[LLVMdev] Supporting multiple PIC/PLT formats per architecture

[Justin Hibbits]

Hi,

PowerPC, and SPARC, supports both -fpic and -fPIC with gcc, and it may
make a difference on other architectures as well. However, llvm
currently does not distinguish between them (see
http://llvm.org/bugs/show_bug.cgi?id=20385).  I currently have both
models working, but are mutually exclusive, since I haven't thought of
a good way of supporting them.  clang has some level of support, with
the '-pic-level' flag, but it's not documented, and I'm not clear if it
even passes this information down to llvm.

PowerPC makes this even more interesting, because it also supports a
'secure PLT' mode, which makes the PLT read-only.  I haven't verified,
but it likely also works on both pic levels (-fpic, -fPIC), so there
needs to be some way of supporting this as well.

One proposal I have is to have a '-pic-level' and '-secure-plt'
arguments for llc, supported by two extra flags in the class
MCCodeGenInfo, PICLevel, and IsSecurePLT, accessible through accessors
as the current flags are handled.  This way, existing code would behave
as normal, and these new capabilities could be handled seamlessly.

Comments?

- Justin