[Mesa-dev] [RFC] Solving the TGSI indirect addressing optimization problem

[Christoph Bumiller]

On 12.03.2013 10:31, Christian König wrote:
> Am 12.03.2013 02:48, schrieb Marek Olšák:
>> On Mon, Mar 11, 2013 at 1:44 PM, Christian König
>> <deathsimple at vodafone.de> wrote:
>>> Hi everybody,
>>>
>>> this problem has been open for quite some time now, with a bunch of
>>> different
>>> opinions and sometimes even patches floating on the list.
>>>
>>> The solutions proposed or implemented so far all more or less
>>> incomplete, so
>>> this approach was designed in mind with both completeness and
>>> compatibility
>>> with existing code.
>>>
>>> Over all it's just an implementation of what Tom Stellard named
>>> solution #4 in
>>> this eMail thread:
>>> http://lists.freedesktop.org/archives/mesa-dev/2013-January/033264.html
>> Hi Christian,
>>
>> this is definitely not the solution #4. According to the TGSI dump
>> Christoph posted, it looks more like #3.
>
> Well, for me the main difference between proposal #3 and #4 is that #3
> tries to identify the declaration to use with the supplied "offset",
> while #4 uses a completely distinct identifier for that.
>
>> The solution #4 completely changes the temporary file such that it
>> becomes two-dimensional with the first index being a literal and the
>> second index being either a literal or ADDR[literal], and it would
>> always be like that regardless of whether drivers support that or not.
>> One-dimensional indexing of TEMP is not allowed. For backward
>> compatibility, the drivers that do not support it would only get a
>> single array declaration TEMP[0][0..n] and TEMP[0][...] would be
>> everywhere in the code.
>
> Ok, then I misunderstood you a bit, but I don't think the difference
> is so much.
>
> What I'm proposing is that we have an optional "ArrayID" attached to
> each declaration and refer to this "ArrayID" in the indirect
> addressing operand. To sum it up declarations should look something
> like this:
>
> DCL TEMP[0..3]        // normal registers
> DCL TEMP[1][4..11]    // indirectly accessed array
> DCL TEMP[2][12..15]    // another indirectly accessed array
> DCL TEMP[16..17] LOCAL    // local registers
>
> While an indirect operand might look like this:
>
> MOV TEMP[16], TEMP[1][ADDR[0].x-13]
>
> On the pro side for this approach is that it is compatible with all
> the existing state trackers and driver, and we don't need to generate
> different code depending on weather or not the driver supports this.
>
>> I don't know much about TGSI internals, so I can't review this. I'd
>> just like to say that TGSI dumps should make sense (2D indexing should
>> be only allowed with 2D declarations) and tgsi_text_translate should
>> be able to do the reverse - convert the dumps back to TGSI tokens.
>
> Completely agree with that, and beside writing documentation testing
> this is still one of the todos with this patchset.
>
> I have to admit that your approach looks a bit cleaner from the high
> above view. The problem with it is that it requires this additional 2D
> index on every operand, and we just don't have enough bits left for
> this. Even with my approach I need to make room for this ArrayID in
> the indirect addressing operand token, and this additional token is
> only there if the operand uses indirect adressing.
>
> Do you think we can live with my approach or is there any major
> downside I currently don't see?
>

I can live with it. I think ... (I hope I don't regret this later; seems
like this doesn't contain less information, then it's ok.)
If the placement of the hint index offends someone, just write it as
"MOV TEMP[16], TEMP(1)[ADDR[0].x-13]" or ...
TEMP[ADDR[0].x-13 : 1] or
TEMP[ADDR[0].x-13 supposedToBeIn [4,11]] or
something ... nicer.

Actually ...
if TEMP[0] is placed at mem[0]
and TEMP[4..1] is placed at, say, mem[0x1000 in bytes]

do I have to
load $register mem[$addr - 0xd0] (no this can't work) or
load $regsiter mem[$addr - 0xd0 + 0x1000] (if you didn't adjust the
offset) or
load $register mem[$addr - 0xd0 + 0x1000 - 0x40] (if you already added
the base TEMP to the immediate offset)

This needs to be documented as well.

> Thanks for the clarification,
> Christian.
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