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Message-ID: <4F681569.3080603@openvz.org>
Date: Tue, 20 Mar 2012 09:28:09 +0400
From: Konstantin Khlebnikov <khlebnikov@...nvz.org>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: Hugh Dickins <hughd@...gle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v3] radix-tree: introduce bit-optimized iterator
Andrew Morton wrote:
> On Mon, 19 Mar 2012 09:19:08 +0400
> Konstantin Khlebnikov<khlebnikov@...nvz.org> wrote:
>
>> This patch implements clean, simple and effective radix-tree iteration routine.
>>
>> Iterating divided into two phases:
>> * search for the next chunk of slots in radix-tree leaf node
>> * iterate through slots in this chunk
>>
>> Main iterator function radix_tree_next_chunk() returns pointer to first slot,
>> and stores in the struct radix_tree_iter index and next-to-last slot for chunk.
>> For tagged-iterating it also construct bit-mask of tags for slots in chunk.
>> All additional logic implemented as static-inline functions and macroses.
>>
>> Also patch adds radix_tree_find_next_bit() static-inline variant of
>> find_next_bit() optimized for small constant size arrays, because
>> find_next_bit() too heavy for searching in an array with one/two long elements.
>>
>> Signed-off-by: Konstantin Khlebnikov<khlebnikov@...nvz.org>
>>
>> ---
>> v3: No functional changes: renaming variables, updating comments, fixing style errors.
>
> Here's what you changed:
>
> --- a/include/linux/radix-tree.h~radix-tree-introduce-bit-optimized-iterator-v3
> +++ a/include/linux/radix-tree.h
> @@ -258,28 +258,76 @@ static inline void radix_tree_preload_en
> preempt_enable();
> }
>
> +/**
> + * struct radix_tree_iter - radix tree iterator state
> + *
> + * @index: index of current slot
> + * @next_index: next-to-last index for this chunk
> + * @tags: bit-mask for tag-iterating
> + *
> + * Radix tree iterator works in terms of "chunks" of slots.
> + * Chunk is sub-interval of slots contained in one radix tree leaf node.
> + * It described by pointer to its first slot and struct radix_tree_iter
> + * which holds chunk position in tree and its size. For tagged iterating
> + * radix_tree_iter also holds slots' bit-mask for one chosen radix tree tag.
> + */
> struct radix_tree_iter {
> - unsigned long index; /* current index, do not overflow it */
> - unsigned long next_index; /* next-to-last index for this chunk */
> - unsigned long tags; /* bitmask for tag-iterating */
> + unsigned long index;
> + unsigned long next_index;
> + unsigned long tags;
> };
>
> #define RADIX_TREE_ITER_TAG_MASK 0x00FF /* tag index in lower byte */
> #define RADIX_TREE_ITER_TAGGED 0x0100 /* lookup tagged slots */
> #define RADIX_TREE_ITER_CONTIG 0x0200 /* stop at first hole */
>
> -void **radix_tree_next_chunk(struct radix_tree_root *root,
> - struct radix_tree_iter *iter, unsigned flags);
> -
> -static inline
> -void **radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
> +/**
> + * radix_tree_iter_init - initialize radix tree iterator
> + *
> + * @iter: pointer to iterator state
> + * @start: iteration starting index
> + * Returns: NULL
> + */
> +static __always_inline void **
> +radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
> {
> - iter->index = 0; /* to bypass next_index overflow protection */
> + /*
> + * Leave iter->tags unitialized. radix_tree_next_chunk()
> + * anyway fill it in case successful tagged chunk lookup.
> + * At unsuccessful or non-tagged lookup nobody cares about it.
> + *
> + * Set index to zero to bypass next_index overflow protection.
> + * See comment inside radix_tree_next_chunk() for details.
> + */
> + iter->index = 0;
> iter->next_index = start;
> return NULL;
> }
>
> -static inline unsigned long radix_tree_chunk_size(struct radix_tree_iter *iter)
> +/**
> + * radix_tree_next_chunk - find next chunk of slots for iteration
> + *
> + * @root: radix tree root
> + * @iter: iterator state
> + * @flags: RADIX_TREE_ITER_* flags and tag index
> + * Returns: pointer to chunk first slot, or NULL if there no more left
> + *
> + * This function lookup next chunk in the radix tree starting from
> + * @iter->next_index, it returns pointer to chunk first slot.
