[LLVMdev] How to represent zero-sized string?

[Bill Wendling]

On Jan 10, 2009, at 1:21 AM, Eli Friedman wrote:

> On Fri, Jan 9, 2009 at 5:40 PM, Evan Cheng <evan.cheng at apple.com>  
> wrote:
>> The difference seems innocent enough. However, in objc if the zero-
>> sized string is part of a cfstring, it causes a problem. The linker
>> expects it in the readonly __cstring section, but llvm puts it in the
>> read / write bss section.
>
> That seems extremely weird... what sort of magic is objc using that
> could possibly care where a string is stored?  Can you give a more
> complete testcase?   It sounds like LLVM isn't modelling something
> which it really should be...
>
The runtime probably has various requirements about this.

>> The problem is llvm represents this as
>>
>> @"\01LC" = internal constant [1 x i8] zeroinitializer
>>
>> CodeGen can tell it should go into a read only section, but it cannot
>> know it's a cstring. Any ideas how I can fix this? If I write the  
>> zero-
>> sized string as c"A\00", bitcode reader still turns it back to
>> zeroinitializer.
>
> LangRef claims that you can specify a section for globals, although I
> can't actually manage to get it to work...
>
We normally specify the sections in the config/darwin.c file for  
various special kinds of OBJC names (see  
darwin_objc_llvm_special_name_section()). Perhaps we could do the same  
for this? Obviously, it's not a special name, but if we could tag the  
global with the correct section then perhaps all will be well.

-bw