Cross Platform Development Best Practices

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Cross Platform Development Best Practices

• Matt Lee, Kev Gee
• Microsoft Game Technology Group

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Agenda

• Code Considerations
• CPU Considerations
• GPU Considerations
• IO Considerations
• Content Considerations
• Data Build System
• Geometry Formats
• Texture Formats
• Shaders
• Audio Considerations

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Compiler Comparison

• VS 2005 front end used for both platforms
• Preprocessor benefits both platforms
• Debugger experience is the same
• Full 2005 IDE support coming
• Xbox 360 optimizing back end added with XDK
  install
• Single solution / MSBuild file can target both
  platforms

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PC CPUs

• Intel Pentium D / AMD Athlon64 X2
• Programming Model
• 2 Cores running _at_ around 3.20 GHz
• 12-KB Execution trace cache
• 16-KB L1 cache, 1 MB L2 cache
• Deep Branch Prediction
• Dynamic data flow analysis
• Speculative Execution
• Little-endian byte ordering
• SIMD instructions
• Quad Core announced for early 2007

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360 Custom CPU

• Custom IBM Processor
• 3 64-bit PowerPC cores running at 3.2 GHz
• Two hardware threads per core
• 32-KB L1 instruction cache data cache, per core
  
• Shared 1-MB L2 cache
• 128-byte cache lines on all caches
• Big-endian byte ordering
• VMX 128 SIMD
• Lots of Registers

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Performance Tools

• Profiling approaches are very similar between PC
  and Xbox 360
• PIX for Xbox 360 PIX for Windows
• Being developed by the same team now
• Use instrumented tools on Xbox 360
• XbPerfView / Tracedump
• Xbox 360 does not have a sampling profiler yet
• Use PC profiling tools
• Intel VTune / AMD Code Analyst / VS Team System
  Profiler
• Attend the Performance Hands on training!

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Focus Your Efforts

• Use performance tools to guide work
• Areas where we have seen platform specific
  efforts reap rewards
• Single Data Pass engine design
• High Frequency Game API Layers
• Use your profiler tools to target the hot spots
• Math Library - Bespoke vs XGMath vs D3DXMath

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Impact on Code Design

• Designing Cross platform APIs
• Use of virtual Functions
• Parameter passing mechanisms
• Pass by reference vs. pass by value
• Typedef vector types and intrinsics
• Math Library Design Case Study
• Use of inlining

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Use of Virtual Functions

• Be careful when using virtual functions to hide
  platform differences
• Virtual function performance on Xbox 360
• Adds branch instruction which is always
  mispredicted!
• Compiler limited in optimizing these
• Make a concrete implementation for Xbox 360
• Avoid virtual functions in inner loops

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Cross Platform Render Example
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Cross Platform Render Example (ctd.)

• class IRenderSystem
• public
• if !defined(_XBOX)
• virtual void Draw()0
• else
• void Draw()
• endif

• void IRenderSystemDraw()
• // 360 Implementation

• D3D9 D3D10 implementations subclass for
  specialization

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Beware Big Constructors

• Ctors can dominate execution time
• Ctors often hidden to casual observer
• Copy ctors add objects to containers
• Arrays of C objects are constructed
• Overloaded operators may construct temporaries
• Consider should ctor init data?
• Example matrix class zeroing all data
• Prefer array initialization

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Inlining

• Careful inlining is in general a Good Thing
• Plan to spend time ensuring the compiler is
  inlining the right stuff
• Use Perf Tools such as VTune / Trace recorder
• Try the inline any suitable option
• Enable link-time code generation
• Consider profile-guided optimization
• Use __forceinline only where necessary

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Consider Passing Native Types by Value

• Xbox 360 has large registers
• 64 bit Native PC does too
• Pass and return these types by value
• int, __int64, float
• Consider these types if targeting SSE / VMX
• __m128 / __vector4, XMVECTOR, XMMATRIX
• Pass structs by pointer or reference
• Help the compiler using _restrict

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Math Library Header (Xbox 360)

• if defined( _XBOX )
• include
• include
• typedef __vector4 XVECTOR
• typedef const XVECTOR XVECTOR_PARAM
• typedef XVECTOR XVECTOR_OUTPARAM
• define XMATHAPI inline
• define VMX128_INTRINSICS
• endif

Pass by value
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Math Library Header (Windows)

• if defined( _WIN32 )
• include
• typedef __m128 XVECTOR
• typedef const XVECTOR XVECTOR_PARAM
• typedef XVECTOR XVECTOR_OUTPARAM
• define XMATHAPI inline
• define SSE_INTRINSICS
• endif