> + * Also it fills @iter with data about chunk: position in the tree (index),
> + * its end (next_index), and construct bit mask for tagged iterating (tags).
> + */
> +void **radix_tree_next_chunk(struct radix_tree_root *root,
> + struct radix_tree_iter *iter, unsigned flags);
> +
> +/**
> + * radix_tree_chunk_size - get current chunk size
> + *
> + * @iter: pointer to radix tree iterator
> + * Returns: current chunk size
> + */
> +static __always_inline unsigned
> +radix_tree_chunk_size(struct radix_tree_iter *iter)
> {
> return iter->next_index - iter->index;
> }
> @@ -287,41 +335,40 @@ static inline unsigned long radix_tree_c
> /**
> * radix_tree_next_slot - find next slot in chunk
> *
> - * @slot pointer to slot
> - * @iter iterator state
> - * @flags RADIX_TREE_ITER_*
> - *
> - * Returns pointer to next slot, or NULL if no more left.
> - */
> -static __always_inline
> -void **radix_tree_next_slot(void **slot, struct radix_tree_iter *iter,
> - unsigned flags)
> + * @slot: pointer to current slot
> + * @iter: pointer to interator state
> + * @flags: RADIX_TREE_ITER_*, should be constant
> + * Returns: pointer to next slot, or NULL if there no more left
> + *
> + * This function updates @iter->index in case successful lookup.
> + * For tagged lookup it also eats @iter->tags.
> + */
> +static __always_inline void **
> +radix_tree_next_slot(void **slot, struct radix_tree_iter *iter, unsigned flags)
> {
> - unsigned size, offset;
> -
> - size = radix_tree_chunk_size(iter) - 1;
> if (flags& RADIX_TREE_ITER_TAGGED) {
> iter->tags>>= 1;
> if (likely(iter->tags& 1ul)) {
> iter->index++;
> return slot + 1;
> }
> - if ((flags& RADIX_TREE_ITER_CONTIG)&& size)
> - return NULL;
> - if (likely(iter->tags)) {
> - offset = __ffs(iter->tags);
> + if (!(flags& RADIX_TREE_ITER_CONTIG)&& likely(iter->tags)) {
> + unsigned offset = __ffs(iter->tags);
> +
> iter->tags>>= offset;
> iter->index += offset + 1;
> return slot + offset + 1;
> }
> } else {
> + unsigned size = radix_tree_chunk_size(iter) - 1;
> +
> while (size--) {
> slot++;
> iter->index++;
> if (likely(*slot))
> return slot;
> if (flags& RADIX_TREE_ITER_CONTIG)
> - return NULL;
> + break;
> }
> }
> return NULL;
> @@ -330,70 +377,79 @@ void **radix_tree_next_slot(void **slot,
> /**
> * radix_tree_for_each_chunk - iterate over chunks
> *
> - * @slot: the void** for pointer to chunk first slot
> - * @root the struct radix_tree_root pointer
> - * @iter the struct radix_tree_iter pointer
> - * @start starting index
> - * @flags RADIX_TREE_ITER_* and tag index
> + * @slot: the void** variable for pointer to chunk first slot
> + * @root: the struct radix_tree_root pointer
> + * @iter: the struct radix_tree_iter pointer
> + * @start: iteration starting index
> + * @flags: RADIX_TREE_ITER_* and tag index
> *
> - * Locks can be released and reasquired between iterations.
> + * Locks can be released and reacquired between iterations.
> */
> #define radix_tree_for_each_chunk(slot, root, iter, start, flags) \
> - for ( slot = radix_tree_iter_init(iter, start) ; \
> - (slot = radix_tree_next_chunk(root, iter, flags)) ; )
> + for (slot = radix_tree_iter_init(iter, start) ; \
> + (slot = radix_tree_next_chunk(root, iter, flags)) ;)
>
> /**
> * radix_tree_for_each_chunk_slot - iterate over slots in one chunk
> *
> - * @slot: the void** for pointer to slot
> - * @iter the struct radix_tree_iter pointer
> - * @flags RADIX_TREE_ITER_*
> + * @slot: the void** variable, at the beginning points to chunk first slot
> + * @iter: the struct radix_tree_iter pointer
> + * @flags: RADIX_TREE_ITER_*, should be constant
> + *
> + * This macro supposed to be nested inside radix_tree_for_each_chunk().
