Re: [Qemu-devel] [PATCH v2 1/2] ppc: Add support for 'mffscrn', 'mffscrni' instructions

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

On 9/16/19 1:02 PM, Paul A. Clarke wrote:
> +#define FP_DRN2         (1ull << FPSCR_DRN2)
> +#define FP_DRN1         (1ull << FPSCR_DRN1)
> +#define FP_DRN0         (1ull << FPSCR_DRN0)
> +#define FP_DRN          (FP_DRN2 | FP_DRN1 | FP_DRN0)

Why not just 7ull << FPSCR_DRN?
Are the individual DRN bits separately useful?
They don't appear to be...

> -#define FP_MODE         FP_RN
> +#define FP_MODE         (FP_DRN | FP_RN)

This, I think, isn't helpful.

> +static void gen_helper_mffscrn(DisasContext *ctx, TCGv_i64 t1)
> +{
> +    TCGv_i64 t0 = tcg_temp_new_i64();
> +    TCGv_i32 mask = tcg_const_i32(0x0001);
> +
> +    gen_reset_fpstatus();
> +    tcg_gen_extu_tl_i64(t0, cpu_fpscr);
> +    tcg_gen_andi_i64(t0, t0, FP_MODE | FP_ENABLES);
> +    set_fpr(rD(ctx->opcode), t0);
> +
> +    /* Mask FPSCR value to clear RN.  */
> +    tcg_gen_andi_i64(t0, t0, ~FP_MODE);

Because here,

> +static void gen_mffscrn(DisasContext *ctx)
> +{
> +    TCGv_i64 t1;
> +
> +    if (unlikely(!(ctx->insns_flags2 & PPC2_ISA300))) {
> +        return gen_mffs(ctx);
> +    }
> +
> +    if (unlikely(!ctx->fpu_enabled)) {
> +        gen_exception(ctx, POWERPC_EXCP_FPU);
> +        return;
> +    }
> +
> +    t1 = tcg_temp_new_i64();
> +    get_fpr(t1, rB(ctx->opcode));
> +    /* Mask FRB to get just RN.  */
> +    tcg_gen_andi_i64(t1, t1, FP_MODE);

and here, we're only interested in RN, not DRN.

Possibly FP_MODE should itself be removed.  It's used
exactly once, in mffsl, which could just as easily
reference FP_RN | FP_DRN.


r~