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Re: [PATCH v5 57/60] target/riscv: vector slide instructions


From: Richard Henderson
Subject: Re: [PATCH v5 57/60] target/riscv: vector slide instructions
Date: Sat, 14 Mar 2020 22:16:44 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 3/12/20 7:58 AM, LIU Zhiwei wrote:
> +#define GEN_VEXT_VSLIDEUP_VX(NAME, ETYPE, H, CLEAR_FN)                    \
> +void HELPER(NAME)(void *vd, void *v0, target_ulong s1, void *vs2,         \
> +        CPURISCVState *env, uint32_t desc)                                \
> +{                                                                         \
> +    uint32_t mlen = vext_mlen(desc);                                      \
> +    uint32_t vlmax = env_archcpu(env)->cfg.vlen / mlen;                   \
> +    uint32_t vm = vext_vm(desc);                                          \
> +    uint32_t vl = env->vl;                                                \
> +    uint32_t offset = s1, i;                                              \
> +                                                                          \
> +    if (offset > vl) {                                                    \
> +        offset = vl;                                                      \
> +    }                                                                     \

This isn't right.

> +    for (i = 0; i < vl; i++) {                                            \
> +        if (((i < offset)) || (!vm && !vext_elem_mask(v0, mlen, i))) {    \
> +            continue;                                                     \
> +        }                                                                 \
> +        *((ETYPE *)vd + H(i)) = *((ETYPE *)vs2 + H(i - offset));          \
> +    }                                                                     \
> +    if (i == 0) {                                                         \
> +        return;                                                           \
> +    }                                                                     \

You need to eliminate vl == 0 first, not last.
Then

    for (i = offset; i < vl; i++)

The types of i and vl need to be extended to target_ulong, so that you don't
incorrectly crop the input offset.

It may be worth special-casing vm=1, or hoisting it out of the loop.  The
operation becomes a memcpy (at least for little-endian) at that point.  See
swap_memmove in arm/sve_helper.c.


> +#define GEN_VEXT_VSLIDEDOWN_VX(NAME, ETYPE, H, CLEAR_FN)                  \
> +void HELPER(NAME)(void *vd, void *v0, target_ulong s1, void *vs2,         \
> +        CPURISCVState *env, uint32_t desc)                                \
> +{                                                                         \
> +    uint32_t mlen = vext_mlen(desc);                                      \
> +    uint32_t vlmax = env_archcpu(env)->cfg.vlen / mlen;                   \
> +    uint32_t vm = vext_vm(desc);                                          \
> +    uint32_t vl = env->vl;                                                \
> +    uint32_t offset = s1, i;                                              \
> +                                                                          \
> +    for (i = 0; i < vl; i++) {                                            \
> +        if (!vm && !vext_elem_mask(v0, mlen, i)) {                        \
> +            continue;                                                     \
> +        }                                                                 \
> +        if (i + offset < vlmax) {                                         \
> +            *((ETYPE *)vd + H(i)) = *((ETYPE *)vs2 + H(i + offset));      \

Again, eliminate vl == 0 first.  In fact, why don't we make that a global
request for all of the patches for the next revision.  Checking for i == 0 last
is silly, and checks for the zero twice: once in the loop bounds and again at
the end.

It is probably worth changing the loop bounds to

    if (offset >= vlmax) {
       max = 0;
    } else {
       max = MIN(vl, vlmax - offset);
    }
    for (i = 0; i < max; ++i)


> +        } else {                                                          \
> +            *((ETYPE *)vd + H(i)) = 0;                                    \
> +        }

Which lets these zeros merge into...

> +    for (; i < vlmax; i++) {                                              \
> +        CLEAR_FN(vd, vl, vl * sizeof(ETYPE), vlmax * sizeof(ETYPE));      \
> +    }                                                                     \

These zeros.

> +#define GEN_VEXT_VSLIDE1UP_VX(NAME, ETYPE, H, CLEAR_FN)                   \
> +void HELPER(NAME)(void *vd, void *v0, target_ulong s1, void *vs2,         \
> +        CPURISCVState *env, uint32_t desc)                                \
> +{                                                                         \
> +    uint32_t mlen = vext_mlen(desc);                                      \
> +    uint32_t vlmax = env_archcpu(env)->cfg.vlen / mlen;                   \
> +    uint32_t vm = vext_vm(desc);                                          \
> +    uint32_t vl = env->vl;                                                \
> +    uint32_t i;                                                           \
> +                                                                          \
> +    for (i = 0; i < vl; i++) {                                            \
> +        if (!vm && !vext_elem_mask(v0, mlen, i)) {                        \
> +            continue;                                                     \
> +        }                                                                 \
> +        if (i == 0) {                                                     \
> +            *((ETYPE *)vd + H(i)) = s1;                                   \
> +        } else {                                                          \
> +            *((ETYPE *)vd + H(i)) = *((ETYPE *)vs2 + H(i - 1));           \
> +        }                                                                 \
> +    }                                                                     \
> +    if (i == 0) {                                                         \
> +        return;                                                           \
> +    }                                                                     \
> +    for (; i < vlmax; i++) {                                              \
> +        CLEAR_FN(vd, vl, vl * sizeof(ETYPE), vlmax * sizeof(ETYPE));      \
> +    }                                                                     \
> +}

As a preference, I think you can do away with this helper.
Simply use the slideup helper with argument 1, and then
afterwards store the integer register into element 0.  You should be able to
re-use code from vmv.s.x for that.

> +#define GEN_VEXT_VSLIDE1DOWN_VX(NAME, ETYPE, H, CLEAR_FN)                 \
> +void HELPER(NAME)(void *vd, void *v0, target_ulong s1, void *vs2,         \
> +        CPURISCVState *env, uint32_t desc)                                \
> +{                                                                         \

Likewise.


r~



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