Re: [Qemu-devel] [PATCH 3/3] tcg/arm: improve constant loading

[andrzej zaborowski]

On 7 January 2011 15:40, Aurelien Jarno <address@hidden> wrote:
> On Fri, Jan 07, 2011 at 01:52:25PM +0100, andrzej zaborowski wrote:
>> Hi,
>>
>> On 6 January 2011 22:54, Aurelien Jarno <address@hidden> wrote:
>> > Improve constant loading in two ways:
>> > - On all ARM versions, it's possible to load 0xffffff00 = -0x100 using
>> >  the mvn rd, #0. Fix the conditions.
>> > - On <= ARMv6 versions, where movw and movt are not available, load the
>> >  constants using mov and orr with rotations depending on the constant
>> >  to load. This is very useful for example to load constants where the
>> >  low byte is 0. This reduce the generated code size by about 7%.
>>
>> That's a nice improvement.  For some instructions using MVN and AND
>> could yield even shorter code and I think with that the optimisation
>> options (except loading from a constant pool) would be exhausted :)
>
> I also did something with MVN and BIC, it works well, but the problem is
> to find the right heuristic to choose between MOV/ORR and MVN/BIC. In my
> tries, it was making the code bigger.

I was thinking of running both without writing the instructions, then
comparing the lengths and then running the better method.  It's
possible that the cost of this outweights the shorter code advantage
though.

>
>> ...
>> >         }
>> > +    } else {
>> > +        int opc = ARITH_MOV;
>> > +        int rn = 0;
>> > +
>> > +        do {
>> > +            int i, rot;
>> > +
>> > +            i = ctz32(arg) & ~1;
>> > +            rot = ((32 - i) << 7) & 0xf00;
>> > +            tcg_out_dat_imm(s, cond, opc, rd, rn, ((arg >> i) & 0xff) | 
>> > rot);
>> > +            arg &= ~(0xff << i);
>> > +
>> > +            opc = ARITH_ORR;
>> > +            rn = rd;
>>
>> I think you could get rid of rn and just use rd from the start of the
>> loop.  Otherwise acked by me too.
>>
>
> What do you mean exactly? rn has to be 0 when opc is ARITH_MOV in order
> to generate a correct ARM instruction.

According to my ARM926 manual rn is ignored for MOV/MVN, perhaps it's
different in later revisions.

Cheers