Confusion in gcc switch case assembly

Question

My Prog.c contains a simple switch case as follows:

switch (x)
{
    case 1:
        p=2;
        break;
    case 2:
        p=3;
        break;
    case 3:
        p=4;
        break;
    case 4:
        p=5;
        break;
    case 5:
        p=6;
        break;
    default:
        break;
}

I compiled this program with

gcc -g -v prog.c

and created objdump with

objdump -S -l a.out>dump_file

For the switch case statement, I have found that one indirect branch has been created (jmp *%eax).

But if I remove any one of the case statement, no indirect branch is created.

gcc version: 5.4.0

I could not understand why this is happening?

It might be useful to show us the output. Currently you could simply do if (x>=1 && x<=5) p=x+1; or jump to an address calculated from x. This does not work if you have a gap in the cases list. — Gerhardh
– Gerhardh, Commented Nov 22, 2018 at 12:39
Compiler optimization decide to use indirect jumps than multiple comparisons. — EsmaeelE
– EsmaeelE, Commented Nov 22, 2018 at 12:40

EsmaeelE · Accepted Answer · 2025-04-17 07:54:33Z

0

This is happen because of Branch Tables. to avoid it compile with

gcc -g -v -fno-jump-tables SO.c

If you using jump tables (a feature in gcc compiler) during compile phase

the size of dump_file is: 9,965 bytes
But without it size is: 11,493 bytes.

Jump tables helps to reduce object code size by optimize and relocate some jump instructions.

edited Apr 17 at 7:54

answered Nov 22, 2018 at 12:47

EsmaeelE

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