[LLVMdev] [lld] adding demangler for symbol resolution

Shankar Easwaran
2014-04-02T18:02:35Z

On 4/2/2014 12:23 PM, Nick Kledzik wrote:
> On Apr 1, 2014, at 9:19 PM, Shankar Easwaran wrote:
>
>> Hi Nick, Bigcheese,
>>
>> When lld is used to link C++ code, it would be required to demangle
symbol names by default/user driven option.
>>
>> The Gnu linker has the following options :-
>>
>> --demangle=[style]
>> --no-demangle
>>
>> I found that clang/llvm-symbolizer use __cxx_demangle function.
>>
>> I would think that lld also need to call the same function, and I think
the way we want to demangle is to have the function in LinkingContext as various
flavors may choose to use different API's to demangle symbol names.
>>
>> The API's that would be in LinkingContext would be :-
>>
>>         * virtual bool canDemangle()  = 0; // Does the flavor provide a
way to demangle symbol names ?
>>         * virtual std::string demangle(StringRef symbolName) = 0; //
demangle the symbol name
>>
>> Thoughts / Suggestions ?
> Wouldn't it be simpler to have one demangle() method that does nothing
(returns input string) if demangling is not available, the string is not a
mangled symbol, or demangling was turned off (--no-demangle).   Then, you just
wrap a demangle() call around every use.
Are you mentioning that one demangle function in LinkingContext ?

One demangle method wouldnt work as the ItaniumABI uses one method to
demangle, ARMCXXABI uses a different method, and MSVC uses a different
one. I am not sure about Mach-O here ?

> The __cxa_demangle function has an odd interface that requires a malloc
allocated block.  Having demangle() return a std::string means yet another
allocation. We might not care if this is just used in diagnostic outputs, but a
more efficient way would be to pass the stream object to demangle and have it
write directly to the stream instead of creating a std::string.
I dont know if diagnostics in clang, already redirect things directly to
a stream.

May be for now, as an initial implementation, we can have a single
demangle function that returns a std::string.

As part of this, I was thinking to cleanup the way the errors are
displayed to the user from the Resolver, we could have functions in
SymbolTable with

raiseError(SymbolErrorKind, filename, symbolname)
raiseError(SymbolErrorKind, filename, symbolname, filename, symbolname)

SymbolErrorKind :
MultipleDefinition
Undefined
GroupError
Note (for tracing)
...


> Seems like a demangling utility might be something to add at the LLVM
level.  Either directly to raw_ostream or a wrapper like format().
I have browsed discussions in llvm related to this to move the demangler
function which is housed in libcxx, and I dont think there is a plan to
move that.

I think the format() specifier would be one thing that would be useful,
but I am not sure on how different linking contexts in lld, could route
calls with a central format specifier.

Can you share more info on this ?

Thanks