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Re: [Qemu-devel] [PATCH 06/24] tcg-mips: Move softmmu slow path out of l
From: |
Richard Henderson |
Subject: |
Re: [Qemu-devel] [PATCH 06/24] tcg-mips: Move softmmu slow path out of line |
Date: |
Sat, 24 May 2014 08:42:00 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 |
On 05/24/2014 02:55 AM, Paolo Bonzini wrote:
> Il 14/05/2014 09:17, Richard Henderson ha scritto:
>> + tcg_out_opc_imm(s, OPC_LW, TCG_REG_A0, TCG_REG_A0, add_off);
>> + tcg_out_opc_reg(s, OPC_AND, TCG_REG_T0, TCG_REG_T0, addrl);
>> +
>> + label_ptr[0] = s->code_ptr;
>> tcg_out_opc_br(s, OPC_BNE, TCG_REG_T0, TCG_REG_AT);
>> - tcg_out_nop(s);
>
> I don't remember mips very well, LW cannot be put in the delay slot? This
> would
> let you fill both delay slots for the 64-bit case. Or is it just that the
> code
> becomes harder to follow due to the TARGET_LONG_BITS == 64 "if"s?
>
> Alternatively, for 64-bit you could use OR+BNE instead of BNE+NOP+BNE. Of
> course this can be done later, this patchset is already a big improvement.
It's MIPS I that had all sorts of problems with scheduling loads. Including
requiring two cycles between load issue and use. TCG doesn't handle any of
that; we require a fully interlocked pipeline. Without looking it up, I'd
guess that was at least MIPS III (circa 1992?).
Mostly that nop is hard to fill because of the if's, and I wanted to fill the
last slot with the addition to make up the full host address.
OR+BNE doesn't help; you need 2 XORs and 1 OR to do a double-word equality
comparison. That's something that might take a bit of measurement to show it's
worthwhile.
r~
- [Qemu-devel] [PATCH 00/24] tcg mips updates, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 02/24] tcg-mips: Constrain the code_gen_buffer to be within one 256mb segment, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 01/24] tcg-mips: Layout executable and code_gen_buffer, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 03/24] tcg-mips: Use J and JAL opcodes, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 06/24] tcg-mips: Move softmmu slow path out of line, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 04/24] tcg-mips: Fill the exit_tb delay slot, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 07/24] tcg-mips: Convert to new qemu_l/st helpers, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 05/24] tcg-mips: Split large ldst offsets, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 09/24] tcg-mips: Rearrange register allocation, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 10/24] tcg-mips: Introduce TCG_TMP0, TCG_TMP1, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 11/24] tcg-mips: Use T9 for TCG_TMP1, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 08/24] tcg-mips: Convert to new_ldst, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 13/24] tcg-mips: Name the opcode enumeration, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 12/24] tcg-mips: Use EXT for AND on mips32r2, Richard Henderson, 2014/05/14
- [Qemu-devel] [PATCH 14/24] tcg-mips: Fix subtract immediate range, Richard Henderson, 2014/05/14