[Mesa-dev] [PATCH] r600g: fix color exports when we have no CBs
Vadim Girlin
vadimgirlin at gmail.com
Wed Aug 28 10:56:28 PDT 2013
On 08/28/2013 01:15 PM, Christian König wrote:
> Well, for this discussion let's just assume that we fixed the delay in
> the upper layers of the stack and the driver sees the shader code as
> soon as the application (if I understood it correctly Vadim has just
> volunteered for the job).
No, I'm not really volunteering to implement that. :)
I'm not even sure if it's possible in reasonable time. In fact it was
more like a theoretical discussion about what would be required for the
early compilation in the driver to make sense.
Perhaps I failed to explain it, but actually my point is that while the
compilation is deferred in upper layers and nobody is going to change
this (if it's possible at all), it doesn't make sense to try compiling
early in the driver. I think we might prefer to defer the compilation in
the driver as well - it doesn't make overall situation any worse, but
can make it better by not compiling unused variants at least.
Vadim
> Also let's assume that shaders are small and having allot of shader
> variants around after they are compiled isn't bad.
>
> In this case the probably best solution is to compile early and try to
> make the shaders as state invariant as possible, e.g. don't do
> optimizations like getting ride of extra exports for case where we don't
> need the alpha test or if it's just a dependency on a boolean then have
> both variants covered by the bytecode and use a bit constant to choose
> between the two etc...
>
> As a second step the driver should create a optimized version of the
> shader in a background thread when we know all the state that is/was
> active when the shader is used.
>
> Of course you need a bit of heuristic for this, cause sometimes it is
> better to switch between shader variants and other times it is better to
> have one variant covering all the different states and just use bit
> constants to choose between them.
>
> Just some thoughts on this topic,
> Christian.
>
> PS: My mail server is once more driving me nuts, please ignore the extra
> copy if you get this mail twice.
>
> Am 28.08.2013 02:07, schrieb Vadim Girlin:
>> On 08/28/2013 02:59 AM, Marek Olšák wrote:
>>> First, you won't really see any significant continual difference in
>>> frame rate no matter how many shader variants you have unless you are
>>> very CPU-bound. The problem is shader compilation on the first use,
>>> that's where you get a big hiccup. Try Skyrim for example: You have to
>>> first look around and see every object that's around you and get
>>> unpleasant stuttering before you can actually go on and play the game.
>>> Yes, this also Wine's fault that it compiles shaders on the first use
>>> too, but we don't have to be as bad as Wine, do we? Valve also
>>> reported shader recompilations on the first use being a serious issue
>>> with open source drivers.
>>
>> I perfectly understand that deferred compilation is exactly the
>> problem that makes the games freeze due to shader compilation on first
>> use when something new appears on the screen, but I don't think we can
>> solve this problem in the *driver* by trying to compile early, because
>> AFAICS currently the shaders are passed to the driver too late anyway,
>> and this happens not only with wine. E.g. when I run Heaven in a
>> window with "MESA_GLSL=dump R600_DEBUG=ps,vs", so that I can see
>> Heaven's window and console output at the same time, what I see is
>> that most of GL dumps happen while Heaven shows splash screen with
>> loading progress, but most of the driver's dumps appear on the first
>> frame and few more times during benchmark. It looks like compilation
>> is deferred somewhere in the stack before the driver, or am I missing
>> something?
>>
>> Vadim
>>
>>
>>>
>>> Marek
>>>
>>> On Tue, Aug 27, 2013 at 11:52 PM, Vadim Girlin
>>> <vadimgirlin at gmail.com> wrote:
>>>> On 08/28/2013 12:43 AM, Marek Olšák wrote:
>>>>>
>>>>> Shader variants are BAD, BAD, BAD. Have you ever played an AAA game
>>>>> with a Mesa driver that likes to compile shader variants on first use?
>>>>> It's HORRIBLE.
