Tiny C Compiler: "error: unknown opcode 'jmp'"

Question

Given this code:

int main(void)
{
    __asm volatile ("jmp %eax");

    return 0;
}

32-bit TCC will complain with:

test.c:3: error: unknown opcode 'jmp'

but the 64-bit version will compile just fine.

What's the problem with the 32 bit code?

@Martin James: From here: "If our assembly statement must execute where we put it, (i.e. must not be moved out of a loop as an optimization), put the keyword volatile after asm and before the ()’s." — hauzer
– hauzer, Commented Aug 29, 2013 at 22:38

Community · Accepted Answer · 2017-05-23 11:54:02Z

1

The solution is to simply add a star (*) before the register, like this:

__asm volatile ("jmp *%eax");

I'm not exactly sure what the star means. According to this SO post:

The star is some syntactical sugar indicating that control is to be passed indirectly, by reference/pointer.

As for why it works with 64-bit TCC, I assume that it's a bug; 64-bit GCC complains with Error: operand type mismatch for 'jmp', as it should.

edited May 23, 2017 at 11:54

CommunityBot

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answered Aug 30, 2013 at 5:28

hauzer

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1 Comment

Peter Cordes Over a year ago

The * means it's an indirect jump, setting IP/EIP/RIP = value from register or memory, rather than jmp rel32 to a label address. TCC 0.9.27 still has this bug; I checked with objdump -d and it emits ff e0 jmp *%rax

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