Re: PATCH: Fix i386 disassembler with index == 0x4 in SIB (Re: objdump bug-report)

["H. J. Lu" <hjl at lucon dot org>]

On Thu, Jan 13, 2005 at 09:08:49AM -0800, H. J. Lu wrote:
> On Thu, Jan 13, 2005 at 02:14:40PM +1030, Alan Modra wrote:
> > On Wed, Jan 12, 2005 at 11:10:52AM -0800, H. J. Lu wrote:
> > > > 	.byte	0x8B, 0x04, 0x63	# effect is: movl (%ebx), %eax	
> > [snip]
> > > >  8048081:	8b 04 63             	mov    (%ebx,2),%eax
> > 
> > I don't agree that this is a problem.  In fact, I think that this
> > disassembly is more accurate than "mov (%ebx),%eax".  Note that gas
> > accepts "mov (%ebx,2),%eax" giving
> > Warning: scale factor of 2 without an index register
> 
> But it generates "8b 03", not "8b 04 63".
> 
> > 
> > Yes, I agree that the effect of executing these byte sequences is the
> > same as "mov (%ebx),%eax", but that's beside the point.  For example,
> > plenty of x86 instructions execute as a nop, but that doesn't mean they
> > should all be disassembled as "nop".  The disassembler ought to reflect
> > the machine encoding as closely as possible, and in this case that means
> > printing the ignored scale factor.
> > 
> > I think this change should be reverted.
> > 
> 
> IA-32 instruction reference manual says when INDEX == 0x4, scaled index
> is "[none]". Displaying "(%ebx,2)" is simply wrong here.
> 

I don't mind to revert my patch and make "mov (%ebx,2),%eax" a valid
instruction for "8b 04 63". I kind of like this approach.


H.J.