Recently I've been taking a foray into the ARM ABI to port the Glasgow Haskell Compiler's internal linker to ARM. One question I've run into is how to handle the case of interworking with R_ARM_JUMP24. This particular relocation could be generated often by GHC as a result of tail call optimization.
According to the ELF for ARM specification, this case requires the generation of veneer code to handle the instruction set switch. My question is where can one reliably place this veneer such that it is within the 2^11 window permitted by the relevant instructions. This is made difficult considering the fact that in the general case .text itself may be over 2^11 bytes long. It seems that the ELF for ARM specification gives a format for veneer symbol naming in appendix B, but compilers are not required to support this. How does one generally handle placing these relocations? Also, I have seen references[1] to an INTERWORK flag encoded somewhere in the ELF, but I haven't been able to find any references to this in the specification. Could someone point me in the right direction here? It has been suggested that the compiler might include an 8 byte vacancy for veneer after functions where this relocation might be necessary. I have seen that GCC has an -mthumb-interwork option and while the documentation does not directly say this is the option's effect, it is suggestive. Can someone explain exactly what this option does? Cheers, - Ben [1] http://gcc.gnu.org/ml/gcc/2004-03/msg00373.html
