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 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.


H.J.