[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Tinycc-devel] Stack Overflow question about jmp
From: |
Thomas Preud'homme |
Subject: |
Re: [Tinycc-devel] Stack Overflow question about jmp |
Date: |
Sun, 22 Sep 2013 19:55:11 +0200 |
User-agent: |
KMail/4.10.5 (Linux/3.10-3-amd64; KDE/4.10.5; x86_64; ; ) |
Le dimanche 22 septembre 2013 18:16:06 grischka a écrit :
> Thomas Preud'homme wrote:
> > So I looked at it and the reason it works on x86_64 but not on i386 target
> > is that for i386 target it only works when I386_ASM_16 is defined (16-bit
> > support). It seems from (i386|x86_64)-asm.h that jmp can take a register
> > (such as %eax) instead of an address (*%eax). I guess in one case
> > (register) it does a jump relative to the beginning of the segment while
> > for the address case it's an absolute jump at the address stored in the
> > register.
>
> I have no idea what the '*' means with "jmp *%eax" but according
> to gcc it doesn't make a difference whether or not it is there.
> (both emit FFE0).
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.
Note that in my case, on x86_64, both syntaxs are refused by gcc, even with
ecx as suggested in the stackoverflow link. 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?
>
> Beyond that
>
> error: unknown opcode 'jmp'
>
> is just a rather bad message. Maybe it could say something like
>
> error: bad operand '%eax' with opcode 'jmp'
A test could be added with a TOK_ASM_allfirst and TOK_ASM_alllast to
distinguish between incorrect operand and incorrect opcode.
>
> --- grischka
Thomas
signature.asc
Description: This is a digitally signed message part.