Re: [PATCH 1/1] tcg: Adjust simd_desc size encoding

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

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Le mar. 1 sept. 2020 00:31, Richard Henderson <richard.henderson@linaro.org>
a écrit :

> With larger vector sizes, it turns out oprsz == maxsz, and we only
> need to represent mismatch for oprsz <= 32.  We do, however, need
> to represent larger oprsz and do so without reducing SIMD_DATA_BITS.
>
> Reduce the size of the oprsz field and increase the maxsz field.
> Steal the oprsz value of 24 to indicate equality with maxsz.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/tcg/tcg-gvec-desc.h | 38 ++++++++++++++++++++++++-------------
>  tcg/tcg-op-gvec.c           | 35 ++++++++++++++++++++++++++--------
>  2 files changed, 52 insertions(+), 21 deletions(-)
>
> diff --git a/include/tcg/tcg-gvec-desc.h b/include/tcg/tcg-gvec-desc.h
> index 0224ac3e78..704bd86454 100644
> --- a/include/tcg/tcg-gvec-desc.h
> +++ b/include/tcg/tcg-gvec-desc.h
> @@ -20,29 +20,41 @@
>  #ifndef TCG_TCG_GVEC_DESC_H
>  #define TCG_TCG_GVEC_DESC_H
>
> -/* ??? These bit widths are set for ARM SVE, maxing out at 256 byte
> vectors. */
> -#define SIMD_OPRSZ_SHIFT   0
> -#define SIMD_OPRSZ_BITS    5
> +/*
> + * This configuration allows MAXSZ to represent 2048 bytes, and
> + * OPRSZ to match MAXSZ, or represent the smaller values 8, 16, or 32.
> + *
> + * Encode this with:
> + *   0, 1, 3 -> 8, 16, 32
> + *   2       -> maxsz
> + *
> + * This steals the input that would otherwise map to 24 to match maxsz.
>

Nice trick :)

+ */
> +#define SIMD_MAXSZ_SHIFT   0
> +#define SIMD_MAXSZ_BITS    8
>
> -#define SIMD_MAXSZ_SHIFT   (SIMD_OPRSZ_SHIFT + SIMD_OPRSZ_BITS)
> -#define SIMD_MAXSZ_BITS    5
> +#define SIMD_OPRSZ_SHIFT   (SIMD_MAXSZ_SHIFT + SIMD_MAXSZ_BITS)
> +#define SIMD_OPRSZ_BITS    2
>
> -#define SIMD_DATA_SHIFT    (SIMD_MAXSZ_SHIFT + SIMD_MAXSZ_BITS)
> +#define SIMD_DATA_SHIFT    (SIMD_OPRSZ_SHIFT + SIMD_OPRSZ_BITS)
>  #define SIMD_DATA_BITS     (32 - SIMD_DATA_SHIFT)
>
>  /* Create a descriptor from components.  */
>  uint32_t simd_desc(uint32_t oprsz, uint32_t maxsz, int32_t data);
>
> -/* Extract the operation size from a descriptor.  */
> -static inline intptr_t simd_oprsz(uint32_t desc)
> -{
> -    return (extract32(desc, SIMD_OPRSZ_SHIFT, SIMD_OPRSZ_BITS) + 1) * 8;
> -}
> -
>  /* Extract the max vector size from a descriptor.  */
>  static inline intptr_t simd_maxsz(uint32_t desc)
>  {
> -    return (extract32(desc, SIMD_MAXSZ_SHIFT, SIMD_MAXSZ_BITS) + 1) * 8;
> +    return extract32(desc, SIMD_MAXSZ_SHIFT, SIMD_MAXSZ_BITS) * 8 + 8;
> +}
> +
> +/* Extract the operation size from a descriptor.  */
> +static inline intptr_t simd_oprsz(uint32_t desc)
> +{
> +    uint32_t f = extract32(desc, SIMD_OPRSZ_SHIFT, SIMD_OPRSZ_BITS);
> +    intptr_t o = f * 8 + 8;
> +    intptr_t m = simd_maxsz(desc);
> +    return f == 2 ? m : o;
>  }
>
>  /* Extract the operation-specific data from a descriptor.  */
> diff --git a/tcg/tcg-op-gvec.c b/tcg/tcg-op-gvec.c
> index 7ebd9e8298..ddbe06b71a 100644
> --- a/tcg/tcg-op-gvec.c
> +++ b/tcg/tcg-op-gvec.c
> @@ -37,11 +37,21 @@ static const TCGOpcode vecop_list_empty[1] = { 0 };
>     of the operand offsets so that we can check them all at once.  */
>  static void check_size_align(uint32_t oprsz, uint32_t maxsz, uint32_t ofs)
>  {
> -    uint32_t opr_align = oprsz >= 16 ? 15 : 7;
> -    uint32_t max_align = maxsz >= 16 || oprsz >= 16 ? 15 : 7;
> -    tcg_debug_assert(oprsz > 0);
> -    tcg_debug_assert(oprsz <= maxsz);
> -    tcg_debug_assert((oprsz & opr_align) == 0);
> +    uint32_t max_align;
> +
> +    switch (oprsz) {
> +    case 8:
> +    case 16:
> +    case 32:
> +        tcg_debug_assert(oprsz <= maxsz);
> +        break;
> +    default:
> +        tcg_debug_assert(oprsz == maxsz);
> +        break;
> +    }
> +    tcg_debug_assert(maxsz <= (8 << SIMD_MAXSZ_BITS));
> +
> +    max_align = maxsz >= 16 ? 15 : 7;
>      tcg_debug_assert((maxsz & max_align) == 0);
>      tcg_debug_assert((ofs & max_align) == 0);
>  }
> @@ -77,12 +87,21 @@ uint32_t simd_desc(uint32_t oprsz, uint32_t maxsz,
> int32_t data)
>  {
>      uint32_t desc = 0;
>
> -    assert(oprsz % 8 == 0 && oprsz <= (8 << SIMD_OPRSZ_BITS));
> -    assert(maxsz % 8 == 0 && maxsz <= (8 << SIMD_MAXSZ_BITS));
> -    assert(data == sextract32(data, 0, SIMD_DATA_BITS));
> +    check_size_align(oprsz, maxsz, 0);
> +    tcg_debug_assert(data == sextract32(data, 0, SIMD_DATA_BITS));
>
>      oprsz = (oprsz / 8) - 1;
>      maxsz = (maxsz / 8) - 1;
> +
> +    /*
> +     * We have just asserted in check_size_align that either
> +     * oprsz is {8,16,32} or matches maxsz.  Encode the final
> +     * case with '2', as that would otherwise map to 24.
> +     */
> +    if (oprsz == maxsz) {
> +        oprsz = 2;
> +    }
>

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

+
>      desc = deposit32(desc, SIMD_OPRSZ_SHIFT, SIMD_OPRSZ_BITS, oprsz);
>      desc = deposit32(desc, SIMD_MAXSZ_SHIFT, SIMD_MAXSZ_BITS, maxsz);
>      desc = deposit32(desc, SIMD_DATA_SHIFT, SIMD_DATA_BITS, data);
> --
> 2.25.1
>
>
>

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