In my last article, I described the first generation of CrossStitch, the shader assembly system used in MechAssault 2. Today I’m going to write about the second generation of CrossStitch, the one used in Fracture.
Development on CrossStitch 2.0 began with the development of Day 1′s Despair Engine. This was right at the end of MechAssault 2, when the Xbox 360 was still an Apple G5 and the Cell processor was going to be powering everything from super computers to kitchen appliances in a few years. It is hard to believe looking back, but at that time we were still debating whether to adopt a high-level shading language for graphics. There were respected voices on the platform side insisting that the performance advantage of writing shaders in assembly language would justify the additional effort. Thankfully I sided with the HLSL proponents, but that left me with the difficult decision of what to do about CrossStitch.
CrossStitch was a relatively simple system targeting a single, very constrained platform. HLSL introduced multiple target profiles, generic shader outputs, and literal constants, not to mention a significantly more complex and powerful language syntax. Adding to that, Despair Engine was intended to be cross-platform, and we didn’t even have specs on some of the platforms we were promising to support. Because of this, we considered the possibility of dispensing with dynamic shader linking entirely and adopting a conventional HLSL pipeline, implementing our broad feature set with a mixture of compile-time, static, and dynamic branching. In the end, however, I had enjoyed so much success with the dynamic shader linking architecture of MechAssault 2, I couldn’t bear to accept either the performance cost of runtime branching or the clunky limitations of precomputing all possible shader permutations.
The decision was made: Despair Engine would feature CrossStitch 2.0. I don’t recall how long it took me to write the first version of CrossStitch 2.0. The early days of Despair development are a blur because we were supporting the completion of MechAssault 2 while bootstrapping an entirely new engine on a constantly shifting development platform and work was always proceeding on all fronts. I know that by December of 2004, however, Despair Engine had a functional implementation of dynamic shader linking in HLSL.
CrossStitch 2.0 is similar in design to its predecessor. It features a front-end compiler that transforms shader fragments into an intermediate binary, and a back-end linker that transforms a chain of fragments into a full shader program. The difference, of course, is that now the front-end compiler parses HLSL syntax and the back-end linker generates HLSL programs. Since CrossStitch 1.0 was mostly limited to vertex shaders with fixed output registers, CrossStitch 2.0 introduced a more flexible model for passing data between pipeline stages. Variables can define and be mapped to named input and output channels; and each shader chain requires an input signature from the stage preceding it and generates an output signature for the stage following it.
CrossStitch’s primary concern is GPU runtime efficiency, so it is nice that shaders are compiled with full knowledge of the data they’ll be receiving either from vertex buffers or interpolators. If, for example, some meshes include per-vertex color and some don’t, the same series of shader fragments will generate separate programs optimized for each case. It turns out that this explicit binding of shader programs to attributes and interpolators is a common requirement of graphics hardware, and making the binding explicit in CrossStitch allows for some handy optimizations on fixed consoles.
The early results from CrossStitch 2.0 were extremely positive. The HLSL syntax was a nice break from assembly, and the dynamic fragment system allowed me to quickly and easily experiment with a wide range of options as our rendering pipeline matured. Just as had happened with MechAssault 2, the feature set of Despair expanded rapidly to become heavily reliant on the capabilities of CrossStitch. The relationship proved circular too. Just as CrossStitch facilitated a growth in Despair’s features, Despair’s features demanded a growth in CrossStitch’s capabilities.
The biggest example of this is Despair’s material editor, Façade. Façade is a graph-based editor that allows content creators to design extremely complex and flexible materials for every asset. The materials are presented as a single pipeline flow, taking generic mesh input attributes and transforming them through a series of operations into a common set of material output attributes. To implement Façade, I both harnessed and extended the power of CrossStitch. Every core node in a Facade material graph is a shader fragment. I added reflection support to the CrossStich compiler, so adding a new type of node to Façade is as simple as creating a new shader fragment and annotating its public-facing variables. Since CrossStitch abstracts away many of the differences between pipeline stages, Façade material graphs don’t differentiate between per-vertex and per-pixel operations. The flow of data between pipeline stages is managed automatically depending on the requirements of the graph.
It was about 6 months after the introduction of Façade when the first cracks in CrossStitch began to appear. The problem was shader compilation times. On MechAssault 2 we measured shader compilation times in microseconds. Loading a brand new level with no cached shader programs in MechAssault 2 might cause a half-second hitch as a hundred new shaders were compiled. If a few new shaders were encountered during actual play, a couple of extra milliseconds in a frame didn’t impact the designers’ ability to evaluate their work. Our initial HLSL shaders were probably a hundred times slower to compile than that on a high-end branch-friendly PC. By the end of 2005 we had moved to proper Xbox 360 development kits and our artists had mastered designing complex effects in Façade. Single shaders were now taking as long as several seconds to compile, and virtually every asset represented a half-dozen unique shaders.
The unexpected 4-5 decimal order of magnitude increase in shader compilation times proved disastrous. CrossStitch was supposed to allow the gameplay programmers, artists, and designers to remain blissfully ignorant of how the graphics feature set was implemented. Now, all of a sudden, everyone on the team was aware of the cost of shader compilation. The pause for shader compilation was long enough that it could easily be mistaken for a crash, and, since it was done entirely on the fly, on-screen notification of the event couldn’t be given until after it was complete. Attempts to make shader compilation asynchronous weren’t very successful because at best objects would pop in seconds after they were supposed to be visible and at worst a subset of the passes in a multipass process would be skipped resulting in unpredictable graphical artifacts. Making matters worse, the long delays at level load were followed by massive hitches as new shaders were encountered during play. It seemed like no matter how many times a designer played a level, new combinations of lighting and effects would be encountered and repeated second-long frame rate hitches would make evaluating the gameplay impossible.
