Re: [Tinycc-devel] "error: invalid displacement" i386 1f 'L..1' does not get resolved

[Michael Matz]

Hello,

On Sun, 17 Apr 2022, Volodymyr Boyko wrote:

> Hi
> I'm trying to assemble the following snippet of code:
>
> .global sigsetjmp
> .global __sigsetjmp
> .type sigsetjmp,@function
> .type __sigsetjmp,@function
> sigsetjmp:
> __sigsetjmp:
>         mov 8(%esp),%ecx
>         jecxz 1f
>
>         mov 4(%esp),%eax
>         popl 24(%eax)
>         mov %ebx,28+8(%eax)
>         mov %eax,%ebx
>
> .hidden ___setjmp
>         call ___setjmp
>
>         pushl 24(%ebx)
>         mov %ebx,4(%esp)
>         mov %eax,8(%esp)
>         mov 28+8(%ebx),%ebx
>
> .hidden __sigsetjmp_tail
>         jmp __sigsetjmp_tail
>
> 1:      jmp ___setjmp
> but getting this error:
> src/signal/i386/sigsetjmp.s:8: error: invalid displacement
> After renaming/moving the label around I assume the issue is that local
> labels cannot be forward;1b and my_label moved to the top do work fine.

Yeah, it's related to that, but specific to the 'jexcz' opcode: the 
special thing about it is that it only comes in an 8-bit displacement form 
_and_ doesn't have an inverse form.  TCC, being single-pass, has to assume 
that forward labels could be anywhere, and hence potentially need a 32bit 
form (no matter that _in this case_ it would turn out to not be 
necessary), so jumps are rewritten into their disp32 forms for such 
labels.  For jecxz that can't be done.  And as the inverse form doesn't 
exist we can't even work around this by emitting something like a 
hypothetical:

  jecxnz +5   ; doesn't exist!
  jmp 1f

The GNU assembler uses multiple passes to figure out that the displacement 
in the testcase turns out to fit into a disp8 field and hence all is well. 
Add a couple more opcodes between the jecxz and the 1: label to make the 
distance larger than 128 and see it erroring out as well.

Supporting this requires some surgery (e.g. to support the _PC8 relocation 
and emitting them for this case only), so is of low priority.


Ciao,
Michael.