On 2/6/20 5:52 PM, Taylor Simpson wrote:
>
>
>> -----Original Message-----
>> From: Richard Henderson <rth7...@gmail.com> On Behalf Of Richard
>> Henderson
>> Sent: Thursday, February 6, 2020 9:35 AM
>> To: Taylor Simpson <tsimp...@quicinc.com>; qemu-devel@nongnu.org
>> Cc: Paolo Bonzini <pbonz...@redhat.com>
>> Subject: Re: [PATCH] Add support for a helper with 7 arguments
>>
>>
>> I think this is quite ugly. I know you've been talking about auto-generating
>> everything but we ought to do better than this.
>>
>> You should be passing values not regnos if you can possibly do so. You
>> should
>> be passing full virtual addresses not N separate components of an address.
>> Predicates should be evaluated earlier so that the helper isn't even called
>> if
>> it's false.
>
> We are passing values, not reg numbers. The generator doesn't know anything
> about the semantics of the instruction...
And this, I claim, is wrong.
If you can generate C for the operation out of line, then you can interpret it
inline as well. Or, make a reasonable decision about what bit to be out of
line and what bit to be a helper.
> It only knows which operands are read, written, or both. So, there's no way
> to combine the 3 operands into a single effective address until we are
> inside the helper. Also, there's no way to know if the instruction is
> predicated or if it just has a predicate as an operand. Also, there are
> instructions where the predicate is used in the false sense.
There certainly is a way to know what the effective address is. From the
pseudo-code that I browsed today, "EA = foo" is a big clue.
There certainly is a way to know if the instruction is predicated. From the
pseudo-code that I browsed today,
if (pred) {
operation;
} else {
NOP;
}
is the clue, vs
Rd = mux(Pu, Rs, Rt).
Of course there are insns where !Pd is used; that doesn't change anything.
>> Combine that with 3.3.1 Packet execution semantics, "dual stores, new-value
>> stores, and slot1 store with slot0 loads have non-parallel execution
>> semantics", and you need no special helper at all:
>>
>> andt0, pv, 1
>> brcondit0, 0, over
>>
>> shlit0, ru, u2
>> addt0, t0, rs
>> qemu_strt, t0, mmu_idx, MO_UB
>> over:
>>
>
> We can't actually do the store here. We have to record it in a side data
> structure in the env and perform the store when the packet commits.
I think that we can do the store immediately -- I give specifics above. Do you
have a counter-example? Admittedly I'm new to browsing the architecture, but I
don't see a legal packet for which you can't just Store Now.
> Currently, there are a total of 35 instructions that need a helper with 7
> arguments. 32 of them are predicated stores, but there are also 3 vscatter
> instructions. For example,
> if (Qs4) vscatter(Rt32, Mu2, Vv32.w).w = Vw32
Ok, vscatter is indeed complex. But I still count 6 operands. Is it the
"slot" one that you're concerned is the 7th? I don't see that as strictly
necessary, since this can only be in slot 0.
r~
