Designing a Modern GPU Interface

Designing a Modern GPU Interface
Brooke Hodgman ( @BrookeHodgman)

Brooke Hodgman ( @BrookeHodgman)
How to make a wrapper for D3D9/11/12, GL2/3/4, GL|ES2/3, Metal, Mantle, Vulkan, GNM & GCM without going (completely) insane Brooke Hodgman ( @BrookeHodgman)

Agenda GPU Interface Pipeline State management Resource management
“wrapper” around the native GPU APIs for every platform Pipeline State management Resource management Shader program management (e.g. Microsoft .fx or Nvidia .cgfx) Q&A

Where does this fit in? Shading Pipeline Scene Manager
Deferred/Forward shading, post-processing, order of passes, high level techniques Scene Manager Spatial partitioning, Camera management, object culling Specific Drawable Types Generic models, Particle systems, Animated meshes, Instanced meshes GPU Interface Lowest level portable rendering API. Just a GPU abstraction. Game Engine Today!

Goals Flexibility Productivity Performance Simplicity
Can do anything that the native APIs let us do. No cutting out features. Productivity Much simpler to use than the native APIs. Less code, and less mental tax. Performance Similar CPU frame-time to hand-written native code. Simplicity Keep the interface as small as possible.

Dog food test ’22’ Don Bradman Cricket (port) Rugby League Live 3
22 is my ‘indie’ project. We started off on D3D9, and have since added support for D3D11 & Mantle, with plans for OpenGL, D3D12 and Vulkan. Big Ant Studios contracted me to port Bradman to PS4 and Xbox One. They hated their current rendering abstraction, so I convinced them to move to this model. All of the high level code (game-level materials, meshes, models, UI’s) was mostly unaffected – we just shifted the rug out from under them. I helped Big Ant a lot on their next project to optimize the architecture and port it to the previous-gen console, allowing a lot of the rendering systems to work across five SKUs. PC, PS4, Xbox One PS4, Xbox One Steam, PS4, PS3, Xbox One, Xbox 360

(Textures, Buffers, Samplers)
The GPU pipeline SM2 Draw: Input Assembler Vertex Shader Rasterizer Pixel Shader Output Merger Resources / Memory (Textures, Buffers, Samplers)

The GPU pipeline + Vertex texture fetch SM3 Draw: Input Assembler Vertex Shader Rasterizer Pixel Shader Output Merger Resources / Memory (Textures, Buffers, Samplers)

The GPU pipeline + Geometry Shader & Stream Out stage + Compute shaders SM4 Draw: Rasterizer Input Assembler Vertex Shader Geometry Shader Pixel Shader Output Merger Stream Out Resources / Memory (Textures, Buffers, Samplers) Dispatch: Compute Shader

The GPU pipeline + Read-Write resources (UAVs) at pixel shader + Tessellation stages SM5 Draw: Rasterizer Input Assembler Vertex Shader Hull Shader Tessellator Domain Shader Geometry Shader Pixel Shader Output Merger Stream Out Resources / Memory (Textures, Buffers, Samplers) Dispatch: Compute Shader

The GPU pipeline + Read-Write resources at every stage SM5+ Draw: Rasterizer Input Assembler Vertex Shader Hull Shader Tessellator Domain Shader Geometry Shader Pixel Shader Output Merger Stream Out Resources / Memory (Textures, Buffers, Samplers) Dispatch: Compute Shader

The GPU pipeline Most common features Draw: Rasterizer Input Assembler Vertex Shader Pixel Shader Output Merger Stream Out Resources / Memory (Textures, Buffers, Samplers) Dispatch: Compute Shader

Buffer / Texture / Sampler
The GPU pipeline Most common features, API view API states: Input Layout Programs Depth / Stencil Blend Raster Rasterizer Draw Command Input Assembler Vertex Shader Pixel Shader Output Merger Stream Out Resource bindings: Buffer Buffer Depth Texture Colour Texture Buffer / Texture / Sampler

The GPU pipeline 2005 - 2015 Most common features, API view Programs
Dispatch Command Compute Shader Buffer / Texture

Stateless Rendering

Native APIs are state machines
Draw(3, TRIANGLES)– Behaviour depends on the current state ??? ??? ??? ??? ??? Rasterizer Input Assembler Vertex Shader Pixel Shader Output Merger Stream Out ??? ??? ??? ??? ???