> + * @slot points to radix tree slot, @iter->index contains its index.
> */
> -#define radix_tree_for_each_chunk_slot(slot, iter, flags) \
> - for ( ; slot ; slot = radix_tree_next_slot(slot, iter, flags) )
> +#define radix_tree_for_each_chunk_slot(slot, iter, flags) \
> + for (; slot ; slot = radix_tree_next_slot(slot, iter, flags))
>
> /**
> - * radix_tree_for_each_slot - iterate over all slots
> + * radix_tree_for_each_slot - iterate over non-empty slots
> *
> - * @slot: the void** for pointer to slot
> - * @root the struct radix_tree_root pointer
> - * @iter the struct radix_tree_iter pointer
> - * @start starting index
> + * @slot: the void** variable for pointer to slot
> + * @root: the struct radix_tree_root pointer
> + * @iter: the struct radix_tree_iter pointer
> + * @start: iteration starting index
> + *
> + * @slot points to radix tree slot, @iter->index contains its index.
> */
> -#define radix_tree_for_each_slot(slot, root, iter, start) \
> - for ( slot = radix_tree_iter_init(iter, start) ; \
> - slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
> - slot = radix_tree_next_slot(slot, iter, 0) )
> +#define radix_tree_for_each_slot(slot, root, iter, start) \
> + for (slot = radix_tree_iter_init(iter, start) ; \
> + slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
> + slot = radix_tree_next_slot(slot, iter, 0))
>
> /**
> - * radix_tree_for_each_contig - iterate over all contiguous slots
> + * radix_tree_for_each_contig - iterate over contiguous slots
> + *
> + * @slot: the void** variable for pointer to slot
> + * @root: the struct radix_tree_root pointer
> + * @iter: the struct radix_tree_iter pointer
> + * @start: iteration starting index
> *
> - * @slot: the void** for pointer to slot
> - * @root the struct radix_tree_root pointer
> - * @iter the struct radix_tree_iter pointer
> - * @start starting index
> + * @slot points to radix tree slot, @iter->index contains its index.
> */
> #define radix_tree_for_each_contig(slot, root, iter, start) \
> - for ( slot = radix_tree_iter_init(iter, start) ; \
> - slot || (slot = radix_tree_next_chunk(root, iter, \
> + for (slot = radix_tree_iter_init(iter, start) ; \
> + slot || (slot = radix_tree_next_chunk(root, iter, \
> RADIX_TREE_ITER_CONTIG)) ; \
> - slot = radix_tree_next_slot(slot, iter, \
> - RADIX_TREE_ITER_CONTIG) )
> + slot = radix_tree_next_slot(slot, iter, \
> + RADIX_TREE_ITER_CONTIG))
>
> /**
> - * radix_tree_for_each_tagged - iterate over all tagged slots
> + * radix_tree_for_each_tagged - iterate over tagged slots
> + *
> + * @slot: the void** variable for pointer to slot
> + * @root: the struct radix_tree_root pointer
> + * @iter: the struct radix_tree_iter pointer
> + * @start: iteration starting index
> + * @tag: tag index
> *
> - * @slot: the void** for pointer to slot
> - * @root the struct radix_tree_root pointer
> - * @iter the struct radix_tree_iter pointer
> - * @start starting index
> - * @tag tag index
> + * @slot points to radix tree slot, @iter->index contains its index.
