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Use more efficient encoding for string headers

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Since all string allocations are pointer-aligned to avoid aligning more
frequent node allocations, we can rely on that in string encoding.

Encoding page offset and block size in sizeof(void*) units increases the
maximum memory page size from 64k to 256k on 32-bit and 512k on 64-bit
platforms.

Fixes #35.
This commit is contained in:
Arseny Kapoulkine 2015-03-05 12:50:29 -08:00
parent 12e137d12f
commit 23060d0954
1 changed files with 11 additions and 7 deletions
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18
src/pugixml.cpp
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@ -400,7 +400,9 @@ PUGI__NS_BEGIN
char_t* allocate_string(size_t length) char_t* allocate_string(size_t length)
{ {
PUGI__STATIC_ASSERT(xml_memory_page_size <= (1 << 16)); static const size_t max_encoded_offset = (1 << 16) * sizeof(void*);
PUGI__STATIC_ASSERT(xml_memory_page_size <= max_encoded_offset);
// allocate memory for string and header block // allocate memory for string and header block
size_t size = sizeof(xml_memory_string_header) + length * sizeof(char_t); size_t size = sizeof(xml_memory_string_header) + length * sizeof(char_t);
@ -416,12 +418,14 @@ PUGI__NS_BEGIN
// setup header // setup header
ptrdiff_t page_offset = reinterpret_cast<char*>(header) - reinterpret_cast<char*>(page) - sizeof(xml_memory_page); ptrdiff_t page_offset = reinterpret_cast<char*>(header) - reinterpret_cast<char*>(page) - sizeof(xml_memory_page);
assert(page_offset >= 0 && page_offset < (1 << 16)); assert(page_offset % sizeof(void*) == 0);
header->page_offset = static_cast<uint16_t>(page_offset); assert(page_offset >= 0 && static_cast<size_t>(page_offset) < max_encoded_offset);
header->page_offset = static_cast<uint16_t>(static_cast<size_t>(page_offset) / sizeof(void*));
// full_size == 0 for large strings that occupy the whole page // full_size == 0 for large strings that occupy the whole page
assert(full_size < (1 << 16) || (page->busy_size == full_size && page_offset == 0)); assert(full_size % sizeof(void*) == 0);
header->full_size = static_cast<uint16_t>(full_size < (1 << 16) ? full_size : 0); assert(full_size < max_encoded_offset || (page->busy_size == full_size && page_offset == 0));
header->full_size = static_cast<uint16_t>(full_size < max_encoded_offset ? full_size / sizeof(void*) : 0);
// round-trip through void* to avoid 'cast increases required alignment of target type' warning // round-trip through void* to avoid 'cast increases required alignment of target type' warning
// header is guaranteed a pointer-sized alignment, which should be enough for char_t // header is guaranteed a pointer-sized alignment, which should be enough for char_t
@ -438,11 +442,11 @@ PUGI__NS_BEGIN
assert(header); assert(header);
// deallocate // deallocate
size_t page_offset = sizeof(xml_memory_page) + header->page_offset; size_t page_offset = sizeof(xml_memory_page) + header->page_offset * sizeof(void*);
xml_memory_page* page = reinterpret_cast<xml_memory_page*>(static_cast<void*>(reinterpret_cast<char*>(header) - page_offset)); xml_memory_page* page = reinterpret_cast<xml_memory_page*>(static_cast<void*>(reinterpret_cast<char*>(header) - page_offset));
// if full_size == 0 then this string occupies the whole page // if full_size == 0 then this string occupies the whole page
size_t full_size = header->full_size == 0 ? page->busy_size : header->full_size; size_t full_size = header->full_size == 0 ? page->busy_size : header->full_size * sizeof(void*);
deallocate_memory(header, full_size, page); deallocate_memory(header, full_size, page);
} }