Re: [Tinycc-devel] Stack Overflow question about jmp

[grischka]

Thomas Preud'homme wrote:
> x86_64-asm.h defines 3 kind of jmp instruction, 2 of them being:
>
> ALT(DEF_ASM_OP1(jmp, 0xff, 4, OPC_MODRM, OPT_INDIR))
> ALT(DEF_ASM_OP1(jmp, 0xff, 0, OPC_JMP | OPC_WL, OPT_REGW))
>
> so basically the star select between the two. *%eax means indirection so first
> kind, %eax means register so second choice.  If gcc treat both syntax as the
> same it means one of the two is incorrect.

Hard to say seen tcc's full spectrum of bugs there:
- emits wrong codes, e.g. for "jmp *(%eax)"
- accepts invalid input, e.g. "jmp *%eax"
- inconsistence wrt. optional forms: accepts "jmp %rax" but does
  not accept "jmp (%rax)"

(all for x86-64)

> Note that in my case, on x86_64, both syntaxs are refused by gcc, even with
> ecx as suggested in the stackoverflow link.

Sure.

> I've tried on i386 and as outputs:
> "Warning: indirect jmp without `*'" so I guess the second alternative is wrong
> and should be removed. Can someone confirm this?

Yes with gcc 4.5.  No with gcc 3.4.

--- grischka