Re: [Qemu-devel] [PATCH v3 06/15] target-tricore: Add instructions of SRC opcode format

[Richard Henderson <rth@twiddle.net>]

On 08/04/2014 07:38 AM, Bastian Koppelmann wrote:
> +static inline void gen_calc_psw_sv_i32(TCGv ret, TCGv arg)
> +{
> +    tcg_gen_xor_tl(ret, ret, arg);
> +}

Not exclusive or, inclusive or.  And there's really no need for a helper for this.

> +static inline void gen_calc_psw_av_i32(TCGv ret, TCGv arg)
> +{
> +    TCGv temp = tcg_temp_new();
> +    tcg_gen_muli_tl(temp, arg, 2);

Strength reduce to tcg_gen_add_tl(temp, arg, arg).

> +    tcg_gen_xor_tl(temp, arg, temp);
> +    tcg_gen_andi_tl(ret, temp, 0x80000000);

No need for the andi if you do as I suggested and only consider the high bit
when reading the value from PSW_AV.

> +static inline void gen_calc_psw_sav_i32(TCGv ret, TCGv arg)
> +{
> +    tcg_gen_xor_tl(ret, ret, arg);
> +}

Again, inclusive or.

> +static inline void gen_add_i32(TCGv ret, TCGv r1, TCGv r2)

I strongly suggest that you name this something else, because you've gone and
confused yourself: this only applies to adds in the data registers.

> +    TCGv t0 = tcg_temp_new_i32();
> +    /* Addition and set V/SV bits */
> +    tcg_gen_movi_tl(t0, 0);
> +    tcg_gen_add2_tl(ret, cpu_PSW_V, r1, t0, r2, t0);

This computation is not overflow, but carry.  As I said, see e.g. the ARM port
where we properly compute overflow as

	R = A + B
	VF = (R ^ A) & ~(A ^ B)

i.e.

	tcg_gen_xor_tl(VF, R, A)
	tcg_gen_xor_tl(tmp, A, B)
	tcg_gen_andc_tl(VF, VF, tmp)

considering only the most significant bit as the overflow.

> +#define OP_COND(insn)\
> +static inline void gen_cond_##insn(int cond, TCGv r1, TCGv r2, TCGv r3, \
> +                                   TCGv r4)                             \
> +{                                                                       \
> +    TCGv temp = tcg_temp_new();                                         \
> +    TCGv temp2 = tcg_temp_new();                                        \
> +    TCGv t0 = tcg_const_i32(0);                                         \
> +                                                                        \
> +    tcg_gen_##insn ## 2_tl(temp, temp2, r1, t0, r2, t0);                \
> +    tcg_gen_movcond_tl(cond, r3, r4, t0, temp, r3);                     \
> +    /* Set PSW_V conditional */                                         \
> +    tcg_gen_movcond_tl(cond, cpu_PSW_V, r4, t0, temp2, cpu_PSW_V);      \
> +    /* Set PSW_SV conditional */                                        \
> +    gen_calc_psw_sv_i32(temp2, cpu_PSW_SV);                             \
> +    tcg_gen_movcond_tl(cond, cpu_PSW_SV, r4, t0, temp2, cpu_PSW_SV);    \
> +    /* calc AV bit */                                                   \
> +    gen_calc_psw_av_i32(temp2, temp);                                   \
> +    tcg_gen_movcond_tl(cond, cpu_PSW_AV, r4, t0, temp2, cpu_PSW_AV);    \
> +    /* calc SAV bit */                                                  \
> +    gen_calc_psw_sav_i32(temp2, cpu_PSW_SAV);                           \
> +    tcg_gen_movcond_tl(cond, cpu_PSW_SAV, r4, t0, temp2, cpu_PSW_SAV);  \
> +                                                                        \
> +    tcg_temp_free(t0);                                                  \
> +    tcg_temp_free(temp);                                                \
> +    tcg_temp_free(temp2);                                               \
> +}                                                                       \
> +                                                                        \
> +static inline void gen_condi_##insn(int cond, TCGv r1, int32_t r2,      \
> +                                    TCGv r3, TCGv r4)                   \
> +{                                                                       \
> +    TCGv temp = tcg_const_i32(r2);                                      \
> +    gen_cond_##insn(cond, r1, temp, r3, r4);                            \
> +    tcg_temp_free(temp);                                                \
> +}
> +
> +OP_COND(add)
> +OP_COND(sub)

BTW, this macro substitution isn't going to work well as is, since there are
different overflow computations for addition and subtraction.

> +static void gen_shaci(TCGv ret, TCGv r1, int32_t shift_count)
> +{
> +    uint32_t msk, msk_start;
> +    TCGv_i64 temp = tcg_temp_new_i64();
> +    TCGv_i64 result = tcg_temp_new_i64();
> +    TCGv_i64 t_0 = tcg_const_i64(0);
> +    TCGv_i64 t_1 = tcg_const_i64(1);
> +    TCGv_i64 t_max = tcg_const_i64(0x7FFFFFFF);
> +    TCGv_i64 t_min = tcg_const_i64(-(0x80000000L));
> +
> +    if (shift_count == 0) {
> +        /* Clear PSW.C */
> +        tcg_gen_movi_tl(cpu_PSW_C, 0);
> +        tcg_gen_mov_tl(ret, r1);
> +    } else if (shift_count > 0) {
> +        tcg_gen_ext_i32_i64(temp, r1);
> +        tcg_gen_shli_i64(result, temp, shift_count);
> +        /* calc carry */
> +        msk_start = 32 - shift_count;
> +        msk = ((1 << shift_count) - 1) << msk_start;
> +        tcg_gen_andi_tl(cpu_PSW_C, r1, msk);

You don't need a 64-bit shift here.

> +    } else {
> +        tcg_gen_ext_i32_i64(temp, r1);
> +        tcg_gen_sari_i64(result, temp, -(shift_count));
> +        /* calc carry */
> +        msk = (1 << (shift_count - 1)) - 1;
> +        tcg_gen_andi_tl(cpu_PSW_C, r1, msk);
> +    }

Likewise, although that does mean you need to handle the special case of -32.

> +    /* calc v/sv bits only if shift happened and write back 64bit result*/
> +    if (shift_count != 0) {
> +        /* v/sv */
> +        tcg_gen_movcond_i64(TCG_COND_GT, temp, result, t_max, t_1, t_0);
> +        tcg_gen_movcond_i64(TCG_COND_LT, temp, result, t_min, t_1, temp);
> +        tcg_gen_trunc_i64_i32(cpu_PSW_V, temp);
> +
> +        gen_calc_psw_sv_i32(cpu_PSW_SV, cpu_PSW_V);
> +        /* write back result */
> +        tcg_gen_trunc_i64_i32(ret, result);
> +    }

Note that right shifts can't overflow, since the magnitude always reduces.

I suppose using the 64-bit shift result is a reasonable way to compute left
shift overflow on a 64-bit host.  It seems like there's an easier way to
compute this that would be better for 32-bit hosts though...

One way is to adjust the comparisons for prior to the shift.  That is,

	R >= 0x7fff_ffff
	= (A << C) >= 0x7fff_ffff
	= A >= (0x7fff_ffff >> C)

Also, using 2 setcond and 1 or is more efficient than 2 movcond with those
constants on most hosts.

> +    case OPC1_16_SRC_ADD_A:
> +        gen_addi_i32(cpu_gpr_a[r1], cpu_gpr_a[r1], const4);
> +        break;

No PSW computation here.


r~