In gdb, how do I disassemble the previous instruction of an address?

Question

We know that disassembling instructions after a given address (inclusive) can be achieved by something like:

x/5i address

which will print 5 instructions, but how do I disassemble the previous instruction?

I am debugging JIT code so things like disassembling a line doesn't work. I could disassemble a random range containing address like:

disas address-10 address+10

but this is very awkward and you'll see (bad) (hopefully not in the middle!) and start to worry that you are not getting something right. What I am looking for is something like:

x/-5i address

, but the above won't work.

Answer 1

x/-5i address doesn't work

On x86, or any architecture with variable instruction size, you can't in general know the address of the start of previous instruction, and so you can't reliably disassemble previous instruction.

What I do (very similar to what you do): x/15i $pc-35. When you step back by sufficient number of bytes (35 here) the instruction stream disassembly usually re-synchronizes, you only see one or two (bad) instructions at the beginning, but instructions around $pc look correct.