BindTexture( t )– plug some resources in BindVertexBuffer( v ) BindRenderTarget( r ) ??? ??? ??? ??? ??? Rasterizer Input Assembler Vertex Shader Pixel Shader Output Merger Stream Out v ??? ??? r t

SetBlend( OPAQUE )– configure some fixed-function bits SetShaderProgram( s )– plug some procedures in ??? s ??? OPAQUE ??? Rasterizer Input Assembler Vertex Shader Pixel Shader Output Merger Stream Out v ??? r t

SetInputLayout( l ) SetRaster( SOLID ) SetDepthTest( DISABLED ) l s DIS- ABLED OPAQUE SOLID Rasterizer Draw(3, TRIANGLES) Input Assembler Vertex Shader Pixel Shader Output Merger Stream Out v r t

State machine issues (and features)
Objects can specify that they “don’t care” about a state (by not setting it) “Don’t care” states can be inherited from the calling logic. SetBlend( Translucent ) House.Draw() Tree.Draw()

State machine issues and features
But… this system of inherited state can be very fragile to code modifications. void House::Draw(){ SetBlend( Opaque ) Draw( TRIANGLES,3 ) } We didn’t configure the fixed-function depth test, which means we inherit it’s configuration from the previous draw. SetBlend( Translucent ) House.Draw() Tree.Draw() – uh oh!

State machine issues and features
It can also lead to inefficiencies as your graphics programmers become pessimistic. void Tree::Draw(){ SetBlend( Opaque ) ... } void House::Draw(){

Stateless Alternative
Simplify the API remove the entire state machine concept! Less mental tax no worrying about leaky states Retain the flexibility of “don’t care” states but remove the fragility that it has in state-machine APIs

Draw Items Pipeline state Resources Primitives
Bundle all native API state and all resource bindings together into a “Draw Item”. Missing / “don’t care” states are always filled in by some form of default value. Pipeline state Resources Primitives Input Layout Depth / Stencil Raster Blend Programs Buffer Texture Sampler Draw Command

Draw Items draw_item = CreateDrawItem( ... ) Submit( draw_item )
– Behaviour depends only on the contents of the draw item draw_item IA VS Raster PS Less Solid Code Layout VB Colour OM Opaque Tex Depth 3 triangles

Leaky states Now impossible to get state “leakage”. Every draw is completely independent and immune to code modifications in other drawing systems. Submit( House.GetDrawItem() ) Submit( Tree.GetDrawItem() ) We didn’t configure the fixed-function depth test, which means we inherit it’s configuration from the previous draw.

State Groups Container for Pipeline States and Resource Bindings.
Plain-old-data, generated by a ‘writer’ object. StateGroupWriter sgw sgw.Begin() sgw.BindTexture( t ) sgw.BindVertexBuffer( v ) sgw.SetBlend( Opaque ) sgw.SetShaderProgram( s ) StateGroup* sg = sgw.End() Blend Programs Buffer Texture

State Group Stacks Allow different systems to contribute pipeline-states and resource bindings. StateGroup* mesh = ... StateGroup* material = ... StateGroup* stack[] = {material, mesh} Input Layout Buffer Blend Programs Raster Texture

State Overrides Stack ordering dictates priority for overrides. Placing a state-group at the front of the array causes it’s values to be chosen in any state conflicts. StateGroup* mesh = ... StateGroup* material = ... StateGroup* override = ... StateGroup* stack[] = {override, material, mesh} Input Layout Buffer Blend Programs Raster Texture Blend

State Overrides Stack ordering dictates priority for overrides. Placing a state-group at the front of the array causes it’s values to be chosen in any state conflicts. StateGroup* stack[] = {override, material, mesh} override material mesh Input Layout Buffer Blend Blend Programs Raster Texture

State Defaults Stack ordering dictates priority for overrides. Placing a state-group at the back of the array causes it’s values to only be chosen as a fall-back. StateGroup* mesh = ... StateGroup* material = ... StateGroup* defaults = ... StateGroup* stack[] = {material, mesh, defaults}

