On Jun 4, 2008, at 12:27 AM, Rafael Espindola wrote:
Is there a specific reason you don't use the LLVM LTO interface?
It seems
to be roughly the same as your proposed interface:
a) it has a simple C interface like your proposed one
b) it is already implemented in one system linker (Apple's), so GCC
would
just provide its own linker plugin and it would work on apple
platforms
c) it is richer than your interface
d) it is battle tested, and exists today
e) it is completely independent of llvm (by design)
f) it is fully documented: http://llvm.org/docs/LinkTimeOptimization.html
Is there something specific you don't like about the LLVM interface?
We are still discussing how we are going to implement, so the API is
still not final. Some things that have been pointed out:
Hey Rafael!
*) Plugins could have other uses and the naming used on the LLVM LTO
interface is LTO specific.
The LLVM interface uses an lto_ prefix. This interface is used by nm/
ar/etc as well as the linker. Is there something specific about lto_
that is bad?
http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm-c/lto.h?revision=HEAD&view=markup
*) We have a normal symbol table on the .o files. It is not clear if
we should assume that it will always be the case. If so, we don't need
the API part that handles that.
This seems like a pretty minor point, but it would be easy to either:
1) make this an optional interface
2) make the plugin implement the symtab interfaces, but query the ELF
symbol table instead of the LTO symbol table if possible.
*) How do you handle the case of multiple symbols with the same name
(say a weak and a strong one)? lto_codegen_add_must_preserve_symbol
has a char* argument. How does it know which symbol we are talking
about?
The lto_symbol_attributes enum specifies linkage.
*) To save memory, one option is to have the plugin exec WPA and WPA
exec the linker again with the new objects. In this case the api
should be a bit different.
That's an interesting idea, but it is very unclear to me whether it
would save a significant amount of memory. Operating system VM
systems are pretty good at paging out data that isn't used (e.g.
the .o files the linker loaded into memory that exist when WPA is
going on).
-Chris