> */
> #define radix_tree_for_each_tagged(slot, root, iter, start, tag) \
> - for ( slot = radix_tree_iter_init(iter, start) ; \
> - slot || (slot = radix_tree_next_chunk(root, iter, \
> + for (slot = radix_tree_iter_init(iter, start) ; \
> + slot || (slot = radix_tree_next_chunk(root, iter, \
> RADIX_TREE_ITER_TAGGED | tag)) ; \
> - slot = radix_tree_next_slot(slot, iter, \
> - RADIX_TREE_ITER_TAGGED) )
> + slot = radix_tree_next_slot(slot, iter, \
> + RADIX_TREE_ITER_TAGGED))
>
> #endif /* _LINUX_RADIX_TREE_H */
> diff -puN lib/radix-tree.c~radix-tree-introduce-bit-optimized-iterator-v3 lib/radix-tree.c
> --- a/lib/radix-tree.c~radix-tree-introduce-bit-optimized-iterator-v3
> +++ a/lib/radix-tree.c
> @@ -150,6 +150,7 @@ static inline int any_tag_set(struct rad
>
> /**
> * radix_tree_find_next_bit - find the next set bit in a memory region
> + *
> * @addr: The address to base the search on
> * @size: The bitmap size in bits
> * @offset: The bitnumber to start searching at
> @@ -158,8 +159,9 @@ static inline int any_tag_set(struct rad
> * Tail bits starting from size to roundup(size, BITS_PER_LONG) must be zero.
> * Returns next bit offset, or size if nothing found.
> */
> -static inline unsigned long radix_tree_find_next_bit(const unsigned long *addr,
> - unsigned long size, unsigned long offset)
> +static __always_inline unsigned long
> +radix_tree_find_next_bit(const unsigned long *addr,
> + unsigned long size, unsigned long offset)
> {
> if (!__builtin_constant_p(size))
> return find_next_bit(addr, size, offset);
> @@ -651,27 +653,26 @@ EXPORT_SYMBOL(radix_tree_tag_get);
> /**
> * radix_tree_next_chunk - find next chunk of slots for iteration
> *
> - * @root: radix tree root
> - * @iter: iterator state
> - * @flags RADIX_TREE_ITER_* flags and tag index
> - *
> - * Returns pointer to first slots in chunk, or NULL if there no more left
> + * @root: radix tree root
> + * @iter: iterator state
> + * @flags: RADIX_TREE_ITER_* flags and tag index
> + * Returns: pointer to chunk first slot, or NULL if iteration is over
> */
> void **radix_tree_next_chunk(struct radix_tree_root *root,
> struct radix_tree_iter *iter, unsigned flags)
> {
> unsigned shift, tag = flags& RADIX_TREE_ITER_TAG_MASK;
> struct radix_tree_node *rnode, *node;
> - unsigned long i, index;
> + unsigned long index, offset;
>
> if ((flags& RADIX_TREE_ITER_TAGGED)&& !root_tag_get(root, tag))
> return NULL;
>
> /*
> - * Catch next_index overflow after ~0UL.
> - * iter->index can be zero only at the beginning.
> - * Because RADIX_TREE_MAP_SHIFT< BITS_PER_LONG we cannot
> - * oveflow iter->next_index in single step.
> + * Catch next_index overflow after ~0UL. iter->index never overflows
> + * during iterating, it can be zero only at the beginning.
> + * And we cannot overflow iter->next_index in single step,
> + * because RADIX_TREE_MAP_SHIFT< BITS_PER_LONG.
> */
> index = iter->next_index;
> if (!index&& iter->index)
> @@ -691,34 +692,37 @@ void **radix_tree_next_chunk(struct radi
>
> restart:
> shift = (rnode->height - 1) * RADIX_TREE_MAP_SHIFT;
> - i = index>> shift;
> + offset = index>> shift;
>
> - /* Index ouside of the tree */
> - if (i>= RADIX_TREE_MAP_SIZE)
> + /* Index outside of the tree */
> + if (offset>= RADIX_TREE_MAP_SIZE)
> return NULL;
>
> node = rnode;
> while (1) {
> if ((flags& RADIX_TREE_ITER_TAGGED) ?
