Am 03.02.2018 um 03:12 schrieb Marek Olšák: > On Sat, Feb 3, 2018 at 2:55 AM, Roland Scheidegger <srol...@vmware.com> wrote: >> Am 03.02.2018 um 00:31 schrieb Marek Olšák: >>> On Sat, Feb 3, 2018 at 12:01 AM, Roland Scheidegger <srol...@vmware.com> >>> wrote: >>>> Am 02.02.2018 um 23:39 schrieb Marek Olšák: >>>>> On Fri, Feb 2, 2018 at 10:26 PM, Roland Scheidegger <srol...@vmware.com> >>>>> wrote: >>>>>> Am 02.02.2018 um 21:48 schrieb Marek Olšák: >>>>>>> Hi, >>>>>>> >>>>>>> This is the second and hopefully final version of 32-bit pointer >>>>>>> support for radeonsi. >>>>>>> >>>>>>> Constant buffer 0 now has restrictions on which buffers can be set >>>>>>> in that slot. >>>>>>> >>>>>>> I plan to push this when my LLVM patch lands in 6.0 (hopefully it >>>>>>> will be accepted there). >>>>>>> >>>>>>> There will also be a dependency on new libdrm (not included in this >>>>>>> series). >>>>>>> >>>>>>> Please review. >>>>>>> >>>>>> >>>>>> From a api cleanliness point of view, I don't like this much. >>>>>> First, you're making the hack case the default and even require it. IMHO >>>>>> a driver should be able to bind ordinary UBOs to all buffer slots. This >>>>>> is really not a nice burden to put on state trackers to do something >>>>>> special for just slot 0. The gallium API should stay reasonable imho, >>>>>> that's a bit too much custom tailoring for GL for my liking. >>>>>> >>>>>> Maybe I'm missing something but I can't quite see why you can't handle >>>>>> this transparently inside the driver. Can't you just create a different >>>>>> shader depending on what kind of buffer is bound or what's the problem? >>>>>> (You wouldn't expect it to change therefore you should not have to >>>>>> recompile.) >>>>> >>>>> We don't recompile shaders in the vast majority of cases. When shader >>>>> compilation stalls rendering, the gaming experience is destroyed. >>>>> >>>>> There is no alternative. Our shader ABI will be set up such that it >>>>> only has 32-bit pointers in shader registers. There are >>>>> performance-related reasons for that. >>>> >>>> That seems to be quite limited, why can't you have a shader ABI which >>>> can do either 32 or 64 bit pointers? >>> >>> Good questions. GCN shaders have only 16 dwords of constant memory >>> (GFX9 has 32). There are no shader resource slots and the pixel shader >>> is the only one to have real inputs. All other stages don't have any >>> shader inputs except for system values. >>> >>> The 16 dwords contain pointers and states to load inputs and load >>> descriptions of resource slots from memory. One of the pointers >>> sometimes points to constant buffer 0. If it's a VS, there are only 13 >>> dwords, because 3 are reserved for baseinstance, basevertex, and >>> drawID. We can also put some other data into that constant memory to >>> skip load instructions. There is a huge incentive to free those >>> precious dwords and use them for something else, like skipping some >>> load instructions. I've been also considering 16-bit pointers (e.g. >>> 32-bit pointers aligned to 64KB). >>> >> >> Ok, so for other buffers you can't really do anything special? You just >> go through a pointer to array-of-pointer lookup? > > By default, the shader gets a pointer that points to a merged list of > constant buffer and shader buffer descriptions in memory. If a shader > only uses constant buffer 0 and no shader buffers, that pointer points > to constant buffer 0 directly. Ahh so you can easily guarantee a 32bit pointer if you use the descriptor list (as that's a driver-internal allocation), in which case the actual buffer address size doesn't matter (?), but not if you use the optimization when only constant buffer 0 is used? Albeit that also means you can't do any such optimization for other cases (say, a simple shader only using UBO 0 as that will end up as constant buffer 1, without a guaranteed 32bit address). So I guess indeed if that optimization is worth it, your options are limited (if you don't want a shader dependency on the actual type of buffer bound).
Roland > >> I thought "proper" apps would just use UBOs for everything these days >> (hence nothing really much need for tuning slot 0). But maybe that's not >> actually true... I can see that you'd want to optimize usage of this >> precious space. I suppose GL doesn't give you much help there with its >> iffy buffer handling. > > Yes, games use UBOs. > > Marek > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
