[PATCH] D13501: [ELF2] Add -wrap switch

[Rui Ueyama via llvm-commits]

We are not necessarily aim 100% precise compatibility with GNU ld in every
detail. GNU ld may have implemented this feature in a way that's natural to
them. I'd really want to implement this feature in a way as the LLD
architecture is designed for. This --wrap option is by nature a bit hacky
option, and users need to understand what they are doing. GNU ld manual
does not mention about the details about how this option is supposed to
work.

On Wed, Oct 7, 2015 at 9:12 AM, Igor Kudrin <ikudrin.dev at gmail.com> wrote:

>
>
> On 07.10.2015 20:22, Rui Ueyama wrote:
>
> On Wed, Oct 7, 2015 at 6:59 AM, Igor Kudrin <ikudrin.dev at gmail.com> wrote:
>
>> ikudrin added a comment.
>>
>> In http://reviews.llvm.org/D13501#261600, @ruiu wrote:
>>
>> > This patch needs redesigning because we don't want to look up hash
>> tables more than once for each symbol. In this patch, names for undefined
>> symbols are looked up twice -- once in the InputFile.cpp and the other in
>> SymbolTable.cpp.
>>
>>
>> We have to look up for different names for defined and undefined symbols,
>> so we can't use just one hash map.
>>
>> In most cases, when the -wrap switch is not used and
>> UndefSymNameReplacement is empty, addition lookup will be very cheap,
>> without calculating a hash value at all. On the other hand, we can use just
>> std::map which expected to work really quick in our case.
>>
>>
> No, we don't have to look up hash table more than once. I thought a bit
> more about this and noticed that the LLD architecture (the separation of
> Symbol and SymbolBody) would really play well here. Symbols are just
> pointers to SymbolBodies, and symbol renaming is as easy as single pointer
> mutation. Here's the idea.
>
> First we resolve all symbols without considering --wrap option (so no
> overhead for --wrap during file parsing and symbol resolution). When symbol
> resolution is done, we basically have three Symbols for --wrap'ed symbol S
> in the symbol table: S, __wrap_S, and __real_S. Let X, Y and Z be their
> SymbolBodies, respectively.
>
> Originally the relationships are
>
>   S -> X
>   __wrap_S -> Y
>   __real_S -> Z
>
> We swap the pointers so that their relationships are
>
>   S -> Y
>   __wrap_S -> Y
>   __real_S -> X
>
> Now you can see that all symbols that would have been resolved to S
> without --wrap are now resolved as if they were __wrap_S. __real_S is also
> properly resolved to the symbol that was once the real symbol pointed to.
> This is the same result as what --wrap expects.
>
>
> Thank you for explaining your idea. It can work in some cases, but I dread
> it's unable to cover all situations.
>
> 1) Suppose we have all three symbols defined. The command like "ld.lld2
> a.o -wrap=S" in your case will eliminate Z and change content for symbols S
> and __real_S (see SymbolTableSection<ELFT>::writeGlobalSymbols). That's not
> right, GNU's ld preserves symbols in this case.
>
> 2) Suppose that S and __wrap_S are defined in different object files,
> which lay in archives. With switch "-wrap=S" (and if we have a reference to
> S) we should not get the content of the first object file at all, but how
> can we avoid parsing it if there is the record 'S' in the SymbolTable?
>
> 3) If we generate DSO and have an unresolved undefined symbol S we have to
> rename it and store as __wrap_S. It'll require more complicated update of
> symbols in the symbol table to support this case.
>
> Finally, my approach, I hope, preserves the original idea of relationships
> in the symbol table. With my case, we will have links like these:
> (Body for undefined S, name is "__wrap_S") -> (Symbol with name
> "__wrap_S") -> (Body for defined symbol "__wrap_S")
> (Body for defined __wrap_S) -> (Symbol with name "__wrap_S") -> (Body for
> defined symbol "__wrap_S")
> (Body for defined S) -> (Symbol with name "S") -> (Body for defined symbol
> "S")
>
> With your proposal we'll end up with links like:
> (Body for undefined S, name is "S") -> (Symbol with name "S") -> (Body for
> defined symbol "__wrap_S")
> (Body for defined __wrap_S) -> (Symbol with name "__wrap_S") -> (Body for
> defined symbol "__wrap_S")
> (Body for defined S) -> (Symbol with name "S") -> (Body for defined symbol
> "__wrap_S")
>
> The last line is a bit unexpected and can be a source for unwitting bugs.
>
>
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