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  Soft-Wired Shaders
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Software rendering is more flexible than hardware rendering, yet most software renderers don't have as many features as modern graphics cards. And when they do, they totally lack performance and are only useful for offline rendering... The main reason for this is that you need lots of control statements to implement the different render states. Function calling and parameter passing in the middle of the pixel pipeline is no rarity in a reference rasterizer's shader interpreter. This causes lots of redundant memory accesses for variables that could have stayed in registers. They often also do not use any advanced instruction sets like MMX and SSE but are written in plain C. Today I present swShader, a software renderer which combines the full flexibility of software rendering with performance comparable to hand-written SIMD assembly. This is possible thanks to soft-wiring technology. Because the code path for a pixel pipeline is fixed as long as the render state doesn't change, all control statements can be eliminated by selecting only the useful arithmetic instructions that are executed for the current state. My SoftWire run-time assembler library ( is used for dynamically generating this code. Its run-time intrinsics make it very easy to use SIMD instructions and perform automatic register allocation to eliminate redundant memory accesses. Because of this total flexibility, it was not that hard to write a compiler for ps 2.0 code. For the parsing I use Alec Panovici's CppCC, which creates much nicer code in my opinion than the more well-known flex/bison duo. I can't repeat enough that writing shaders for the CPU allows total freedom and ps 2.0 is a restriction. It's even possible to be ahead of features supported in hardware. Also, some interesting algorithms which are hard to implement, inefficient and/or expensive in hardware can be much simpler in software. You can add a new feature to a software renderer in a matter of hours, while developing an integrated circuit takes months. So free your mind and create what you want to create! For more information and the newest version, please check out Read Readme.html before use. Note that in its current state it's only an alpha release, a proof-of-concept if you want. So don't be surprised to find tons of unimplemented methods, and bugs. If you discover any strange behaviour that you think I am not aware of, please report it on the sourceforge page. Of course performance isn't comparable to dedicated hardware, but contrary to the reference rasterizer it is real-time for simple applications. With an optimized clipper and rasterizer, improved register allocator, a peephole optimizer and a scheduler I hope to nearly double performance in the future... You can read all the implementation details in my article for the upcoming ShaderX 2 book: Enjoy! Nicolas "Nick" Capens

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