Generate unaligned memory access exception in PowerPC

Question

I have a huge source code that works on PowerPC. I need to port it to ARM. But, ARM generates h/w exception on unaligned memory accesses. So, I want to find all the possible instances where the unaligned memory access exception could occur. I've considered the following options.

1. Use -Wcast-align in gcc which would throw warnings for unaligned access.
2. Make the PowerPC generate unaligned exception. For ARM, there is an option /proc/cpu/alignment by which the user can decide how to handle the exception. But, there is no such option for PowerPC.

My questions are,

1. Is there a way to make the PowerPC generate unaligned memory access exception?
2. Is there a better way to find out all occurences of unaligned memory access in the source code?

Answer 1

1. It depends on your POWERPC processor. High end server processors like POWER8 will almost never generate alignment exceptions. That being said, often there is a HID SPR bit to make alignment exceptions occur more often. Either way, under Linux, the kernel will handle them and the user won't see it, other than a performance loss. You can set the prctl(PR_UNALIGN_SIGBUS) and this will make the kernel generate a SIGBUS, rather than handle them.

2. In linux with perf events you can use the alignment-faults events. eg "perf stat -e alignment-faults testcase". Also if you turn on CONFIG_PPC_EMULATED_STATS you will get a debugfs entry called "emulated_instructions" which has a entry for unaligned accesses.