Re: [Qemu-devel] [PATCH v6 2/7] target/mips: Support R5900 specific three-operand MULT and MULTU

[Maciej W. Rozycki]

On Sun, 30 Sep 2018, Philippe Mathieu-Daudé wrote:

> >> I'd use:
> >>
> >>            MIPS_INVAL("mul/div Toshiba");
> > 
> >  But just like `gen_mul_vr54xx' this function doesn't handle division!
> 
> Per the commit message, I understood this function would eventually
> handle "the R5900 specific pipeline 1 instruction variants MULT1,
> MULTU1, DIV1, DIVU1, ..."

 Toshiba division instructions do not support the extra operand and always 
return their result in the MD accumulator only.  The same applies to the 
pipeline 1 instruction variants, so I've been thinking that a different 
handler would better be used for DIV1 and DIVU1.  But maybe we can fold it 
together after all and just force `rd' to be 0 at the call site for 
DIV1/DIVU1.

 I'm not sure at this point which approach would be most beneficial, but 
as it stands the function does not handle division operations.  If it 
starts in the future, then we can rename/update it accordingly.

 Note that while the R5900 (shouldn't that be called TX59 actually?) or 
TX79 do not implement DMULT or DMULTU, the Tx49 does and they do support 
the extra `rd' operand there[1].  Still no DMADD or DMADDU though.

References:

[1] "64-Bit TX System RISC TX49/H2, TX49/H3, TX49/H4 Core Architecture", 
    Rev 1.0, Toshiba Corporation, September 2004, Table 5-8 "Extensions to 
    the ISA: Multiply and Divide Instructions", p. 5-7

  Maciej