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

[andrzej zaborowski]

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 :)

...
>         }
> +    } 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.

Best regards