[LLVMdev] [lld] ELF/Linux support

[Oleg Ranevskyy]

Hello LLVM community,

I am evaluating profitability of using the LLVM lld for linking our 
projects.

As a test, I tried to build the latest Boost with clang / lld on X86_64 
Ubuntu and faced a problem at the very first step of doing this. One of 
the Boost's auxiliary tools refers to the "environ" variable to access 
the environment variables. "environ" is defined in glibc and is actually 
a weak alias for the "__environ" variable. Here is a fragment of 
posix/environ.c:

char **__environ = NULL; // var pointing to env vars array
weak_alias (__environ, environ) // and two weak aliases to it
weak_alias (__environ, _environ)

When linking a program that accesses "environ" with binutils' ld, the 
symbol is correctly marked as a weak object:
   > nm a.out | grep environ
      0000000000601040 B __environ@@GLIBC_2.2.5
      0000000000601040 *V *environ@@GLIBC_2.2.5

LLVM lld marks it as uninitialized data from the BSS section:
      0000000000401168 *B* environ

Readelf also shows different symbols (readelf -s a.out | grep environ):
   - ld:
      4: 0000000000601040     8 OBJECT  WEAK   DEFAULT   25 
_environ at GLIBC_2.2.5 (2)
      5: 0000000000601040     8 OBJECT  WEAK   DEFAULT   25 
environ at GLIBC_2.2.5 (2)        <- environ is a weak object
      6: 0000000000601040     8 OBJECT  GLOBAL DEFAULT   25 
__environ at GLIBC_2.2.5 (2)   <- __environ itself is present as a global
     54: 0000000000601040     8 OBJECT  WEAK   DEFAULT   25 
environ@@GLIBC_2.2.5
     63: 0000000000601040     8 OBJECT  GLOBAL DEFAULT   25 
__environ@@GLIBC_2.2.5

   - lld
          1: 0000000000401168     8 OBJECT  GLOBAL DEFAULT   22 
environ    <- no __environ reference. Environ is a global, not weak
          8: 0000000000000000     8 OBJECT  GLOBAL DEFAULT  UND environ
        37: 0000000000401168     8 OBJECT  GLOBAL DEFAULT   22 environ

This leads to "environ" being nil when running the lld-linked binary.

Is it a known problem?
What is the fullness of ELF/Linux support?
Where can I find the features list that are expected to work and what 
work is still in progress?

Thank you!

Kind regards,
Oleg
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