State Defaults Stack ordering dictates priority for overrides. Placing a state-group at the back of the array causes it’s values to only be chosen as a fall-back. StateGroup* stack[] = {material, mesh, defaults} material mesh defaults Blend Programs Input Layout Buffer Input Layout Blend Raster Raster Texture Depth / Stencil Programs

Compiling a Draw Item Given a stack and a draw command, they can be pre-compiled into a draw item. override material mesh defaults Blend Blend Programs Input Layout Buffer Input Layout Blend Raster Raster Texture Depth / Stencil Programs StateGroup* stack[] = {override, material, mesh, defaults} DrawCommand command = { 3, TRIANGLES } DrawItem* draw = Compile( stack, command ) Draw Command

Compiling a Draw Item draw
Given a stack and a draw command, they can be pre-compiled into a draw item. Depth / Stencil Raster Programs Input Layout Buffer Blend Texture ??? Draw Command override material mesh defaults Blend Blend Programs Input Layout Buffer Input Layout Blend Raster Rasterizer Input Assembler Vertex Shader Pixel Shader Output Merger Raster Texture Depth / Stencil Programs Stream Out Fancy animating slide… Resolve conflicts Keep remaining states and resources Copy into template StateGroup* stack[] = {override, material, mesh, defaults} DrawCommand command = { 3, TRIANGLES } DrawItem* draw = Compile( stack, command ) Draw Command

Render Passes Draw Items defined all of the pipeline state except for the Depth/Stencil Target and Render Targets. Render Passes define these destination resources, plus the default and override state groups. RenderPass* pass = CreatePass( depth, color, defaults, override ) StateGroup* stack[] = { material, mesh } DrawCommand command = { 3, TRIANGLES } DrawItem* draw = Compile( stack, command, pass ) DrawItem* draws[] = { draw } Submit( pass, draws ) StateGroup* stack[] = {override, material, mesh, defaults} DrawCommand command = { 3, TRIANGLES } DrawItem* draw = Compile( stack, command )

Resource Bindings

Resource ID’s (and state ID’s)
Similar to GL, we use small integer types to refer to resource allocations & views. No reference counting – a higher level of the engine can ‘wrap’ reference counting around this simple ‘integer handle’ scheme if necessary (a la std::shared_ptr). Helps decouple platform-specific types from the client code. This can be a significant memory saving per compiled Draw Item – Pointers are 64 bits! Most resource IDs should fit in <16 bits Some kinds of state IDs might fit in <8 bits! (how many blend modes do you really use?)

Resource slots Most resource binding points are arrays
Conflicts are resolved per individual array elements override material Sampler 1 Sampler 2 Blend Programs Sampler 0 Sampler 1 StateGroup* stack[] = {override, material}

Resource slots Resource slots aren’t named, only numbered?
Sampler 0, Sampler 1, Sampler 2… Constant Buffer 0, Constant Buffer 1, Constant Buffer2… Using this assumption at this level of the engine greatly simplifies development. Our ‘shader programs’ struct can use a sampler bitmask of 0x05 to indicate that it uses sampler slot #0 and slot #2 (i.e. ((1<<0)|(1<<2)) == 0x5) The State Group conflict / merging system is built on super simple integer comparisons.

Resource slots Using numbered slots requires defining convention.
Constant Buffer 0 is always used for the per-camera matrix data. Constant Buffer 1 is always used for lighting data etc. This is actually quite useful for “magic” engine-generated data, which always conforms to a known (hard-coded) structure. …such as camera matrices, which you want to automatically plug into every object. These are also a good use for the defaults/overrides state groups!

Resource slots Using named resources requires reflection.
To bind data to a named slot, simply use the shader reflection system. Check with the object’s shader to discover the number that’s associated with that name. This is useful for less rigidly defined structures, such as materials, which may change often during development and vary from object to object.

