Re: GCC 4.1.2 generates different pentium instructions

[fafa <fafa at freesurf dot ch>]

Am 21.03.2007, 23:38 Uhr, schrieb Ian Lance Taylor <iant@google.com>:

> "H. J. Lu" <hjl@lucon.org> writes:
>
> > On Wed, Mar 21, 2007 at 09:19:44PM +0100, fafa wrote:
> > > Hi all,
> > >
> > > I noticed that G++ 4.1.2 (on a Pentium 4) generates different  
> > instructions
> > > for
> > >   lea    0x0(%esi),%esi
> > > or
> > >   lea    0x0(%edi),%edi
> > > with the same meaning but different encoding depending on the switch
> > > "-momit-leaf-frame-pointer".
> > >
> >
> > They are generated by assembler for different alignment adjustments.
>
> To expand on that, note that those instructions do nothing.  They are
> nops inserted for alignment purposes.  The size of the instruction
> varies depending upon how many bytes it has to take up.
>
> Ian

I see. But why not simple "nop" instructions ?
Is it just for the compactness of the listing, or does it
optimize the instruction prefetching of the CPU ?

Maett