[Patch] [MIPS] Change opcode membership for the ssnop and ehb instructions

Maciej W. Rozycki macro@codesourcery.com
Tue May 25 02:08:00 GMT 2010


On Sun, 23 May 2010, Richard Sandiford wrote:

> > This patch changes the opcode membership for the ssnop instruction to 
> > I1 from I32 | N55.  It also changes the opcode membership of the ehb 
> > instruction to I32 from I33.  For those ASE's which don't support ehb 
> > and/or ssnop, the instruction executes as a nop.  This change make the 
> > disassembly read more cleanly.  Does this look okay to install?
> 
> I'm not convinced this is a good idea.  Do we really want:
> 
> 	.set mips32
>         ehb
> 
> to be acceptable?

 Well, EHB is an assembly idiom for "SLL $0, $0, 3" and likewise SSNOP for 
"SLL $0, $0, 1".  Both instructions were chosen deliberately to execute 
correctly back to the MIPS I ISA.  I see no harm with assembling them and, 
more importantly, they are much more readable in `objdump' or GDB where 
you don't have the luxury of a per-instruction ISA annotation.

 With an example hazard-clearing sequence like this:

 ssnop; ssnop; ehb

(EHB executes as SSNOP on pre-MIPS-r2 processors) I see no gain with 
interspersing this code with .set directives (or even wrapping it all).

 What's the cause of your concern?

  Maciej



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