Input Assembler (D3D11) Input Assembler Index buffer Vertex buffer(s)
Binding slots: Input Assembler Input Layout (Formats, strides for each element) Index buffer (Buffer + offset) Vertex buffer(s) AKA Vertex Attribute Object, Vertex Declaration, etc… Input layouts are problematic, as they’re a link between mesh data layout and the vertex shader. This forces your mesh management code to be strongly coupled with your material code. Some of the vertex buffers being bound will be used for per-vertex attributes, and some will be used for per-instance. Input Assembler Vertex Shader Stream Out Rasterizer Pixel Shader Depth / Stencil Raster Programs Input Layout Buffer Output Merger Blend API states: Resource bindings: Buffer / Texture / Sampler Depth Texture Colour Texture Draw Command

Input Layouts and Vertex Shaders
Input layouts tell the VS where to find the vertex attributes. Stream #0 data: Position 1 Position 2 Position 3 Stream #1 data: TexCoord 1 Normal 1 TexCoord 2 Normal 2 TexCoord3 Normal 3 Offset: Stride: VS_Input_Full reads every attribute that’s defined by this stream format. VS_Input_Thin does not. struct VS_Input_Full { float3 p : Position; float2 t : TexCoord; float3 n : Normal } struct VS_Input_Thin { float3 p : Position; }

Input Layouts and Vertex Shaders
Lua config files define stream formats (memory layouts for buffers) and vertex formats (VS input structures). StreamFormat("example_stream", { [VertexStream(0)] = { Float32, 3, Position }, }, [VertexStream(1)] = { Float32, 3, Normal }, { Float32, 2, TexCoord, 0 }, }) VertexFormat("VS_Input_Full", { { "p", float3, Position }, { "t", float2, TexCoord, 0 }, { "n", float3, Normal }, }) InputLayout( "example_stream", "VS_Input_Full" ) InputLayout( "example_stream", "VS_Input_Thin" ) InputLayout( "simple_stream", "VS_Input_Thin" )

Input Assembler (Simplified)
Vertex Data Index buffer (Buffer ID + offset) Vertex buffer(s) Stream Format Input Layout (now hidden from the user) Instance Data Binding slots: Vertex Attribute Object, Vertex Declaration, etc…

Shader Resources (D3D11) Constant Buffer View(s) Buffer
Binding slots: Pixel Shader Constant Buffer View(s) Buffer Shader Resource View(s) Texture Sampler(s) Unordered Access view(s) …repeat for other shader stages GL notes – UAV==SSBO, CBV+Buffer==UBO, SRV+Buffer==‘Buffer Texture’, SRV+Texture+Sampler==Texture Input Assembler Vertex Shader Stream Out Rasterizer Pixel Shader Depth / Stencil Raster Programs Input Layout Buffer Output Merger Blend API states: Resource bindings: Buffer / Texture / Sampler Depth Texture Colour Texture Draw Command

Resource Lists D3D11 allows for 128 texture slots per shader stage.
Can we still allow the user to access a hundred textures without the overhead of managing a hundred binding points? How did APIs already solve this for constants / uniforms? Resource lists are constant buffers (UBOs) for texture bindings. Similar to “bindless” resources. Ports well to Mantle/Vulkan/D3D12 descriptor lists! Only a small number of resource list binding points required. Resource List Diffuse Map ID Normal Map ID Specular Map ID

Shader Resources (simplified)
Binding slots: Shader Stages (all) Constant Buffer ID(s) Resource List ID(s) Buffer ID / Texture ID Sampler ID(s) Unordered Access view(s) (Buffer ID / Texture ID) We’re using ID’s instead of pointers to view objects. We’ve merged all the shader stages together – the ability to bind 128 textures to the pixel shader and a different 128 textures to the vertex shader did not seem worth it, when compared to the simplicity gained by removing the need to manually specify which stage you’re binding to. Often, constant-buffers are used by several stages, and it’s a pain in the arse to issue to keep track of this and remember to bind it to every stage… On the modern APIs (d3d12/etc), this “all-stage” binding system also lets us share the same descriptor tables across stages if we desire. Resource lists blah

Draw Item Resources Draw Item
Final size of each draw item is usually <1 cache line Draw Item Resource List Buffer ID / Texture ID Constant Buffer ID(s) Raster ID Blend ID 2 – 256 bytes Depth / Stencil Program ID Resource List ID(s) Draw commands not only specify the number of primitives to draw (and the type), they also specify a vertex offset, which allows many draw items to share the same IA config. Input Assembler Config ID Sampler ID(s) Input Assembler Config Draw Command Vertex Data Unordered Access view(s) (Buffer ID / Texture ID) Instance Data 20 – 128 bytes 32 – 80 bytes

