Toru Nishimura wrote:

> Simon Burge wrote;
> 
> >> Could someone MIPS knowledgeful out there provide me the handy rule of
> >> thumb to use ".set noreorder" for the purpose of protecting
> >> instruction sequencing? 
> >
> > Do you mean "when to use it", or "what does it do"?
> 
> "When to use it."  Various locore stuff in arch/mips/mips looks over-
> protected by noreorder directive.  That was my initial concern.

Yes, I agree.  One ".set noreorder" at the top of each .S file seems a
little excessive!  I suspect mainly the m?c[01] and tlb instructions as
well as timing code (do we have asm delay() anywhere?) need it.

> > Normally the assembler is free to reorder instructions so that it can
> > avoid pipeline delays and fill the load/store/branch delay slot.  ".set
> > noreorder" is typically used for accessing system and device type
> > registers in a set order.
> 
> Mach code suggests the designer(s) used noreorder conservatively.  I'm
> not sure it does matter for behaviour difference between GCC and MIPS
> u-code based compiler.

I just had a quick look at locore*.o with the ".set noreorder" commented
out, and there doesn't appear to be that much change.  I say "doesn't
appear" because all the delay slots got rearranged since ".set reorder"
fills the delay slot for you.  It will be a lot of manual movement to
rearrage for the delay slots if we want to fix this up (which I think
will be a good thing).

Simon.