In defense of NIR
Submitted by Roy Schestowitz on Friday 28th of January 2022 02:23:20 AM
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Shortly after I joined the Mesa team at Intel in the summer of 2014, I was sitting in the cube area asking Ken questions, trying to figure out how Mesa was put together, and I asked, “Why don’t you use LLVM?” Suddenly, all eyes turned towards Ken and myself and I realized I’d poked a bear. Ken calmly explained a bunch of the packaging/shipping issues around having your compiler in a different project as well as issues radeonsi had run into with apps bundling their own LLVM that didn’t work. But for the more technical question of whether or not it was a good idea, his answer was something about trade-offs and how it’s really not clear if LLVM would really gain them much.
That same summer, Connor Abbott showed up as our intern and started developing NIR. By the end of the summer, he had a bunch of data structures a few mostly untested passes, and a validator. He also had most of a GLSL IR to NIR pass which mostly passed validation. Later that year, after Connor had gone off to school, I took over NIR, finished the Intel scalar back-end NIR consumer, fixed piles of bugs, and wrote out-of-SSA and a bunch of optimization passes to get it to the point where we could finally land it in the tree at the end of 2014. Initially, it was only a few Intel folks and Emma Anholt (Broadcom, at the time) who were all that interested in NIR. Today, it’s integral to the Mesa project and at the core of every driver that’s still seeing active development. Over the past seven years, we (the Mesa community) have poured thousands of man hours (probably millions of engineering dollars) into NIR and it’s gone from something only capable of handling fragment shaders to supporting full Vulkan 1.2 plus ray-tracing (task and mesh are coming) along with OpenCL 1.2 compute.
Was it worth it? That’s the multi-million dollar (literally) question. 2014 was a simpler time. Compute shaders were still newish and people didn’t use them for all that much more than they would have used a fancy fragment shader for a couple years earlier. More advanced features like Vulkan’s variable pointers weren’t even on the horizon. Had I known at the time how much work we’d have to put into NIR to keep up, I may have said, “Nah, this is too much effort; let’s just use LLVM.” If I had, I think it would have made the wrong call.
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