Re: [PATCH] Add support for a helper with 7 arguments

[Richard Henderson]

On 2/8/20 5:08 AM, Taylor Simpson wrote:
>> {
>>   r6 = memb(r1)
>>   r7 = memb(r2)
>> }
>>
>> qemu_ld   t0, r1, MO_UB, mmu_idx
>> qemu_ld   t1, r2, MO_UB, mmu_idx
>> mov       r6, t0
>> mov       r7, t1
>>
> 
> Here is the TCG we generate currently.
>  movi_i32 tmp0,$0x0
>  add_i32 loc2,r1,tmp0
>  qemu_ld_i32 loc3,loc2,sb,0
>  mov_i32 new_value,loc3
>  movi_i32 tmp0,$0x0
>  add_i32 loc2,r2,tmp0
>  qemu_ld_i32 loc3,loc2,sb,0
>  mov_i32 new_value,loc3
>  mov_i32 r6,new_value
>  mov_i32 r7,new_value
> I could work on eliminating the add of zero and the extra copies.  Is TCG
> able to optimize these before emitting the host code?
We can optimize them.

However, you should prefer to use tcg_gen_addi_* over tcg_gen_add_* (etc) when
you know that one operand is constant.  This will optimize away the add zero
immediately as opposed to allocating memory and walking the data structures to
eliminate it later.

Why are you using a local temporary for EA?  That should be dead immediately
after this slot is complete.

What's with two temporaries both named "new_value"?

>> For a conditional load in slot 0, we can load directly into the final
>> destination register and skip the temporary.
> 
> In general, there will be lots of checks we would need to perform before
> concluding that an instruction can write directly into the destination.  For
> example, we have to make sure no other instruction later in the packet reads
> r7.

Which is of course all trivial for slot 0, being last.


r~