Re: PATCH: Add --alt-nops=short|long to x86/x86-64 assemblers

[Nick Clifton <nickc at redhat dot com>]

Hi H. J.

> > I am not familiar with the x86 instruction set, so please can you 
> > explain why it is necessary to have these two different versions of the 
> > nop and why the user has to select one ?  ie if the space to be padded 
> > is up to 10 bytes why can't the assembler just use the "short" version 
> > automatically and if it is longer than 10 bytes use the "long" version ?
>
> x86/x86-64 assemblers need to fill the text section from 1 to
> 15 bytes for alignment. We have simple nop instructions for 1 to
> 10 bytes. For 11 to 15 bytes, we can add 0x66 prefix repeatedly
> to 10 byte nop to get 11-15byte nops. However some processors
> prefer simple nops. That is, 3 0x66 prefixes on the 10 byte nop
> are slower on those processors than a 6 byte nop + a 7 byte nop.

Does the assembler not know the target processor variant ?  ie is the 
assembler unable to make an informed choice of which type of nop to use ?

> User can use --alt-nops=short to avoid repeated 0x66 prefixes.
> Maybe we can use something other than short|long. But I don't
> have a better name.

How about "--allow-long-nop-sequences" and its inverse 
"--no-allow-long-nop-sequences" ?  The default would presumably be to 
allow the long sequences since this is faster on more modern processors, 
yes ?

I would also suggest that you extend the description in your patch to 
c-i386.texi to cover the explanation you just gave me, unless you are 
sure that all x86 assembler programmers would know about these two types 
of nop.

Cheers
  Nick