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

["Maciej W. Rozycki" <macro@mips.com>]

On Sat, 10 Feb 2018, Fredrik Noring wrote:

> The short loop bug under certain conditions causes loops to execute
> only once or twice. GCC 2.95 that shipped with Sony PS2 Linux had a
> patch with the following note:
> 
>     On the R5900, we must ensure that the compiler never generates
>     loops that satisfy all of the following conditions:
> 
>     - a loop consists of less than equal to six instructions
>       (including the branch delay slot);
>     - a loop contains only one conditional branch instruction at
>       the end of the loop;
>     - a loop does not contain any other branch or jump instructions;
>     - a branch delay slot of the loop is not NOP (EE 2.9 or later).
> 
>     We need to do this because of a bug in the chip.
> 
> Signed-off-by: Fredrik Noring <noring@nocrew.org>
> ---
> 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).

  Maciej