State Group Resources Final look at actual State Group members (all optional) State Group Constant Buffer ID(s) Raster ID Depth / Stencil Blend ID Resource List ID(s) Vertex Data Instance Data Sampler ID(s) Technique ID Shader Options Draw Item Unordered Access view(s) (Buffer ID / Texture ID) Program ID

Shaders

Program management Out of the box, shaders are hard to manage.
One program = Pixel Shader + Vertex Shader (+Geometry + Tessellation…) Most objects/materials require more than one program. Deferred rendering – write GBuffer attributes. Forward rendering – compute all shading and lighting. Shadow mapping – write depth only. Material LOD – enable disable features (e.g. normal mapping at a distance). Loop unrolling – compile the shader once for each value of N. All of these programs grouped together form a single Technique.

Techniques, Passes, Options, Permutations…
A technique is a single shader file (Effect in MS lingo) Each technique contains several passes Gbuffer, Forward, Depth-Only, etc… Each pass can contain several options Normal Mapping (y/n), Number of lights [0..8), etc… For each technique, for each pass, for each permutation of options, pre- compile the shader source file into a program Careful – each 1-bit option doubles the number of programs!

[FX] syntax All the APIs we use (except mobile/Mac/Linx) use a shader language that is close enough to HLSL that we can just write all our shader code in HLSL! A header file full of #defines is enough to smooth over the small differences in syntax. However, resource declaration syntax varies widely. Not all platforms support constant buffers (We support prev-gen / D3D9 / GL2 era). Not all platforms support Resource Lists. Not all platforms support separate Textures and Samplers 

[FX] syntax Small amount of code generation used to smooth over these issues. We search for comment blocks of the pattern /*[FX] … */ and execute their contents as Lua code. The Lua VM has been pre-registered with functions such as below, to create a domain-specific-language for declaring shader resources and techniques/passes/options: CBuffer( slot, stages, name, values ) TextureList( slot, stages, name, values ) Option( name, range ) Pass( slot, name, parameters )

[FX] Examples CBuffer( 0, Pixel, 'Material', { { g_emissive = float }, }) TextureList( 0, Pixel, 'Material', { { Tex2D, 's_Diffuse', 'Linear' }, }) Sampler(0, {Pixel,Vertex}, 'Linear', { MinFilter = Linear, MagFilter = Linear, MipFilter = Linear, AddressU = Wrap, AddressV = Wrap, AddressW = Wrap, })

[FX] Examples Pass( 0, 'Opaque', { vertexShader = 'vs_main'; pixelShader = 'ps_main'; vertexLayout = { 'VS_Input_Full' }; pixelOptions = ‘LightCount'; })

Shader Options Shader options are all packed together into a bitmask.
Option( 'NormalMapped' ) -- pick a bit for me (use reflection!) Option( 'NormalMapped', {id=3} ) -- mask == 0x8 (i.e. 1<<3) Option( 'LightCount', {id=4, min=1, max=4} ) 0x00 / == LightCount: 1 0x10 / == LightCount: 2 0x20 / == LightCount: 3 0x30 / == LightCount: 4 1 to 4 inclusive is 4 values. 4 values requires 2 bits. Start bit is #4, will also use bit #5. To encode, subtract min, shift left 4 places. To decode, shift right 4 places, mask off two bits, add min.

Shader Options Given a pass with: Option( 'NormalMapped', {id=0} )
Option( 'LightCount', {id=4, min=1, max=4} ) The permutations would be: 0x00 / == NormalMapped: 0, LightCount: 1 0x01 / == NormalMapped: 1, LightCount: 1 0x10 / == NormalMapped: 0, LightCount: 2 0x11 / == NormalMapped: 1, LightCount: 2 0x20 / == NormalMapped: 0, LightCount: 3 0x21 / == NormalMapped: 1, LightCount: 3 0x30 / == NormalMapped: 0, LightCount: 4 0x31 / == NormalMapped: 1, LightCount: 4 We would compile the shader code 8 times, with different #define values for ‘NormalMapped’ and ‘LightCount’

Program selection I lied earlier – I said that a Render Pass has just a depth-texture, render- target(s), defaults state group and overrides state group. A Render Pass also specifies a “shader pass” integer. Look up the technique, then look up the right pass within the technique… … and then you’ve got a potentially long list of permutations  State Group Technique ID Shader Options Render Pass Pass ID Draw Item Program ID Step 1 Step 2 Profit!

