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

[Paul Cercueil]

Hi Franz,

I'm not really sure what could be the problem, sorry.

One thing I noticed though, is that you use jit_forward(). You actually 
do not need to use that.

Try this:

label = jit_label();
jump = jit_jmpi();

jit_addi(JIT_R0, JIT_R0, 42);

jit_patch(jump);

jit_addi(JIT_R0, JIT_R0, 42);

jump2 = jit_jmpi();
jit_patch_at(jump2, label);

jit_retr(JIT_R0);

A second thing to try: replace your jit_label() with a jit_indirect(). 
I actually always do that for backwards branches.

Cheers,
-Paul


Le dim., janv. 23 2022 at 13:00:13 +0100, Franz Flasch 
<franz.flasch@gmx.at> a écrit :
> 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