Something had to be done and fast.
Simple optimization was never an option, because almost the entire cost of compilation was in the HLSL compiler itself. Instead I focused my efforts on the CrossStitch shader cache. The local cache was made smarter and more efficient, and extended so that multiple caches could be processed simultaneously. That allowed the QA staff to start checking in their shader caches, which meant tested assets came bundled with all their requisite shaders. Of course content creators frequently work with untested assets, so there was still a lot of unnecessary redundant shader compilation going on.
To further improve things we introduced a network shader cache. Shaders were still compiled on-target, but when a missing shader was encountered it would be fetched from a network server before being compiled locally. Clients updated servers with newly compiled shaders, and since Day 1 has multiple offices and supports distributed development, multiple servers had to be smart enough to act as proxies for one another.
With improvements to the shader cache, life with dynamic, on-the-fly shader compilation was tolerable but not great. The caching system has only had a few bugs in its lifetime, but it is far more complicated than you might expect and only really understood by a couple of people. Consequently, a sort of mythology has developed around the shader cache. Just as programmers will suggest a full rebuild to one another as a possible solution to an inexplicable code bug, content creators and testers can be heard asking each other, “have you tried deleting your shader cache?”
At the same time as I was making improvements to the shader cache, I was also working towards the goal of having all shaders needed for an asset compiled at the time the asset was loaded. I figured compiling shaders at load time would solve the in-game hitching problem and it also seemed like a necessary step towards my eventual goal of moving shader compilation offline. Unfortunately, doing that without fundamentally changing the nature and usage of CrossStitch was equivalent to solving the halting problem. CrossStitch exposes literally billions of possible shader programs to the content, taking advantage of the fact that only a small fraction of those will actually be used. Which fraction, however, is determined by a mind-bending, platform-specific tangle of artist content, lua script, and C++ code.
I remember feeling pretty pleased with myself at the end of MechAssault 2 when I learned that Far Cry required a 430 megabyte shader cache compared to MA2′s svelte 500 kilobyte cache. That satisfaction evaporated pretty quickly during the man-weeks I spent tracking down unpredicted shader combinations in Fracture.
Even so, by the time we entered full production on Fracture, shader compilation was about as good as it was ever going to get. A nightly build process loaded every production level and generated a fresh cache. The build process updated the network shader cache in addition to updating the shader cache distributed with resources, so the team had a nearly perfect cache to start each day with.
As if the time costs of shader compilation weren’t enough, CrossStitch suffered from an even worse problem on the Xbox 360. Fracture’s terrain system implemented a splatting system that composited multiple Facade materials into a localized über material, and then decomposed the über material into multiple passes according the register, sampler, and instruction limits of the target profile. The result was some truly insane shader programs.
A few Fracture terrain shaders took over 30 seconds to compile and consumed over 160 megabytes of memory in the process. Since the Xbox 360 development kits have no spare memory, this posed a major problem. There were times when the content creators would generate a shader that could not be compiled on target without running out of memory and crashing. It has only happened three times in five years, but we’ve actually had to run the game in a special, minimal memory mode in order to free up enough memory to compile a necessary shader for a particularly complex piece of content. Once the shader is present in the network cache, the offending content can be checked in and the rest of the team is none the wiser.
Such things are not unusual in game development, but it still kills me to be responsible for such a god-awful hack of a process.
And yet, CrossStitch continues to earn its keep. Having our own compiler bridging the gap between our shader code and the platform compiler has proved to be a very powerful thing. When we added support for the Playstation 3, Chris modified the CrossStitch back-end to compensate for little differences in the Cg compiler. When I began to worry that some of our shaders were interpolator bound on the Xbox 360, the CrossStitch back-end was modified to perform automatic interpolator packing. When I added support for Direct3D 10 and several texture formats went missing, CrossStitch allowed me to emulate the missing texture formats in late-bound fragments. There doesn’t seem to be a problem CrossStitch can’t solve, except, of course, for the staggering inefficiency of its on-target, on-the-fly compilation.
For our next project I’m going to remove CrossStitch from Despair. I’m going to do it with a scalpel if possible, but I’ll do it with a chainsaw if necessary. I’m nervous about it, because despite my angst and my disillusionment with dynamic shader compilation, Day 1′s artists are almost universally fans of the Despair renderer. They see Façade and the other elements of Despair graphics as a powerful and flexible package that lets them flex their artistic muscles. I can’t take that away from them, but I also can’t bear to write another line of code to work around the costs of on-the-fly shader compilation.
It is clear to me now what I didn’t want to accept five years ago. Everyone who has a say in it sees the evolution of GPU programs paralleling the evolution of CPU programs: code is static and data is dynamic. CrossStitch has had a good run, but fighting the prevailing trends is never a happy enterprise. Frameworks like DirectX Effects and CgFx have become far more full-featured and production-ready than I expected, and I’m reasonably confident I can find a way to map the majority of Despair’s graphics features onto them. Whatever I come up with, it will draw a clear line between the engine and its shaders and ensure that shaders can be compiled wherever and whenever future platforms demand.