> - !test_bit(i, node->tags[tag]) :
> - !node->slots[i]) {
> + !test_bit(offset, node->tags[tag]) :
> + !node->slots[offset]) {
> /* Hole detected */
> if (flags& RADIX_TREE_ITER_CONTIG)
> return NULL;
>
> if (flags& RADIX_TREE_ITER_TAGGED)
> - i = radix_tree_find_next_bit(node->tags[tag],
> - RADIX_TREE_MAP_SIZE, i + 1);
> + offset = radix_tree_find_next_bit(
> + node->tags[tag],
> + RADIX_TREE_MAP_SIZE,
> + offset + 1);
> else
> - while (++i< RADIX_TREE_MAP_SIZE&&
> - !node->slots[i]);
> -
> + while (++offset< RADIX_TREE_MAP_SIZE) {
> + if (node->slots[offset])
> + break;
> + }
> index&= ~((RADIX_TREE_MAP_SIZE<< shift) - 1);
> - index += i<< shift;
> + index += offset<< shift;
> /* Overflow after ~0UL */
> if (!index)
> return NULL;
> - if (i == RADIX_TREE_MAP_SIZE)
> + if (offset == RADIX_TREE_MAP_SIZE)
> goto restart;
> }
>
> @@ -726,23 +730,23 @@ restart:
> if (!shift)
> break;
>
> - node = rcu_dereference_raw(node->slots[i]);
> + node = rcu_dereference_raw(node->slots[offset]);
> if (node == NULL)
> goto restart;
> shift -= RADIX_TREE_MAP_SHIFT;
> - i = (index>> shift)& RADIX_TREE_MAP_MASK;
> + offset = (index>> shift)& RADIX_TREE_MAP_MASK;
> }
>
> /* Update the iterator state */
> iter->index = index;
> iter->next_index = (index | RADIX_TREE_MAP_MASK) + 1;
>
> - /* Construct iter->tags bitmask from node->tags[tag] array */
> + /* Construct iter->tags bit-mask from node->tags[tag] array */
> if (flags& RADIX_TREE_ITER_TAGGED) {
> unsigned tag_long, tag_bit;
>
> - tag_long = i / BITS_PER_LONG;
> - tag_bit = i % BITS_PER_LONG;
> + tag_long = offset / BITS_PER_LONG;
> + tag_bit = offset % BITS_PER_LONG;
> iter->tags = node->tags[tag][tag_long]>> tag_bit;
> /* This never happens if RADIX_TREE_TAG_LONGS == 1 */
> if (tag_long< RADIX_TREE_TAG_LONGS - 1) {
> @@ -755,7 +759,7 @@ restart:
> }
> }
>
> - return node->slots + i;
> + return node->slots + offset;
> }
> EXPORT_SYMBOL(radix_tree_next_chunk);
>
> _
>
>
>
> And here are some changes I made to that:
>
> --- a/include/linux/radix-tree.h~radix-tree-introduce-bit-optimized-iterator-v3-fix
> +++ a/include/linux/radix-tree.h
> @@ -265,11 +265,12 @@ static inline void radix_tree_preload_en
> * @next_index: next-to-last index for this chunk
> * @tags: bit-mask for tag-iterating
> *
> - * Radix tree iterator works in terms of "chunks" of slots.
> - * Chunk is sub-interval of slots contained in one radix tree leaf node.
> - * It described by pointer to its first slot and struct radix_tree_iter
> - * which holds chunk position in tree and its size. For tagged iterating
> - * radix_tree_iter also holds slots' bit-mask for one chosen radix tree tag.
> + * This radix tree iterator works in terms of "chunks" of slots. A chunk is a
> + * subinterval of slots contained within one radix tree leaf node. It is
> + * described by a pointer to its first slot and a struct radix_tree_iter
> + * which holds the chunk's position in the tree and its size. For tagged
> + * iteration radix_tree_iter also holds the slots' bit-mask for one chosen
> + * radix tree tag.
> */
> struct radix_tree_iter {
> unsigned long index;
> @@ -292,12 +293,12 @@ static __always_inline void **
> radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
> {
> /*
> - * Leave iter->tags unitialized. radix_tree_next_chunk()
> - * anyway fill it in case successful tagged chunk lookup.
> - * At unsuccessful or non-tagged lookup nobody cares about it.
> + * Leave iter->tags uninitialized. radix_tree_next_chunk() will fill it
> + * in the case of a successful tagged chunk lookup. If the lookup was
> + * unsuccessful or non-tagged then nobody cares about ->tags.
> *
> * Set index to zero to bypass next_index overflow protection.
> - * See comment inside radix_tree_next_chunk() for details.
> + * See the comment in radix_tree_next_chunk() for details.
