commit f927f91ce03a63d5c8a2c1d9c62a4d813e48bc44
author Rui Ueyama <ruiu@google.com> Fri May 17 01:55:20 2019 +0000
committer Rui Ueyama <ruiu@google.com> Fri May 17 01:55:20 2019 +0000
tree 5ea8b87b3e8543f62da7c75e8c8dad392a9814f4
parent ccffa5973513a9285526f831e16e9be899c7ecbd

Move symbol resolution code out of SymbolTable class.

This is the last patch of the series of patches to make it possible to
resolve symbols without asking SymbolTable to do so.

The main point of this patch is the introduction of
`elf::resolveSymbol(Symbol *Old, Symbol *New)`. That function resolves
or merges given symbols by examining symbol types and call
replaceSymbol (which memcpy's New to Old) if necessary.

With the new function, we have now separated symbol resolution from
symbol lookup. If you already have a Symbol pointer, you can directly
resolve the symbol without asking SymbolTable to do that.

Now that the nice abstraction become available, I can start working on
performance improvement of the linker. As a starter, I'm thinking of
making --{start,end}-lib faster.

--{start,end}-lib is currently unnecessarily slow because it looks up
the symbol table twice for each symbol.

 - The first hash table lookup/insertion occurs when we instantiate a
   LazyObject file to insert LazyObject symbols.

 - The second hash table lookup/insertion occurs when we create an
   ObjFile from LazyObject file. That overwrites LazyObject symbols
   with Defined symbols.

I think it is not too hard to see how we can now eliminate the second
hash table lookup. We can keep LazyObject symbols in Step 1, and then
call elf::resolveSymbol() to do Step 2.

Differential Revision: https://reviews.llvm.org/D61898

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@360975 91177308-0d34-0410-b5e6-96231b3b80d8