Re: [Qemu-devel] [PATCH] tcg/mips: Add support for mips64el backend

["Jin Guojie" <jinguojie@loongson.cn>]

Richard,

I have studied your V2 patch

https://lists.nongnu.org/archive/html/qemu-devel/2016-02/msg02969.html

. Though I have not tested this patch on Loongson machine, I feel this 
patch has implemented MIPS64 ISA very completely, including big/little
endian, N32 and N64 ABI. The use of #if is more clean. Many corner cases
are well handled. My patch is only a subset of yours.

I wonder why your patch have not be merged into the mainstream.
If I had seen it before, I don't need to waste time reinventing my patch.

Since tcg target for MIPS64 is of great use for developers, I 
really hope this feature can be merged into mainstream.

Is your v2 patch still in review process? Is there chance for this
patch to be merged in a not so long term? Or should other code
work should be done before being merged?

I want listen to your advice. Should I test your v2 patch on Loongson 
and use it? Or whether it is worth modifying my patch and resubmit it 
according to your review comments?

Jin Guojie

------------------ Original ------------------
From:  "Richard Henderson";<rth@twiddle.net>;
Send time: Sunday, Nov 13, 2016 3:56 PM
To: "jinguojie"<jinguojie@loongson.cn>; "qemu-devel"<qemu-devel@nongnu.org>; 
Cc: "Aurelien Jarno"<aurelien@aurel32.net>; 
Subject:  Re: [Qemu-devel] [PATCH] tcg/mips: Add support for mips64el backend



On 11/11/2016 08:20 AM, jinguojie@loongson.cn wrote:
> From: Jin Guojie <jinguojie@loongson.cn>
>
> This patch implements TCG mips64r2(little-endian) translation backend.
> Tested on Loongson 3A2000(a MIPS64-compatible CPU) with Fedora Linux 21 Remix.
> linux-0.2.img.bz2 runs well.
> The performance is nearly 10 times higher than tci mode.
>
> https://en.wikipedia.org/wiki/Loongson
> http://www.loongnix.org/index.php/Loongnix
>
> Cc: Aurelien Jarno <aurelien@aurel32.net>
> Signed-off-by: Jin Guojie <jinguojie@loongson.cn>

Have you seen

https://lists.nongnu.org/archive/html/qemu-devel/2016-02/msg01910.html

?  I know there are bugs in that patch set, but I would like any mips64 support 
to look like that.  In particular, reduce the use of #if to an absolute minimum.

> +#if UINTPTR_MAX == UINT32_MAX
> +# define TCG_TARGET_REG_BITS 32
> +#elif UINTPTR_MAX == UINT64_MAX
> +# define TCG_TARGET_REG_BITS 64
> +#endif

There are two mips64 abi's.  You're only allowing 64-bit code to be generated 
for n64, and not n32.

> +#undef use_movnz_instructions
> +#undef use_mips32_instructions
> +#undef use_mips32r6_instructions
> +
> +#define use_movnz_instructions  0
> +#define use_mips32_instructions  0
> +#define use_mips32r6_instructions  0

Why?  Certainly we should be able to generate code for mips64r2 and mips64r6.

> +#if TCG_TARGET_REG_BITS == 64
> +static const TCGReg tcg_target_call_oarg_regs[1] = {
> +    TCG_REG_V0,
> +};
> +#else
>  static const TCGReg tcg_target_call_oarg_regs[2] = {
>      TCG_REG_V0,
>      TCG_REG_V1
>  };
> +#endif

This change would be incorrect if we ever enhance tcg to handle __int128_t.  In 
the meantime it doesn't matter, and can be left unchanged.

> @@ -459,7 +502,15 @@ static inline void tcg_out_mov(TCGContext *s, TCGType type,
>  {
>      /* Simple reg-reg move, optimising out the 'do nothing' case */
>      if (ret != arg) {
> +#if TCG_TARGET_REG_BITS == 64
> +        if (type == TCG_TYPE_I32) {
> +            tcg_out_opc_reg(s, OPC_ADDU, ret, arg, TCG_REG_ZERO);
> +        } else {
> +            tcg_out_opc_reg(s, OPC_DADDU, ret, arg, TCG_REG_ZERO);
> +        }
> +#else
>          tcg_out_opc_reg(s, OPC_ADDU, ret, arg, TCG_REG_ZERO);
> +#endif
>      }

This is why a proper mips assembler uses OPC_OR.

