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

[Igor Kudrin via llvm-commits]

They have quite a good explanation of the option's behavior and both ld 
and gold produce the same result. The option should only change the 
resolution target of the specified symbols and shouldn't do anything 
else; otherwise, the generated result will be unpredictable. In 
particular, we should neither change the name of a defined symbol nor 
remove it.

OK, I can suggest a bit different approach. What if we add a field to 
the Symbol class to redirect undefined symbol bodies to another Symbol? 
It'll require a small update in resolve() and maybe in a few other 
places, but the additional hash map will be gone. What do you think?

On 07.10.2015 22:38, Rui Ueyama wrote:
> 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 
> <mailto: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 <mailto: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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