Pass by reference
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Math Library Function

• XVECTOR XMATHAPI XVectorAdd( XVECTOR_PARAM vA,
• XVECTOR_PARAM vB )
• if defined( VMX128_INTRINSICS )
• return __vaddfp( vA, vB )
• elif defined( SSE_INTRINSICS )
• return _mm_add_ps( vA, vB )
• endif

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Threading

• Why Multithread?
• Necessary to take full advantage of modern CPUs
• Attend the Multi-threading talk later today
• Covers synchronization prims and lockless sync
  methods
• See Also
• Talks from Intel and AMD (GDC2005 / GDC-E)
• OpenMP C, not C, useful in limited
  circumstances
• Concur C, see
• http//microsoft.sitestream.com/PDC05/TLN/TLN309_f
  iles/Default.htmnopreload1autostart1

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D3D Architectural Differences

• D3D9 draw call cost is higher on Windows than on
  Xbox 360
• 360 is optimized for a Single GPU target
• D3D10 improves draw call cost by design on
  Windows
• Very important to carefully manage the number of
  batches submitted
• This can have an impact on content creation
• This work will help with 360 performance too

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Agenda

• Code Considerations
• CPU Considerations
• GPU Considerations
• IO Considerations
• Content Considerations
• Data Build System
• Geometry Formats
• Texture Formats
• Shaders
• Audio Considerations

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PC GPUs

• Wide variety of available Direct3D9 H/W
• CAPs and Shader Models abstract over feature
  differences
• GPUs that are approximately equivalent
  performance to the Xbox 360 GPU
• ATi X1900 / NVidia 7800 GTX
• Shader Model 3.0 support
• Direct3D10 Standardizes feature set
• H/W Scales on performance instead

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Xbox 360 Custom GPU

• Direct3D 9.0 compatible
• High-Level Shader Language (HLSL) 3.0 support
• 10 MB Embedded DRAM
• Frame Buffer with 256 GB/sec bandwidth
• Hardware scaling for display resolution matching
• 48 shader ALUs shared between pixel and vertex
  shading (unified shaders)
• Up to 8 simultaneous contexts (threads) in-flight
  at once
• Changing shaders or render state can be cheap,
  since a new context can be started up easily
• Hardware tesselator
• N-patches, triangular patches, and rectangular
  patches
• For non continuous / adaptive cases trade memory
  for this feature on PC systems

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Explicit Resolve Control

• Copies surface data from EDRAM to a texture in
  system memory
• Required for render-to-texture and presentation
  to the screen
• Can perform MSAA sample averaging or resolve
  individual samples
• Can perform format conversions and biasing
• Cannot do rescaling or resampling of any kind
• This can Impact your Xbox 360 engine design as it
  adds an extra step to common operations.

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Agenda

• Code Considerations
• CPU Considerations
• GPU Considerations
• IO Considerations
• Content Considerations
• Geometry
• Textures
• Shaders
• Audio data

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Use Native File I/O Routines

• Only native routines support key features
• Asynchronous I/O
• Completion routines
• Prefer CreateFile and ReadFile
• Guaranteed as fast or faster than any other
  alternatives
• Avoid fopen, fread, C iostreams

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Use Asynchronous File I/O

• File read/write operations block by default
• Async operations allows the game to do other
  interesting work
• CreateFile with FILE_FLAG_OVERLAPPED
• Use FILE_FLAG_NO_BUFFERING, too
• Guarantees no intermediate buffering
• Use OVERLAPPED struct to determine when operation
  is complete
• See CreateFile docs for details

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Memory Mapped File I/O

• Fastest way to load data on Windows
• However, the 32 bit address space is getting
  tight
• This is a great 64 bit feature add! ?
• Memory Mapped I/O not supported on 360
• No HDD backed Virtual Memory management system

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Universal Gaming Controller

• XInput is the same API for Xbox 360 and Windows
• The Microsoft universal controller is a reference
  design which can be leveraged by other hardware
  manufacturers
• XP Driver available from Windows Update
• Support is built in to Xbox 360 and Windows
  Vista

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Agenda

• Code Considerations
• CPU Considerations
• GPU Considerations
• IO Considerations
• Content Considerations
• Data Build System
• Geometry Formats
• Texture Formats
• Shaders
• Audio Considerations

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Data Build System

• Add a data build / processing phase to your
  production system
• Compile, optimize and compress data according to
  multiple target platform requirements
• Easier and faster to handle endian-ness and other
  format conversions offline
• Data packing process can occur here too
• Invest time in making the build fast
• Artists need to rapidly iterate to make quality
  content
• Incremental builds can really help reduce the
  buildtime
• Try the XNA build tools
• Copies of XNA build CTP are available NOW!