>>>>
>>>>
>>>> I don't think that shader variants are bad, but it's definitely bad
>>>> when we
>>>> are compiling variants that are never used. Currently glxgears
>>>> compiles 18
>>>> ps/vs shaders. In my branch with initial GS support [1] I switched
>>>> handling
>>>> of the shaders to deferred compilation, that is, shaders are
>>>> compiled only
>>>> before the actual draw. I found later that it's not really required
>>>> for GS,
>>>> but IIRC this change results in only 5 shaders being compiled for
>>>> glxgears
>>>> instead of 18. It seems most of the useless variants are results of
>>>> state
>>>> changes between creation of the shader state (initial compilation) and
>>>> actual draw call.
>>>>
>>>> I had some concerns about increased overhead with those changes, and
>>>> it's
>>>> actually noticeable with drawoverhead demo, but I didn't see any
>>>> regressions
>>>> with a few real apps that I tested, e.g. glxgears even showed slightly
>>>> better performance with these changes. Probably I also implemented
>>>> it in a
>>>> not very optimal way (I was mostly concentrated on GS support) and the
>>>> overhead can be reduced.
>>>>
>>>> One more thing is duplicate shaders, I've analyzed shader dumps from
>>>> Unigine
>>>> Heaven 3.0 some time ago and found that from about 320 compiled
>>>> shaders,
>>>> only about 180 (50%) were unique, others were duplicates (detected by
>>>> comparing the bytecode dumps for them in an automated way), maybe
>>>> they had
>>>> different shader keys (which still resulted in the same bytecode),
>>>> but I
>>>> suspect duplicate pipe shaders were also involved. Unfortunately I
>>>> didn't
>>>> have a time to investigate it more thoroughly since then.
>>>>
>>>> So my point is that we don't really need to eliminate shader
>>>> variants, first
>>>> we need to eliminate compilation of unused variants and duplicate
>>>> shaders.
>>>> Also we might want to consider offloading of the compilation to
>>>> separate
>>>> thread(s) and caching of shader binaries between runs.
>>>>
>>>> Vadim
>>>>
>>>> [1] http://cgit.freedesktop.org/~vadimg/mesa/log/?h=r600-geom-shaders
>>>>
>>>>
>>>>>
>>>>> What the patch does is probably the right solution. At least
>>>>> alpha-test state changes don't cause shader recompilation and
>>>>> re-binding, which also negatively affects performance. Ideally we
>>>>> shouldn't depend on the framebuffer state at all, but we need to
>>>>> emulate the TGSI property FS_COLOR0_WRITES_ALL_CBUFS. I think we
>>>>> should always be fine with key.nr_cbufs forced to 8 for any shader
>>>>> without that property. I expect app developers to do the right thing
>>>>> and not write outputs they don't need.
>>>>>
>>>>> Marek
>>>>>
>>>>> On Tue, Aug 27, 2013 at 9:00 PM, Roland Scheidegger
>>>>> <sroland at vmware.com>
>>>>> wrote:
>>>>>>
>>>>>> Not that I'm qualified to review r600 code, but couldn't you create
>>>>>> different shader variants depending on whether you need alpha
>>>>>> test? At
>>>>>> least I would assume shader exports aren't free.
>>>>>>
>>>>>> Roland
>>>>>>
>>>>>> Am 27.08.2013 19:56, schrieb Vadim Girlin:
>>>>>>>
>>>>>>> We need to export at least one color if the shader writes it,
>>>>>>> even when nr_cbufs==0.
>>>>>>>
>>>>>>> Signed-off-by: Vadim Girlin <vadimgirlin at gmail.com>
>>>>>>> ---
>>>>>>>
>>>>>>> Tested on evergreen with multiple combinations of backends - no
>>>>>>> regressions,
>>>>>>> fixes some tests:
>>>>>>>
>>>>>>> default - fixes fb-alphatest-nocolor and
>>>>>>> fb_alphatest-nocolor-ff
>>>>>>> default+sb - fixes fb-alphatest-nocolor and
>>>>>>> fb_alphatest-nocolor-ff
>>>>>>> llvm - fixes about 25 tests related to depth/stencil
>>>>>>> llvm+sb - fixes about 300 tests (llvm's depth/stencil
>>>>>>> issues and
>>>>>>> regressions cased by reordering of exports in sb)
>>>>>>>
>>>>>>> With this patch, there are no regressions with default+sb vs
>>>>>>> default.
