Re: [Qemu-devel] [PATCH v5 08/19] tcg-arm: Improve constant generation

[Aurelien Jarno <aurelien@aurel32.net>]

On Sun, Mar 31, 2013 at 03:34:54PM -0700, Richard Henderson wrote:
> Try fully rotated arguments to mov and mvn before trying movt
> or full decomposition.  Begin decomposition with mvn when it
> looks like it'll help.  Examples include
> 
> -:        mov   r9, #0x00000fa0
> -:        orr   r9, r9, #0x000ee000
> -:        orr   r9, r9, #0x0ff00000
> -:        orr   r9, r9, #0xf0000000
> +:        mvn   r9, #0x0000005f
> +:        eor   r9, r9, #0x00011000
> 
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/arm/tcg-target.c | 67 ++++++++++++++++++++++++++++++++++------------------
>  1 file changed, 44 insertions(+), 23 deletions(-)
> 
> diff --git a/tcg/arm/tcg-target.c b/tcg/arm/tcg-target.c
> index 9e8c97c..1f38795 100644
> --- a/tcg/arm/tcg-target.c
> +++ b/tcg/arm/tcg-target.c
> @@ -427,15 +427,31 @@ static inline void tcg_out_dat_imm(TCGContext *s,
>                      (rn << 16) | (rd << 12) | im);
>  }
>  
> -static inline void tcg_out_movi32(TCGContext *s,
> -                int cond, int rd, uint32_t arg)
> -{
> -    /* TODO: This is very suboptimal, we can easily have a constant
> -     * pool somewhere after all the instructions.  */
> -    if ((int)arg < 0 && (int)arg >= -0x100) {
> -        tcg_out_dat_imm(s, cond, ARITH_MVN, rd, 0, (~arg) & 0xff);
> -    } else if (use_armv7_instructions) {
> -        /* use movw/movt */
> +static void tcg_out_movi32(TCGContext *s, int cond, int rd, uint32_t arg)
> +{
> +    int rot, opc, rn;
> +
> +    /* For armv7, make sure not to use movw+movt when mov/mvn would do.
> +       Speed things up by only checking when movt would be required.
> +       Prior to armv7, have one go at fully rotated immediates before
> +       doing the decomposition thing below.  */
> +    if (!use_armv7_instructions || (arg & 0xffff0000)) {
> +        rot = encode_imm(arg);
> +        if (rot >= 0) {
> +            tcg_out_dat_imm(s, cond, ARITH_MOV, rd, 0,
> +                            rotl(arg, rot) | (rot << 7));
> +            return;
> +        }
> +        rot = encode_imm(~arg);
> +        if (rot >= 0) {
> +            tcg_out_dat_imm(s, cond, ARITH_MVN, rd, 0,
> +                            rotl(~arg, rot) | (rot << 7));
> +            return;
> +        }
> +    }
> +
> +    /* Use movw + movt.  */
> +    if (use_armv7_instructions) {
>          /* movw */
>          tcg_out32(s, (cond << 28) | 0x03000000 | (rd << 12)
>                    | ((arg << 4) & 0x000f0000) | (arg & 0xfff));
> @@ -444,22 +460,27 @@ static inline void tcg_out_movi32(TCGContext *s,
>              tcg_out32(s, (cond << 28) | 0x03400000 | (rd << 12)
>                        | ((arg >> 12) & 0x000f0000) | ((arg >> 16) & 0xfff));
>          }
> -    } 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);
> +        return;
> +    }
>  
> -            opc = ARITH_ORR;
> -            rn = rd;
> -        } while (arg);
> +    /* TODO: This is very suboptimal, we can easily have a constant
> +       pool somewhere after all the instructions.  */
> +    opc = ARITH_MOV;
> +    rn = 0;
> +    /* If we have lots of leading 1's, we can shorten the sequence by
> +       beginning with mvn and then clearing higher bits with eor.  */
> +    if (clz32(~arg) > clz32(arg)) {
> +        opc = ARITH_MVN, arg = ~arg;
>      }
> +    do {
> +        int 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_EOR;
> +        rn = rd;
> +    } while (arg);
>  }
>  
>  static inline void tcg_out_dat_rI(TCGContext *s, int cond, int opc, TCGArg dst,

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net