I want to emulate the system with prohibited unaligned memory accesses on the x86/x86_64. Is there some debugging tool or special mode to do this?
I want to run many (CPU-intensive) tests on the several x86/x86_64 PCs when working with software (C/C++) designed for SPARC or some other similar CPU. But my access to Sparc is limited.
As I know, Sparc always checks alignment in memory reads and writes to be natural (reading a byte from any address, but reading a 4-byte word only allowed when address is divisible by 4).
May be Valgrind or PIN has such mode? Or special mode of compiler? I’m searching for Linux non-commercial tool, but windows tools allowed too.
or may be there is secret CPU flag in EFLAGS?
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Answer
It’s tricky and I haven’t done it personally, but I think you can do it in the following way:
x86_64 CPUs (specifically I’ve checked Intel Corei7 but I guess others as well) have a performance counter MISALIGN_MEM_REF which counter misaligned memory references.
So first of all, you can run your program and use “perf” tool under Linux to get a count of the number of misaligned access your code has done.
A more tricky and interesting hack would be to write a kernel module that programs the performance counter to generate an interrupt on overflow and get it to overflow the first unaligned load/store. Respond to this interrupt in your kernel module but sending a signal to your process.
This will, in effect, turn the x86_64 into a core that doesn’t support unaligned access.
This wont be simple though – beside your code, the system libraries also use unaligned accesses, so it will be tricky to separate them from your own code.