const {
return common().growth_left();
}
size_t growth_left() const { return common().growth_left(); }

GrowthInfo& growth_info() {
return common().growth_info();
}
GrowthInfo growth_info() const {
return common().growth_info();
}
GrowthInfo& growth_info() { return common().growth_info(); }
GrowthInfo growth_info() const { return common().growth_info(); }

// Prefetch the heap-allocated memory region to resolve potential TLB and
// cache misses. This is intended to overlap with execution of calculating the
Expand Down Expand Up @@ -3694,16 +3689,16 @@ struct HashtableFreeFunctionsAccess {
[[maybe_unused]] const size_t original_size_for_assert = c->size();
size_t num_deleted = 0;
using SlotType = typename Set::slot_type;
IterateOverFullSlots(
c->common(), sizeof(SlotType),
[&](const ctrl_t* ctrl, void* slot_void) {
auto* slot = static_cast<SlotType*>(slot_void);
if (pred(Set::PolicyTraits::element(slot))) {
c->destroy(slot);
EraseMetaOnlyLarge(c->common(), ctrl, sizeof(*slot));
++num_deleted;
}
});
IterateOverFullSlots(c->common(), sizeof(SlotType),
[&](const ctrl_t* ctrl, void* slot_void) {
auto* slot = static_cast<SlotType*>(slot_void);
if (pred(Set::PolicyTraits::element(slot))) {
c->destroy(slot);
EraseMetaOnlyLarge(c->common(), ctrl,
sizeof(*slot));
++num_deleted;
}
});
// NOTE: IterateOverFullSlots allow removal of the current element, so we
// verify the size additionally here.
ABSL_SWISSTABLE_ASSERT(original_size_for_assert - num_deleted ==
Expand Down