Shader Options - runtime
Conflict/merging of shader options ‘state’ is implemented a little differently. State Group State Group U32 value U32 mask value = 0x04 mask = 0x0F Shader Options Merged Options = 0x84 Instead of a simple all-or-nothing conflict resolution, each ‘Shader Options’ State Group member contains a bitfield value, and a mask. The mask allows us to resolve conflicts on each individual bit. Technique ID State Group value = 0x80 mask = 0xF0 Render Pass Pass ID

Permutation selection
When compiling your permutations, sort them by CountBitsSet(options_bitmask) such that permutations with more options bits set appear earlier in the array. At runtime, the user creates their own bitmask of requested features. Linearly search through the permutations list, stop when: (requested_options & permutation_options) == permutation_options i.e. stop as soon as you’re not delivering options that weren’t asked for. You won’t necessarily be able to satisfy the user’s request exactly, but this algorithm will give them the program that enables as many of their requests as possible. The sort order of ties doesn’t matter. Requested features bitmask may or may not actually match any particular program that exists. The final permutation in the list will always have a bitmask of zero, which always satisfies the condition and terminates the loop.

Permutation Selection (code)
int SelectProgramsIndex( u32 techniqueId, u32 passId, u32 featuresRequested ) { Technique& technique = techniques[techniqueId]; List<Pass>& passes = technique.passes; Pass& pass = passes[passId]; List<Permutation>& permutations = pass.permutations; for( int i = 0, end = permutations.count; i != end; ++i ) Permutation& permutation = permutations[i]; if( (featuresRequested & permutation.features) == permutation.features ) return permutation.bindingIdx; } return -1; For those of you downloading the slides…

Q&A? @BrookeHodgman

Thanks! @BrookeHodgman

Bonus slides That I was going to write but then I didn’t

GLSL notes GL + GLSL are just specifications – vendors create implementations (which are all broken) Validate your shaders using the Khronos reference compiler*. Don’t ship your source files. Implement a pre-processor for #include, etc. Obfuscate your shipping code if you feel the need. No guarantees that every vendor will optimize (or compile) your code properly! Implement a GLSL->AST->GLSL optimizing compiler. Or better: a HLSL->AST->GLSL optimizing compiler! Automate this! *

Draw sorting Write a function that hashes a compiled Draw Item.
More expensive state changes should be associated with more significant bits in the output. Draw Item Constant Buffer ID(s) Resource List ID(s) Sampler ID(s) Unordered Access view(s) (Buffer ID / Texture ID) Depth / Stencil Raster ID Program ID Blend ID Draw Command Input Assembler Config ID IA Config Shader & pipeline state Textures Hash 0x Sorting key

Transparent Draw sorting
Alpha-blended geometry must be rendered from back to front. Don’t use the draw item’s hash, use it’s distance from the camera. Depth Distance ~*(u32*)distance 0xABCDEF12 Sorting key

Hybrid Draw sorting 0xABCD1357 0x12345678
For opaque geometry to make use of Hi-Z, you want to render front-to-back. However, you also want to sort by state to reduce CPU costs. Compromise by using a hybrid Distance Coarse Depth Original Hash Merge 0xABCD1357 0x New sorting key Original Sorting key

Redundant state filtering
Each draw item is a very compact structure, containing state IDs. XOR’ing two draw items creates a bitmask that highlights any changes. Masking out sections of that bitmask and comparing them to zero lets you quickly check if a state has changed since the previous draw item.

Resource Management

Data conditioning / compilation

Shader compilation fun

Devices, contexts & command lists

Devices

Contexts

Multithreading on old APIs

Higher level layer examples

Scene manager

Materials

Lighting

Designing a Modern GPU Interface

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