Re: [Qemu-devel] [PATCH 2/2] target-mips: Implement IEEE 754-2008 functionality for R6 and MSA instructions

[Richard Henderson <rth@twiddle.net>]

On 03/31/2016 04:55 AM, Aleksandar Markovic wrote:
> Hi, Richard, what would you think about this approach:
> 
> Functionality of <ABS|NEG>.<S|D> and <CVT|FLOOR|CEIL|TRUNC|ROUND>.<L|W>.<S|D>
> instructions is dependent on flags ABS2008 and NAN2008 in FCR31. There are
> MIPS architectures (for example mips32r5) that allow implementations
> with different values of these flags. So, in order to detect the desired
> behavior in translate-time, insn_flags field can't be used - and, therefore,
> it makes sense to add two new members to the MIPS's DisasContext:
> 
> typedef struct DisasContext {
>     . . .
>     bool nan2008;
>     bool abs2008;
> } DisasContext;
> 
> Their initialization could be in gen_intermediate_code_internal():
> 
>     ctx.nan2008 = (env->active_fpu.fcr31 >> FCR31_NAN2008) & 1;
>     ctx.abs2008 = (env->active_fpu.fcr31 >> FCR31_ABS2008) & 1;
> 
> Now, ABS.D (and all <ABS|NEG>.<S|D>) handling might look like this:
> 
>     case OPC_ABS_D:
>         check_cp1_registers(ctx, fs | fd);
>         {
>             TCGv_i64 fp0 = tcg_temp_new_i64();
> 
>             gen_load_fpr64(ctx, fp0, fs);
>             if (ctx->abs2008) {
>                 tcg_gen_andi_i64(fp0, fp0, 0x7fffffffffffffffULL);
>             } else {
>                 gen_helper_float_abs_d(fp0, fp0);
>             }
>             gen_store_fpr64(ctx, fp0, fd);
>             tcg_temp_free_i64(fp0);
>         }
>         opn = "abs.d";
>         break;
> 
> Here, 2008-style ABS.D is implemented inline, without a helper, and
> gen_helper_float_abs_d() is an old pre-2008 helper that would be intact
> (the same as it is currently) with this change.

Yes, that's exactly what I had in mind.

> On the other hand, CVT.L.D (and all <CVT|FLOOR|CEIL|TRUNC|ROUND>.<L|W>.<S|D>)
> handling would take this form:
> 
>     case OPC_CVT_L_D:
>         check_cp1_64bitmode(ctx);
>         {
>             TCGv_i64 fp0 = tcg_temp_new_i64();
> 
>             gen_load_fpr64(ctx, fp0, fs);
>             if (ctx->nan2008) {
>                 gen_helper_float_cvt_2008_l_d(fp0, cpu_env, fp0);
>             } else {
>                 gen_helper_float_cvt_l_d(fp0, cpu_env, fp0);
>             }
>             gen_store_fpr64(ctx, fp0, fd);
>             tcg_temp_free_i64(fp0);
>         }
>         opn = "cvt.l.d";
>         break;
> 
> Function helper_float_cvt_2008_l_d() is a new, only-2008-style helper for
> CVT.L.D and would look like this:
> 
> uint64_t helper_float_cvt_2008_l_d(CPUMIPSState *env, uint64_t fdt0)
> {
>     uint64_t dt2;
> 
>     dt2 = float64_to_int64(fdt0, &env->active_fpu.fp_status);
>     if (get_float_exception_flags(&env->active_fpu.fp_status)
>             & (float_flag_invalid | float_flag_overflow)) {
>         dt2 = DBL_TO_INT64_OVERFLOW(fdt0)
>     }
>     update_fcr31(env, GETPC());
>     return dt2;
> }

That looks fine as well.



r~