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..

Can you call a dummy function that reads it off the stack? I don't know MIPS, but just an idea. — FatalError, Mar 11 '13 at 04:07

Zach Riggle · Accepted Answer · 2013-03-11T04:15:05.950

8

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.

edited Mar 11 '13 at 04:15

answered Mar 11 '13 at 04:09

Zach Riggle

2,975
19
26

1

You both are right, I choose you since you are the first one ^_^ – Randy Mar 11 '13 at 05:09
Thanks! (Need 15 chars to post) – Zach Riggle Mar 11 '13 at 05:10

score 3 · Answer 2 · answered Mar 11 '13 at 04:18

3

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.

answered Mar 11 '13 at 04:18

Grady Player

14,399
2
48
76

score 2 · Answer 3 · answered May 23 '15 at 12:07

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.

A more portable way would be to use GCC's builtins to retrieve the return address: `__builtin_extract_return_addr (__builtin_return_address (0))` instead of the inline assembly. — stefanct, Dec 22 '22 at 20:34

how to get current PC register value on MIPS arch?

3 Answers3