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Optimize and refactor compact_pointer implementations

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Clarify the offset applied when encoding the pointer difference.
Make decoding diff slightly more clear - no effect on performance.

Adjust branch weighting in compact_string encoding - 0.5% faster.
Use uint16_t in compact_pointer_parent - 2% faster.
This commit is contained in:
Arseny Kapoulkine
2015-04-22 07:34:43 -07:00
parent 33b2efe318
commit 4649914447
1 changed files with 23 additions and 18 deletions
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41
src/pugixml.cpp
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@@ -760,7 +760,12 @@ PUGI__NS_BEGIN
{
if (value)
{
ptrdiff_t offset = ((reinterpret_cast<char*>(value) - reinterpret_cast<char*>(this) + int(compact_alignment - 1)) >> compact_alignment_log2) - start;
// value is guaranteed to be compact-aligned; this is not
// our decoding is based on this aligned to compact alignment downwards (see operator T*)
// so for negative offsets (e.g. -3) we need to adjust the diff by compact_alignment - 1 to
// compensate for arithmetic shift behavior for negative values
ptrdiff_t diff = reinterpret_cast<char*>(value) - reinterpret_cast<char*>(this);
ptrdiff_t offset = ((diff + int(compact_alignment - 1)) >> compact_alignment_log2) - start;
if (static_cast<uintptr_t>(offset) <= 253)
_data = static_cast<unsigned char>(offset + 1);
@@ -783,7 +788,7 @@ PUGI__NS_BEGIN
{
uintptr_t base = reinterpret_cast<uintptr_t>(this) & ~(compact_alignment - 1);
return reinterpret_cast<T*>(base + ((_data - (1 - start)) << compact_alignment_log2));
return reinterpret_cast<T*>(base + ((_data - 1 + start) << compact_alignment_log2));
}
else
return compact_get_value<header_offset, T>(this);
@@ -804,7 +809,7 @@ PUGI__NS_BEGIN
template <typename T, int header_offset> class compact_pointer_parent
{
public:
compact_pointer_parent(): _data0(0), _data1(0)
compact_pointer_parent(): _data(0)
{
}
@@ -817,12 +822,16 @@ PUGI__NS_BEGIN
{
if (value)
{
ptrdiff_t offset = ((reinterpret_cast<char*>(value) - reinterpret_cast<char*>(this) + int(compact_alignment - 1)) >> compact_alignment_log2) + 65533;
// value is guaranteed to be compact-aligned; this is not
// our decoding is based on this aligned to compact alignment downwards (see operator T*)
// so for negative offsets (e.g. -3) we need to adjust the diff by compact_alignment - 1 to
// compensate for arithmetic shift behavior for negative values
ptrdiff_t diff = reinterpret_cast<char*>(value) - reinterpret_cast<char*>(this);
ptrdiff_t offset = ((diff + int(compact_alignment - 1)) >> compact_alignment_log2) + 65533;
if (static_cast<uintptr_t>(offset) <= 65533)
{
_data0 = static_cast<unsigned char>(offset + 1);
_data1 = static_cast<unsigned char>((offset + 1) >> 8);
_data = static_cast<unsigned short>(offset + 1);
}
else
{
@@ -833,28 +842,25 @@ PUGI__NS_BEGIN
if (page->compact_shared_parent == value)
{
_data0 = 254;
_data1 = 255;
_data = 65534;
}
else
{
compact_set_value<header_offset>(this, value);
_data0 = 255;
_data1 = 255;
_data = 65535;
}
}
}
else
{
_data0 = 0;
_data1 = 0;
_data = 0;
}
}
operator T*() const
{
int data = _data0 + (_data1 << 8);
int data = _data;
if (data)
{
@@ -862,7 +868,7 @@ PUGI__NS_BEGIN
{
uintptr_t base = reinterpret_cast<uintptr_t>(this) & ~(compact_alignment - 1);
return reinterpret_cast<T*>(base + ((data - (1 + 65533)) << compact_alignment_log2));
return reinterpret_cast<T*>(base + ((data - 1 - 65533) << compact_alignment_log2));
}
else if (data == 65534)
return static_cast<T*>(compact_get_page(this, header_offset)->compact_shared_parent);
@@ -879,8 +885,7 @@ PUGI__NS_BEGIN
}
private:
unsigned char _data0;
unsigned char _data1;
uint16_t _data;
};
template <int header_offset> class compact_string
@@ -901,12 +906,12 @@ PUGI__NS_BEGIN
{
xml_memory_page* page = compact_get_page(this, header_offset);
if (page->compact_string_base == 0)
if (PUGI__UNLIKELY(page->compact_string_base == 0))
page->compact_string_base = value;
ptrdiff_t offset = value - page->compact_string_base;
if (static_cast<uintptr_t>(offset) >= 16777213)
if (PUGI__UNLIKELY(static_cast<uintptr_t>(offset) >= 16777213))
{
compact_set_value<header_offset>(this, value);