Re: [Qemu-devel] [RFC PATCH v2 0/9] target/ppc: convert VMX instructions to use TCG vector operations

[Richard Henderson <richard.henderson@linaro.org>]

On 12/17/18 4:23 AM, Mark Cave-Ayland wrote:
> NOTE: there are a lot of instructions that cannot (yet) be optimised to use TCG vector
> operations, however it struck me that there may be some potential for converting
> saturating add/sub and cmp instructions if there were a mechanism to return a set of
> flags indicating the result of the saturation/comparison.

There are also a lot of instructions that can be converted, but aren't:

* vspltis[bhw] can use tcg_gen_gvec_dup{8,16,32}i.

* vsplt{b,h,w} can use tcg_gen_gvec_dup_mem.

  Note that you'll need something like vec_reg_offset from
  target/arm/translate-a64.h to compute the offset of the
  specific byte/word/long from which we are to splat.

* vmr should be handled by having tcg_gen_gvec_or notice aofs == bofs.
  For ARM, we do special case this during translation.
  But since tcg/tcg-op.c does these things for tcg_gen_or_i64,
  we should probably handle the same set of transformations.

* vnot would need to be handled by actually adding a tcg_gen_gvec_nor
  and then also noticing aofs == bofs.

For saturation, I think the easiest thing to do is represent SAT as a
ppc_avr_t.  We notice saturation by also computing normal arithmetic and
comparing to see if they differ.  E.g.

    tcg_gen_gvec_add(vece, offsetof_avr_tmp,
                     offsetof(ra), offsetof(rb), 16, 16);
    tcg_gen_gvec_ssadd(vece, offsetof(rt),
                       offsetof(ra), offsetof(rb), 16, 16);
    tcg_gen_gvec_cmp(TCG_COND_NE, vece, offsetof_avr_tmp,
                     offsetof_avr_tmp, offsetof(rt), 16, 16);
    tcg_gen_gvec_or(vece, offsetof_avr_sat, offsetof_avr_sat,
                    offsetof_avr_tmp, 16, 16);

You only need to convert the ppc_avr_t to a single bit when reading VSCR.

For comparisons... that's tricky.  I wonder if there's anything better than

    tcg_gen_gvec_cmp(TCG_COND_FOO, vece, offsetof(rt),
                     offsetof(ra), offsetof(rb), 16, 16);
    if (rc) {
        TCGv_i64 hi, lo, t, f;

        tcg_gen_ld_i64(hi, cpu_env, offsetof(rt));
        tcg_gen_ld_i64(lo, cpu_env, offsetof(rt) + 8);

        tcg_gen_and_i64(t, hi, lo);
        tcg_gen_or_i64(f, hi, lo);
        tcg_gen_setcondi_i64(TCG_COND_EQ, t, t, -1);
        tcg_gen_setcondi_i64(TCG_COND_EQ, f, f, 0);

        // truncate to i32, shift, or, and set to cr6.
    }


r~