> */
> iter->index = 0;
> iter->next_index = start;
> @@ -312,10 +313,10 @@ radix_tree_iter_init(struct radix_tree_i
> * @flags: RADIX_TREE_ITER_* flags and tag index
> * Returns: pointer to chunk first slot, or NULL if there no more left
> *
> - * This function lookup next chunk in the radix tree starting from
> - * @iter->next_index, it returns pointer to chunk first slot.
> + * This function looks up the next chunk in the radix tree starting from
> + * @iter->next_index. It returns a pointer to the chunk's first slot.
> * Also it fills @iter with data about chunk: position in the tree (index),
> - * its end (next_index), and construct bit mask for tagged iterating (tags).
> + * its end (next_index), and constructs a bit mask for tagged iterating (tags).
> */
> void **radix_tree_next_chunk(struct radix_tree_root *root,
> struct radix_tree_iter *iter, unsigned flags);
> @@ -340,7 +341,7 @@ radix_tree_chunk_size(struct radix_tree_
> * @flags: RADIX_TREE_ITER_*, should be constant
> * Returns: pointer to next slot, or NULL if there no more left
> *
> - * This function updates @iter->index in case successful lookup.
> + * This function updates @iter->index in the case of a successful lookup.
> * For tagged lookup it also eats @iter->tags.
> */
> static __always_inline void **
> @@ -396,8 +397,8 @@ radix_tree_next_slot(void **slot, struct
> * @iter: the struct radix_tree_iter pointer
> * @flags: RADIX_TREE_ITER_*, should be constant
> *
> - * This macro supposed to be nested inside radix_tree_for_each_chunk().
> - * @slot points to radix tree slot, @iter->index contains its index.
> + * This macro is designed to be nested inside radix_tree_for_each_chunk().
> + * @slot points to the radix tree slot, @iter->index contains its index.
> */
> #define radix_tree_for_each_chunk_slot(slot, iter, flags) \
> for (; slot ; slot = radix_tree_next_slot(slot, iter, flags))
> diff -puN lib/radix-tree.c~radix-tree-introduce-bit-optimized-iterator-v3-fix lib/radix-tree.c
> --- a/lib/radix-tree.c~radix-tree-introduce-bit-optimized-iterator-v3-fix
> +++ a/lib/radix-tree.c
> @@ -670,8 +670,8 @@ void **radix_tree_next_chunk(struct radi
>
> /*
> * Catch next_index overflow after ~0UL. iter->index never overflows
> - * during iterating, it can be zero only at the beginning.
> - * And we cannot overflow iter->next_index in single step,
> + * during iterating; it can be zero only at the beginning.
> + * And we cannot overflow iter->next_index in a single step,
> * because RADIX_TREE_MAP_SHIFT< BITS_PER_LONG.
> */
> index = iter->next_index;
> _
>
>
Yes, I screwed up with the spelling, again =(
Please merge your changes into final patch =)
>
> The comment over radix_tree_next_slot() is a bit terse: "eats
> @iter->tags". Can you please explain what's going on with iter->tags?
> afacit it gets copied from the radix-tree node's relevant tag field and
> then gets right-shifted as we advance across the radix-tree node, so
> that bit 0 of iter->tags always reflects the state of the tag bit for
> the slot which we're currently processing?
Yes, radix_tree_next_slot() search next tag with __ffs() and shift bits to right.
So bit 0 represent tag for current slot.
Also these is fast-path for testing tag for slot next after current,
because this is much likely case and this test works much faster than __ffs().
Usually bit 0 is set, except case if rcu-protected radix_tree_next_chink() is raced
with it's clearing, so this function saw this bit once in lookup loop. But when it comes
to iter->tags constructing this bit already cleared, but we expose this slot into iteration anyway.
iter->tags is for internal use, so this is ok, but maybe we should document this corner case.
>
> Why was it done this way, rather than simply testing the appropriate
> bit within the radix-tree node's tag? That would do away with
> iter->tags altogether?
>
Because nested loop (which uses radix_tree_next_slot()) is inlined into outer code,
so here no pointers to radix_tree_node and even its type declaration.
Also this approach makes inlined code smaller and faster, because we prepare data for processing.
On 32-bit systems iterating is little bit twisted, because not all tags fits
into single unsigned long iter->tags field, but it still faster than old implementation.
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