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Geometry Compression

• Offline Compression of Geometry
• Provides wins across all platforms
• Disk I/O wins as well as GPU wins
• The compression approach is likely to be target
  specific
• PC is usually a superset of the consoles in this
  area
• D3D9 CAPs / limitations to consider
• 16 bit Normals - D3DDECLTYPE_FLOAT16_2

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Compressing Textures

• Wide variety of Texture Compression Tools
• ATI Compressinator
• DirectX SDK DDS tools
• NVIDIA Photoshop DDS Export
• Compression tools for 360 (xgraphics.lib)
• Supports endian swap of texture formats
• Build your own too!
• Make them fit your content.

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Texture Formats

• DXT / DXGI_FORMAT_BC
• BC Block Compressed
• Standard DXT formats across all platforms
• DXN / DXGI_FORMAT_BC5 / BC5u
• 2-component format with 8 bits of precision per
  component
• Great for normal maps
• DXT3A / DXT5A
• Single component textures made from a DXT3/DXT5
  alpha block
• 4 bits of precision
• Xbox 360 / D3D9 Only

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Texture Arrays

• Texture arrays
• generalized version of cube maps
• D3D9 emulate using a texture atlas
• Xbox 360
• Up to 64 surfaces within a texture, optional
  MIPmaps for each surface
• Surface is indexed with a 0..1 z coordinate in
  a 3D texture fetch
• D3D10 supports this as a standard feature
• Up to 512 surfaces within a texture
• Bindable as rendertarget, with per-primitive
  array index selection

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Custom Vertex Fetch / Vertex Texture

• D3D9 Vertex Texture implementations use
  intrinsics
• tex2dlod()
• 360 supports explicit instructions for this
• D3D10 supports this as a standard feature
• Load() from buffer (VB, IB, etc.) at any stage
• Sample() from texture at any stage

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Effects

• D3DX FX and FX Lite co-exist easily
• define around the texture sampler differences
• Preshaders are not supported on FX Lite
• We advise that these should be optimized to
  native code for D3D9 Effects

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HLSL Development

• Set up your engine and tools for rapid shader
  development and iteration
• Compile shaders offline for performance,
• maybe allow run-time recompilation during
  development
• Be careful with shader generation tools
• Perf needs to be considered
• Schedule / Plan work for this

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Cross-Platform HLSL Consideration

• Texture access instruction considerations
• Xbox 360 has native tfetch / getWeights features
• Constant texel offsets (-8.0 to 7.5 in 0.5
  increments)
• Independent of texture size
• Direct3D 10 supports integer texture offsets when
  fetching
• Direct3D 10 supports getdimensions() natively
• Equivalent to getWeights
• Direct3D 9 can emulate tfetch getWeights
  behavior using a shader constant for texture
  dimensions

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HLSL Example
float2 g_invTexSize float2( 1/512.0f,
1/512.0f) float2 getWeights2D( float2 texCoord
) return frac( texCoord / g_invTexSize
) float4 tex2DOffset( sampler t, float2
texCoord, float2 offset ) texCoord offset
g_invTexSize return tex2D( t, texCoord )
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Shader management

• Find a balance between übershaders and
  specialized shader libraries
• Dynamic/static branching versus static
  compilation
• Small shader libraries can be built and stored
  inside a single Effect file
• One technique per shader configuration
• Larger shader libraries
• Hash table populated with configurations
• Streaming code can load could shader groups on
  demand
• Profile-guided content generation
• Avoid compiling shaders at run time
• Compiled shaders compress very well

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Audio Considerations

• XACT
• (Microsoft Cross-Platform Audio Creation Tool)
• API and authoring tool parity
• author once, deploy to both platforms
• Primary difference wave compression
• ADPCM on Windows vs. Xbox 360 native XMA support
• XMA controllable quality setting (varies,
  typically 6-141)
• ADPCM Static 3.51 compression
• Likely need to trade memory for bit rate.
• On Windows, can use hard disk streaming to
  balance lower compression rates if needed

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Call To Action!

• Design your games, engines and production systems
  with cross platform development in mind
• (PC / Xbox 360 / Other)
• Invest in making your data build system fast
• Take advantage of each platforms strengths
• Target a D3D10 content design point and fallback
  to D3D9, D3D9,
• Provide feedback on how we can make production
  easier
• Attend the XACT, HLSL, SM4.0 and Performance
  Hands On Labs

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Questions?
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