Re: [Qemu-devel] [PATCH 06/24] tcg-mips: Move softmmu slow path out of line

[Richard Henderson]

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~