Re: [PATCH] Support for MIPS R5900 (Sony Playstation 2)

["Maciej W. Rozycki" <macro at codesourcery dot com>]

On Tue, 8 Jan 2013, "Jürgen Urban" wrote:

> >  As a side note -- I've noticed elsewhere in this change that the R5900 
> > supports single-precision FP only.  This is similar to what the 
> > R4640/R4650 does (up to supporting 32 single registers in the FR mode -- 
> > is that any different to the R5900?), however up till now we did nothing 
> > explicit for such a setup, except from respecting the -mfp32 option.  
> > This change introduces extra bits however, like hardwiring -mfp32 -- 
> > perhaps they could be generalised in some way to cover the R4640/R4650 as 
> > well? -- again, for consistency.
> 
> The patch for R5900 currently sets -mfp32 with the line:
> 
> if (mips_opts.arch == CPU_R5900)
>   {
>     mips_opts.fp32 = 1;
>   }
> else
>   ...
> 
> This avoids updating the FPU to 64 bit when setting higher mips ISA level.
> The R4640/R4650 can be added here. Is this what you want?

 I've thought of a more general approach, for example marking the missing 
instructions to support the unimplemented double format for the 
R4640/R4650 as well (or perhaps the other way round -- whenever -mfp32 is 
active rather than for any specific processor, as otherwise these 
instructions are in a sense valid, as the MIPS FPU by definition allows 
deferring any operation to software emulation).  There may be something 
else too.

 In any case, please don't feel compelled to make these changes -- unless 
you really want to.  I just raised it as a general point in discussion.

  Maciej