>>>>>>> There is one regression with llvm+sb vs llvm -
>>>>>>> fs-texturegrad-miplevels,
>>>>>>> AFAICS it's a problem with llvm backend uncovered by sb -
>>>>>>> SET_GRADIENTS_V/H
>>>>>>> instructions are not placed in the same TEX clause with
>>>>>>> corresponding
>>>>>>> SAMPLE_G.
>>>>>>>
>>>>>>> src/gallium/drivers/r600/r600_shader.c | 7 ++++---
>>>>>>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> diff --git a/src/gallium/drivers/r600/r600_shader.c
>>>>>>> b/src/gallium/drivers/r600/r600_shader.c
>>>>>>> index 300b5c4..f7eab76 100644
>>>>>>> --- a/src/gallium/drivers/r600/r600_shader.c
>>>>>>> +++ b/src/gallium/drivers/r600/r600_shader.c
>>>>>>> @@ -918,6 +918,7 @@ static int r600_shader_from_tgsi(struct
>>>>>>> r600_screen
>>>>>>> *rscreen,
>>>>>>> unsigned opcode;
>>>>>>> int i, j, k, r = 0;
>>>>>>> int next_pos_base = 60, next_param_base = 0;
>>>>>>> + int max_color_exports = MAX2(key.nr_cbufs, 1);
>>>>>>> /* Declarations used by llvm code */
>>>>>>> bool use_llvm = false;
>>>>>>> bool indirect_gprs;
>>>>>>> @@ -1130,7 +1131,7 @@ static int r600_shader_from_tgsi(struct
>>>>>>> r600_screen *rscreen,
>>>>>>> radeon_llvm_ctx.face_gpr = ctx.face_gpr;
>>>>>>> radeon_llvm_ctx.r600_inputs = ctx.shader->input;
>>>>>>> radeon_llvm_ctx.r600_outputs = ctx.shader->output;
>>>>>>> - radeon_llvm_ctx.color_buffer_count =
>>>>>>> MAX2(key.nr_cbufs ,
>>>>>>> 1);
>>>>>>> + radeon_llvm_ctx.color_buffer_count =
>>>>>>> max_color_exports;
>>>>>>> radeon_llvm_ctx.chip_class = ctx.bc->chip_class;
>>>>>>> radeon_llvm_ctx.fs_color_all =
>>>>>>> shader->fs_write_all &&
>>>>>>> (rscreen->chip_class >= EVERGREEN);
>>>>>>> radeon_llvm_ctx.stream_outputs = &so;
>>>>>>> @@ -1440,7 +1441,7 @@ static int r600_shader_from_tgsi(struct
>>>>>>> r600_screen *rscreen,
>>>>>>> case TGSI_PROCESSOR_FRAGMENT:
>>>>>>> if (shader->output[i].name ==
>>>>>>> TGSI_SEMANTIC_COLOR) {
>>>>>>> /* never export more colors than the
>>>>>>> number of CBs */
>>>>>>> - if (shader->output[i].sid >=
>>>>>>> key.nr_cbufs)
>>>>>>> {
>>>>>>> + if (shader->output[i].sid >=
>>>>>>> max_color_exports) {
>>>>>>> /* skip export */
>>>>>>> j--;
>>>>>>> continue;
>>>>>>> @@ -1450,7 +1451,7 @@ static int r600_shader_from_tgsi(struct
>>>>>>> r600_screen *rscreen,
>>>>>>> output[j].type =
>>>>>>> V_SQ_CF_ALLOC_EXPORT_WORD0_SQ_EXPORT_PIXEL;
>>>>>>> shader->nr_ps_color_exports++;
>>>>>>> if (shader->fs_write_all &&
>>>>>>> (rscreen->chip_class >= EVERGREEN)) {
>>>>>>> - for (k = 1; k <
>>>>>>> key.nr_cbufs; k++)
>>>>>>> {
>>>>>>> + for (k = 1; k <
>>>>>>> max_color_exports;
>>>>>>> k++) {
>>>>>>> j++;
>>>>>>> memset(&output[j], 0,
>>>>>>> sizeof(struct r600_bytecode_output));
>>>>>>> output[j].gpr =
>>>>>>> shader->output[i].gpr;
>>>>>>>
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