What building an LLM inference engine from scratch taught me about compiler design

the insight that started this project hit me while i was finishing a bytecode-compiled language i'd written in C i'd spent months building a hand-written lexer, a single-pass Pratt compiler, a stack VM with 35 opcodes, and a mark-and-sweep garbage collector. and right near the end i had this realization: an LLM inference engine is the same problem. it's a graph-compile plus memory-plan plus kernel-schedule problem. i'd just built one so i decided to find out if that was actually true the project the result is ignis, a from-scratch LLM inference engine in Rust. i used it specifically to see how far the compiler analogy held up. the dependency count ended up at 2: memmap2 (to mmap the weight blob off disk) and fancy-regex (for one look-ahead in the BPE tokenizer). everything else is hand-written, because the whole point was to understand what's actually happening the compiler analogy holds up better than i expected the interesting part of any inference engine isn't loading the weights or doing matrix math. it's what happens between "here's a compute graph" and "here's an efficient execution plan." that's a compiler problem ignis builds an SSA (static single assignment) IR of the entire Qwen2 forward pass. every operation in the transformer (the RMSNorm layers, the SwiGLU activations, the attention projections, all of it) becomes a node in the graph with explicit data dependencies then fusion passes run over the graph. the intuition is simple: if operation B always and only reads the output of operation A, you can merge them into one op and eliminate the intermediate buffer. in practice this fused 49 RMSNorm ops and 24 SwiGLU ops, bringing the total from 435 operations down to 362 that part felt expected. the liveness analysis surprised me the liveness analysis after fusion, the graph still needs activation buffers: scratch memory to hold intermediate results as the plan executes. the naive approach allocates one buffer per node. the smarter approach asks: which buffer