Re: [RFC] MIPS: R5900: Workaround for the short loop bug

[Fredrik Noring <noring@nocrew.org>]

Many thanks for your prompt and detailed reviews, Maciej,

> > The exact NOP placements in this patch are provisional. Request for comment
> > on the method to use. I believe there are at least three alternatives:
> > 
> > 1. Add #ifdefs or macros in the source code (similar to this patch).
> > 2. Modify the assembler to automatically insert NOPs as required.
> > 3. Avoid assembly and use C versions of memcpy etc. instead.
> > 
> > This change has been ported from v2.6 patches.
> 
>  I can't tell if this is a porting artefact or whether the reason is 
> different, but many of these loops contain more than 6 instructions 
> already, or need fewer than 3 NOPs.  Please review accordingly.
> 
>  Also can't this be handled automagically by GAS instead?  We have similar 
> workarounds already implemented, see e.g. `-mfix-vr4130'.  Otherwise this 
> is looking to me like a candidate for a maintenance nightmare (which the 
> problem with getting loop instruction counts wrong in your patch is a sign 
> of).

As noted above, please ignore the NOP details which just barely survived
from v2.6 (according to the principle that too many NOPs still work, whereas
too few crash badly), especially since I very much agree with you that it is
unreasonable to maintain such NOPs by hand and would rather proceed with
alternative (2) [from the list above] that is to modify the assembler instead.

Meanwhile, is it possible to run with alternative (3) that is to use C
fallbacks for the R5900, provided the performance penalty is reasonable?

Fredrik