how to get current PC register value on MIPS arch?

Question

I'd like to do backtrace on MIPS. Then, I face one problem: how do I get the current PC register value, since it doesn't belong to 32 normal registers.. Thanks for your suggestion..

Answer 1

Make a subroutine that looks somewhat like:

.text 
.globl GetIP 

GetIP:
move $v0, $ra
jr $ra

And then call the routine; it'll give you the address of the first instruction after the call.

Answer 2

after a jal call it will be copied to the ra register... so you could store ra, then jal to the next line, read ra, restore ra.

Answer 3

Although this question isn't tagged c, I figured it might be useful to share a solution utilizing inline assembly in gcc.

__attribute__((noinline)) static void *get_pc(void)
{
    void *pc;
    asm volatile ("move %0, $ra" : "=r"(pc));
    return pc;
}

Of course, the gist of the solution is the same as the currently accepted answer. Since the function is very small, it is a good candidate for inlining when optimization is turned on. However, if that function were inlined, its return value would be invalid : it would simply return some value of ra in the calling function, since a jal or jalr wouldn't be generated, and ra thus not set to the instruction following jal/jalr. This is why __attribute__((noinline)) is essential in this case.