Re: [RFC PATCH] MIPS: optimise 32-bit do_div() with constant divisor

[David Daney <ddaney.cavm@gmail.com>]

On 11/07/2014 07:00 AM, Måns Rullgård wrote:
[...]
>
>> As for access to hi/lo, I tried to explicitly put a variable in the lo
>> register.  Which sort of works for very simple cases but as expected it's
>> easy to get GCC to spill its RTL guts because it runs out of spill
>> registers.  It maybe can be made to work but I'd feel nervous about its
>> stability unless a GCC guru approved this method.
>
> The "x" constraint can be used to move a double-word to/from the hi/lo
> registers.  On DSP targets, the "ka" constraint provides access to the
> three additional hi/lo pairs while on a non-DSP targets it degenerates
> to "x".  The "ka" constraint is available since gcc 4.3.0.  I see no
> reason not to allow this extra flexibility.
>

What would the performance penalty be to hand code the assembly so that 
only mips32 instructions are used (i.e. no MADD), and transfers from 
hi/lo were all explicitly coded so that there were no hi/lo register 
constraints, but only clobbers of "hi", "lo"?

That would give you something usable on any 32-bit CPU with any version 
of GCC.

David Daney