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| -rw-r--r-- | lld/ELF/SyntheticSections.cpp | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp index 946914619bc..a10fa48991c 100644 --- a/lld/ELF/SyntheticSections.cpp +++ b/lld/ELF/SyntheticSections.cpp @@ -1413,6 +1413,37 @@ template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *Buf) { } } +// .hash and .gnu.hash sections contain on-disk hash tables that map +// symbol names to their dynamic symbol table indices. Their purpose +// is to help the dynamic linker resolve symbols quickly. If ELF files +// don't have them, the dynamic linker has to do linear search on all +// dynamic symbols, which makes programs slower. Therefore, a .hash +// section is added to a DSO by default. A .gnu.hash is added if you +// give the -hash-style=gnu or -hash-style=both option. +// +// The Unix semantics of resolving dynamic symbols is somewhat expensive. +// Each ELF file has a list of DSOs that the ELF file depends on and a +// list of dynamic symbols that need to be resolved from any of the +// DSOs. That means resolving all dynamic symbols takes O(m)*O(n) +// where m is the number of DSOs and n is the number of dynamic +// symbols. For modern large programs, both m and n are large. So +// making each step faster by using hash tables substiantially +// improves time to load programs. +// +// (Note that this is not the only way to design the shared library. +// For instance, the Windows DLL takes a different approach. On +// Windows, each dynamic symbol has a name of DLL from which the symbol +// has to be resolved. That makes the cost of symbol resolution O(n). +// This disables some hacky techniques you can use on Unix such as +// LD_PRELOAD, but this is arguably better semantics than the Unix ones.) +// +// Due to historical reasons, we have two different hash tables, .hash +// and .gnu.hash. They are for the same purpose, and .gnu.hash is a new +// and better version of .hash. .hash is just an on-disk hash table, but +// .gnu.hash has a bloom filter in addition to a hash table to skip +// DSOs very quickly. If you are sure that your dynamic linker knows +// about .gnu.hash, you want to specify -hash-style=gnu. Otherwise, a +// safe bet is to specify -hash-style=both for backward compatibilty. template <class ELFT> GnuHashTableSection<ELFT>::GnuHashTableSection() : SyntheticSection(SHF_ALLOC, SHT_GNU_HASH, sizeof(uintX_t), ".gnu.hash") { |
