Re: [Qemu-devel] [PATCH v4 22/33] tcg-aarch64: Use MOVN in tcg_out_movi

[Claudio Fontana <claudio.fontana@huawei.com>]

On 14.09.2013 23:54, Richard Henderson wrote:
> When profitable, initialize the register with MOVN instead of MOVZ,
> before setting the remaining lanes with MOVK.
> 
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/aarch64/tcg-target.c | 88 +++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 75 insertions(+), 13 deletions(-)
> 
> diff --git a/tcg/aarch64/tcg-target.c b/tcg/aarch64/tcg-target.c
> index f9319ed..cecda05 100644
> --- a/tcg/aarch64/tcg-target.c
> +++ b/tcg/aarch64/tcg-target.c
> @@ -559,24 +559,86 @@ static void tcg_out_movi(TCGContext *s, TCGType type, TCGReg rd,
>                           tcg_target_long value)
>  {
>      AArch64Insn insn;
> -
> -    if (type == TCG_TYPE_I32) {
> +    int i, wantinv, shift;
> +    tcg_target_long svalue = value;
> +    tcg_target_long ivalue, imask;
> +
> +    /* For 32-bit values, discard potential garbage in value.  For 64-bit
> +       values within [2**31, 2**32-1], we can create smaller sequences by
> +       interpreting this as a negative 32-bit number, while ensuring that
> +       the high 32 bits are cleared by setting SF=0.  */
> +    if (type == TCG_TYPE_I32 || (value & ~0xffffffffull) == 0) {
> +        svalue = (int32_t)value;
>          value = (uint32_t)value;
> +        type = TCG_TYPE_I32;
> +    }
> +
> +    /* Would it take fewer insns to begin with MOVN?  For the value and its
> +       inverse, count the number of 16-bit lanes that are 0.  For the benefit
> +       of 32-bit quantities, compare the zero-extended normal value vs the
> +       sign-extended inverted value.  For example,
> +            v = 0x00000000f100ffff, zeros = 2
> +           ~v = 0xffffffff0eff0000, zeros = 1
> +          ~sv = 0x000000000eff0000, zeros = 3
> +       By using ~sv we see that 3 > 2, leading us to emit just a single insn
> +       "movn ret, 0x0eff, lsl #16".  */
> +
> +    ivalue = ~svalue;
> +    imask = 0;
> +    wantinv = 0;
> +
> +    /* ??? This can be done in the simd unit without a loop:
> +        // Move value and ivalue into V0 and V1 respectively.
> +        mov     v0.d[0], value
> +        mov     v1.d[0], ivalue
> +        // Compare each 16-bit lane vs 0, producing -1 for true.
> +        cmeq    v0.4h, v0.4h, #0
> +        cmeq    v1.4h, v1.4h, #0
> +        mov     imask, v1.d[0]
> +        // Sum the comparisons, producing 0 to -4.
> +        addv    h0, v0.4h
> +        addv    h1, v1.4h
> +        // Subtract the two, forming a positive wantinv result.
> +        sub     v0.4h, v0.4h, v1.4h
> +        smov    wantinv, v0.h[0]
> +     */
> +    for (i = 0; i < 64; i += 16) {
> +        tcg_target_long mask = 0xffffull << i;
> +        if ((value & mask) == 0) {
> +            wantinv -= 1;
> +        }
> +        if ((ivalue & mask) == 0) {
> +            wantinv += 1;
> +            imask |= mask;
> +        }
>      }
>  
> -    /* count trailing zeros in 16 bit steps, mapping 64 to 0. Emit the
> -       first MOVZ with the half-word immediate skipping the zeros, with a shift
> -       (LSL) equal to this number. Then all next instructions use MOVKs.
> -       Zero the processed half-word in the value, continue until empty.
> -       We build the final result 16bits at a time with up to 4 instructions,
> -       but do not emit instructions for 16bit zero holes. */
> +    /* If we had more 0xffff than 0x0000, invert VALUE and use MOVN.  */
>      insn = INSN_MOVZ;
> -    do {
> -        unsigned shift = ctz64(value) & (63 & -16);
> -        tcg_fmt_Rd_uimm(s, insn, shift >= 32, rd, value >> shift, shift);
> +    if (wantinv > 0) {
> +        value = ivalue;
> +        insn = INSN_MOVN;
> +    }
> +
> +    /* Find the lowest lane that is not 0x0000.  */
> +    shift = ctz64(value) & (63 & -16);
> +    tcg_fmt_Rd_uimm(s, insn, type, rd, value >> shift, shift);
> +
> +    if (wantinv > 0) {
> +        /* Re-invert the value, so MOVK sees non-inverted bits.  */
> +        value = ~value;
> +        /* Clear out all the 0xffff lanes.  */
> +        value ^= imask;
> +    }
> +    /* Clear out the lane that we just set.  */
> +    value &= ~(0xffffUL << shift);
> +
> +    /* Iterate until all lanes have been set, and thus cleared from VALUE.  */
> +    while (value) {
> +        shift = ctz64(value) & (63 & -16);
> +        tcg_fmt_Rd_uimm(s, INSN_MOVK, type, rd, value >> shift, shift);
>          value &= ~(0xffffUL << shift);
> -        insn = INSN_MOVK;
> -    } while (value);
> +    }
>  }
>  
>  static inline void tcg_out_ldst_r(TCGContext *s,
> 

I agree in general with the approach "lets see if it is more convenient to start with MOVN".
The existing implementation is, although not easy, leaner.
Can we make it a little this one a little bit leaner?
I'll think myself about it as well.

C.