[Mesa-dev] Allowing the reading of outputs for some drivers

[Marek Olšák]

On Mon, Nov 14, 2011 at 2:49 PM, Vadim Girlin <vadimgirlin at gmail.com> wrote:
> On Mon, 2011-11-14 at 05:22 -0800, Jose Fonseca wrote:
>>
>> ----- Original Message -----
>> > Hi,
>> >
>> > I found some problem with glsl_to_tgsi: remove_output_reads function.
>> > It's replacing outputs with temps, producing incorrect results with
>> > relative addressing. You can see it e.g. with
>> > "vs-varying-array-mat2-col-rd.shader_test". Here is a dump:
>> >
>> > > VERT
>> > > DCL IN[0]
>> > > DCL OUT[0], POSITION
>> > > DCL OUT[1], GENERIC[12]
>> > > DCL OUT[2], GENERIC[13]
>> > > DCL OUT[3], GENERIC[14]
>> > > DCL OUT[4], GENERIC[15]
>> > > DCL OUT[5], GENERIC[16]
>> > > DCL OUT[6], GENERIC[17]
>> > > DCL OUT[7], GENERIC[18]
>> > > DCL CONST[0..5]
>> > > DCL TEMP[0..1]
>> > > DCL ADDR[0]
>> > > IMM FLT32 {    1.0000,     2.0000,     3.0000,     4.0000}
>> > > IMM FLT32 {    5.0000,     6.0000,     7.0000,     8.0000}
>> > > IMM FLT32 {    9.0000,    10.0000,    11.0000,    12.0000}
>> > > IMM FLT32 {    0.0000,     1.0000,     0.0000,     0.0000}
>> > >   0: MUL TEMP[0], CONST[2], IN[0].xxxx
>> > >   1: MAD TEMP[1], CONST[3], IN[0].yyyy, TEMP[0]
>> > >   2: MAD TEMP[1], CONST[4], IN[0].zzzz, TEMP[1]
>> > >   3: MAD OUT[0], CONST[5], IN[0].wwww, TEMP[1]
>> > >   4: MOV OUT[2], IMM[0].xyyy
>> > >   5: MOV OUT[3], IMM[0].zwww
>> > >   6: MOV TEMP[0], IMM[1].xyyy
>> >
>> > OUT[2-7] is a "varying mat2x2[3] m;", OUT[4] is replaced with the
>> > temp
>> > in the instruction 6.
>> >
>> > >   7: MOV OUT[5], IMM[1].zwww
>> > >   8: MOV OUT[6], IMM[2].xyyy
>> > >   9: MOV OUT[7], IMM[2].zwww
>> > >  10: ARL ADDR[0].x, CONST[1].xxxx
>> > >  11: SNE TEMP[1], TEMP[ADDR[0].x].xyyy, CONST[0].xyyy
>> >
>> > Instruction 11 contains the read with the relative addressing using
>> > this temp, which is incorrect.
>> >
>> > I'm not sure how to fix it,
>>
>> The way to fix this is to allocate a consecutive range of temps at start, when there are indirect writes to output registers and at least one read.
>>
>> > but AFAICS at least for r600g this step
>> > could be skipped completely - r600 can read outputs without any
>> > problem, they are located in the general-purpose registers. Removing
>> > calls to remove_output_reads and assert(src.File != TGSI_FILE_OUTPUT)
>> > in the ureg_emit_src produces correct result and test passes on
>> > evergreen (total number of fixed tests is about 60).
>> >
>> > Probably it makes sense to make this step optional and ask the driver
>> > whether to use it, if I'm not missing something?
>>
>> The drawback of doing this is that TGSI will look even more different between drivers. This means that when somebody makes a change to the state tracker, and tested with one driver, it may break other drivers. This also means that comparing a driver to llvmpipe/softpipe will be less meaningfull, as different paths will be taken in the state tracker.
>>
>
> I think if it's needed to compare the drivers, than it's possible to
> switch of the the cap for debugging. I see the drawbacks, but this is
> also about performance. Currently with r600g (and possibly with other
> drivers) we have to spend some time for unneccessary shader
> modification, to get less efficient shader code as a result.

I am 100% sure nobody will turn on/off CAPs just to test something.

I gotta agree with José on this one.

The shader backend in a driver is primarily responsible for the
quality of final shader code, not the state tracker, although the
state tracker can help a lot.

r600g already allocates outputs in the temporary area, so it's not
like remove_output_reads would make any difference for that driver,
right?* Why not fix remove_output_reads such that it works with
indirect addressing? That would also fix _all the other drivers_ which
don't support output reads.

* Lack of proper register allocation is not an excuse, especially if
new temporaries are allocated by a driver for whatever reasons.

Marek

>
> By the way, which drivers do not support reading outputs? I haven't done
> a full piglit run with llvmpipe, but IIRR the single test mentioned
> above was also fixed for llvmpipe without this output replacement.
>
> Vadim
>
>> I'm not sure if for this particular issue a new cap is worth or not. Just pointing out that there are downsides of breaking orthogonality between state tracker and driver. It should not be done lightly.
>>
>>
>> Jose
>
>
>
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