[Mesa-dev] [PATCH] r600g: fix color exports when we have no CBs
Marek Olšák
maraeo at gmail.com
Tue Aug 27 15:59:02 PDT 2013
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.
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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