Arm has two instruction sets, Arm and Thumb, and an execution mode for each.
Executing Arm code in Thumb mode or vice-versa will likely result in an
Illegal instruction exception.
Furthermore, Haskell code compiled via LLVM was generating Arm instructions
while C code compiled via GCC was generating Thumb code by default. When
these two object code types were being linked by the system linker, all was
fine, because the system linker knows how to jump and call from one
instruction set to the other.
The first problem was with GHCi's object code loader which did not know
about Thumb vs Arm. When loading an object file StgCRun would jump
into the loaded object which could change the mode causing a crash after
it returned. This was fixed by forcing all C code to generate Arm
instructions by passing -marm to GCC.
The second problem was the mkJumpToAddr function which was generating
Thumb instructions. Changing that to generate Arm instructions instead
results in a working GHCi on Arm.