Accessing a register without using inline assembly with gcc

Question

I want to read the stack pointer register value without writing inline assembly.The reason I want to do this is because I want to assign the stack pointer register value to an element of an array and I find it cumbersome to access an array using inline assembly. So I would want to do something like that.

register "rsp" long rsp_alias; <--- How do I achieve something like that in gcc?
long current_rsp_value[NUM_OF_THREADS];

current_rsp_value[tid] = rsp_alias;

Is there anything like that possible with gcc?

Will taking the address of a local variable be good enough for your application? — chill
– chill, Commented Nov 20, 2011 at 11:49

Mat · Accepted Answer · 2015-08-09 05:50:40Z

21

There's a shortcut:

register long rsp asm ("rsp");

Demo:

#include<stdio.h>

void foo(void)
{
    register long rsp asm ("rsp");
    printf("RSP: %lx\n", rsp);
}

int main()
{
    register long rsp asm ("rsp");
    printf("RSP: %lx\n", rsp);
    foo();
    return 0;
}

Gives:

 $ gdb ./a.out 
GNU gdb (Gentoo 7.2 p1) 7.2
...
Reading symbols from /home/user/tmp/a.out...done.
(gdb) break foo
Breakpoint 1 at 0x400538: file t.c, line 7.
(gdb) r
Starting program: /home/user/tmp/a.out 
RSP: 7fffffffdb90

Breakpoint 1, foo () at t.c:7
7       printf("RSP: %lx\n", rsp);
(gdb) info registers
....
rsp            0x7fffffffdb80   0x7fffffffdb80
....
(gdb) n
RSP: 7fffffffdb80
8   }

Taken from the Variables in Specified Registers documentation.

edited Aug 9, 2015 at 5:50

answered Nov 20, 2011 at 11:46

Mat

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4 Comments

MetallicPriest Over a year ago

Thanks Mat, this was the answer I was looking for.

Ciro Santilli OurBigBook.com Over a year ago

Isn't this just a shorthand for inline assembly? :-) I think the docs have moved to: gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Explicit-Reg-Vars.html

Sudip Das Over a year ago

how do i access accumulator data ??

Noah Over a year ago

Woth noting GCC very explicitly does not support this. Pinning register is only supported for use in extended assembly: gcc.gnu.org/onlinedocs/gcc-6.1.0/gcc/…

Anthony Blake · Accepted Answer · 2011-11-20 12:11:47Z

9

register const long rsp_alias asm volatile("rsp");

edited Nov 20, 2011 at 12:11

answered Nov 20, 2011 at 11:46

Anthony Blake

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5 Comments

MetallicPriest Over a year ago

Christoph, where should the const be placed, before register or after it?

Mat Over a year ago

@MetallicPriest: between register and long would be the most "natural" way, and indeed, it's a good idea.

Anthony Blake Over a year ago

yup -- I've noticed compiler optimizing away the selected register, thats why I included volatile.

Eric Over a year ago

I get a syntax error with volatile there (gcc 4.3)

Craig Ringer Over a year ago

@Eric same, and ` warning: optimization may eliminate reads and/or writes to register variables [-Wvolatile-register-var]` if I move it to a syntatically valid position.

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