Re: Issue when using jit_label() before a jump or branch

[Franz Flasch]

The lightning version used is the latest official release: 2.1.3

I use it on a x86_64 machine.

Regards,
Franz

On 1/23/22 1:00 PM, Franz Flasch wrote:

Hi!

I think there is kind of an issue in GNU lightning when using a label as jump target if this label was created immediately before a jump or branch. Here is an example:

    /* get a label here */
    label = jit_label();

    /* Prepare forward jump */
    jump = jit_jmpi();
    forward = jit_forward();
    jit_patch_at(jump, forward);

    /* Some random instructions */
    jit_addi(JIT_R0, JIT_R0, 42);
   
    /* Link forward jump */
    jit_link(forward);

    /* Some random instructions */
    jit_addi(JIT_R0, JIT_R0, 42);

    /* Now prepare backward jump to first label */
    jump2 = jit_jmpi();
    jit_patch_at(jump2, label);

    jit_retr(JIT_R0);

The above instructions result into the following GNU lightning instructions:

    L0: %rax /* prolog */
        jmpi L2
    L4: %rax
        addi %rax %rax 0x2a
    L2: %rax
        addi %rax %rax 0x2a
        jmpi L2
    L5: %rax
        retr %rax
         \__ live %rax
         \__ ret
    L3: /* epilog */

As you can see the second jump is wrong. It jumps back to L2 even though it should jump to L0 (before the first jump). When I use the label from the first jump like this

    jit_patch_at(jump2, jump);

it works as expected.

But why can't I use a normal label here? Is this the intended behavior or is this a bug?

If there is another instruction between jit_label() and jit_jmpi() it also works as expected.

The same behavior also occurs with branches.

I could just use the first jump label to solve this issue, however my code generation engine currently does not know about this in advance so I rely on normal labels to work correctly.

Thanks,
Franz