BSD ld (was Re: MCLinker and llvm-config)

[David Chisnall]

On 28 Jul 2012, at 15:15, Pedro Giffuni wrote:

> The Elftoolchain project has been developing a BSD ld:
> 
> http://sourceforge.net/apps/trac/elftoolchain/ 
> 
> 
> http://sourceforge.net/apps/trac/elftoolchain/browser/trunk/ld 
> 
> 
> I thought that would be the official FreeBSD implementation.

There is not yet any consensus as to which linker will be the replacement, nor on whether the replacement in 10.0 will be the one that we use long-term.  The requirements for 10.0 are, I believe:

- Actually exists
- Capable of linking base on all tier 1 platforms (which, in 10.0, hopefully will include ARMv7)
- Cross-linking support (ideally without having to build multiple versions of the linker binary)
- Permissive license

The longer-term requirements are:

- Good integration with the rest of the toolchain, including
    - LTO support
    - Code reuse
- Ability to link all (or, at least initially, most) ports
- Easy to add support for new architectures
- Active upstream community
- Good performance (both in terms of linkage speed and generated binaries)

Unfortunately, in general the ELF toolchain project is duplicating a lot of the work done in LLVM and not in an especially reusable way.  For example, when you add a compiler back end in LLVM you get a disassembler for free and an assembler with a small amount of effort, which means that an LLVM-based objdump and as will share code with the compiler, meaning better testing and few places for bugs to hide.  It also means that when we get compiler support for a new platform from the compiler, we get those tools for free.  For example, llvm-objdump can disassemble the CHERI variant of the MIPS instruction set correctly (including identifying relocation types), but no other objdump can, and this did not require any objdump-specific code to be written.  

The linker's ELF generation support is similarly overlapping with that of the compiler, and I would much rather that we have a single implementation in the base system than two. 

David