>  }
>
> @@ -470,12 +521,21 @@ static inline void tcg_out_movi(TCGContext *s, TCGType type,
>          tcg_out_opc_imm(s, OPC_ADDIU, reg, TCG_REG_ZERO, arg);
>      } else if (arg == (uint16_t)arg) {
>          tcg_out_opc_imm(s, OPC_ORI, reg, TCG_REG_ZERO, arg);
> -    } else {
> +    } else if (arg == (int32_t)arg) {
>          tcg_out_opc_imm(s, OPC_LUI, reg, TCG_REG_ZERO, arg >> 16);
>          if (arg & 0xffff) {
>              tcg_out_opc_imm(s, OPC_ORI, reg, reg, arg & 0xffff);
>          }
>      }
> +#if TCG_TARGET_REG_BITS == 64
> +    /* 64-bit imm */
> +    else {
> +        tcg_out_opc_imm(s, OPC_LUI, reg, 0, (arg >> 32) & 0xffff);
> +        tcg_out_opc_imm(s, OPC_ORI, reg, reg, (arg >> 16) & 0xffff);
> +        tcg_out_opc_imm_64(s, OPC_DSLL, reg, reg, 16);
> +        tcg_out_opc_imm(s, OPC_ORI, reg, reg, arg & 0xffff);
> +    }
> +#endif

This is only a 48-bit immediate.

>  }
>
>  static inline void tcg_out_bswap16(TCGContext *s, TCGReg ret, TCGReg arg)
> @@ -566,7 +626,11 @@ static void tcg_out_ldst(TCGContext *s, MIPSInsn opc, TCGReg data,
>      if (ofs != lo) {
>          tcg_out_movi(s, TCG_TYPE_PTR, TCG_TMP0, ofs - lo);
>          if (addr != TCG_REG_ZERO) {
> +#if TCG_TARGET_REG_BITS == 64
> +            tcg_out_opc_reg(s, OPC_DADDU, TCG_TMP0, TCG_TMP0, addr);
> +#else
>              tcg_out_opc_reg(s, OPC_ADDU, TCG_TMP0, TCG_TMP0, addr);
> +#endif

See my patchset where I introduce OPC_PADDU to avoid this and other similar ifdefs.

> @@ -1163,6 +1276,7 @@ static void tcg_out_qemu_ld_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
>      tcg_out_mov(s, TCG_TYPE_PTR, tcg_target_call_iarg_regs[0], TCG_AREG0);
>
>      v0 = l->datalo_reg;
> +#if TCG_TARGET_REG_BITS == 32
>      if ((opc & MO_SIZE) == MO_64) {
>          /* We eliminated V0 from the possible output registers, so it
>             cannot be clobbered here.  So we must move V1 first.  */
> @@ -1173,11 +1287,21 @@ static void tcg_out_qemu_ld_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
>              tcg_out_mov(s, TCG_TYPE_I32, l->datahi_reg, TCG_REG_V1);
>          }
>      }
> +#endif
>
>      reloc_pc16(s->code_ptr, l->raddr);
>      tcg_out_opc_br(s, OPC_BEQ, TCG_REG_ZERO, TCG_REG_ZERO);
>      /* delay slot */
> +#if TCG_TARGET_REG_BITS == 32
>      tcg_out_mov(s, TCG_TYPE_REG, v0, TCG_REG_V0);
> +#else
> +    /* ext unsigned long(32) -> 64-bit */
> +    if ((opc & MO_SIZE) == MO_32) {
> +        tcg_out_mov(s, TCG_TYPE_I32, v0, TCG_REG_V0);
> +    } else {
> +        tcg_out_mov(s, TCG_TYPE_REG, v0, TCG_REG_V0);
> +    }
> +#endif

This is incorrect, as you're not passing down whether the operation is a 32-bit 
load into a 32-bit temporary, or a 32-bit load into a 64-bit temporary.  I.e. 
the difference between

   unsigned int x = *(unsigned int *)ptr;
and
   unsigned long long x = *(unsigned int *)ptr;


r~