how to know where is the register variable stored?

Question

I knew that register variables are stored in CPU registers.

And the same variables are stored in stack if the CPU registers are busy/full.

how can i know that the variable is stored in stack or CPU register?

Answer 1

I am agree with Mr. Unwind's answer, but upto some extend this way may be helpful to you:

file name x.c:

int main(){
    register int i=0; 
    i++;
    printf("%d",i);
} 

Assemble code:

~$ gcc x.c -S  

output file name is x.s.

In my case ebx register is used, which may be difference at different compilation time.

~$ cat x.s
    .file   "x.c"
    .section    .rodata
.LC0:
    .string "%d"
    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    pushl   %ebx
    subl    $28, %esp
    movl    $0, %ebx
    addl    $1, %ebx             // because i++
    movl    $.LC0, %eax
    movl    %ebx, 4(%esp)
    movl    %eax, (%esp)
    call    printf
    addl    $28, %esp
    popl    %ebx
    movl    %ebp, %esp
    popl    %ebp
    ret    

You can also disassemble your executable using objdunp:

$ gcc x.c -o x 
$ objdump x -d  

Partial assembly output using objdump command:

080483c4 <main>:
 80483c4:   55                      push   %ebp
 80483c5:   89 e5                   mov    %esp,%ebp
 80483c7:   83 e4 f0                and    $0xfffffff0,%esp
 80483ca:   53                      push   %ebx
 80483cb:   83 ec 1c                sub    $0x1c,%esp
 80483ce:   bb 00 00 00 00          mov    $0x0,%ebx
 80483d3:   83 c3 01                add    $0x1,%ebx          //due to i++
 80483d6:   b8 b0 84 04 08          mov    $0x80484b0,%eax
 80483db:   89 5c 24 04             mov    %ebx,0x4(%esp)
 80483df:   89 04 24                mov    %eax,(%esp)
 80483e2:   e8 0d ff ff ff          call   80482f4 <printf@plt>
 80483e7:   83 c4 1c                add    $0x1c,%esp
 80483ea:   5b                      pop    %ebx
 80483eb:   89 ec                   mov    %ebp,%esp
 80483ed:   5d                      pop    %ebp
 80483ee:   c3                      ret    
 80483ef:   90                      nop

%ebx register reserved for register variable.

Answer 2

No, you can't.

It's decided by the compiler, and might change between compilations if, for instance, the surrounding code changes the register pressure or if compiler flags are changed.