compiler: improve target features support

misc/featuregen/README.md

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+# Featuregen
+
+This directory contains a python and CPP script that generates the needed information
+for features regarding microarchitecture and target features of the compiler.
+
+It is not pretty! But LLVM has no way to query this information with their C API.
+
+It generates these globals (intended for `src/build_settings.cpp`:
+
+- `target_microarch_list`: an array of strings indexed by the architecture, each string is a comma-seperated list of microarchitectures available on that architecture
+- `target_features_list`: an array of strings indexed by the architecture, each string is a comma-seperated list of target features available on that architecture
+- `target_microarch_counts`: an array of ints indexed by the architecture, each int represents the amount of microarchitectures available on that target, intended for easier iteration of the next global
+- `microarch_features_list`: an array of a tuple like struct where the first string is a microarchitecture and the second is a comma-seperated list of all features that are enabled by default for it
+
+In order to get the default features for a microarchitecture there is a small CPP program that takes
+a target triple and microarchitecture and spits out the default features, this is then parsed by the python script.
+
+This should be ran each time we update LLVM to stay in sync.
+
+If there are minor differences (like the Odin user using LLVM 14 and this table being generated on LLVM 17) it
+does not impact much at all, the only thing it will do is make LLVM print a message that the feature is ignored (if it was added between 14 and 17 in this case).
+
+## Usage
+
+1. Make sure the table of architectures at the top of the python script is up-to-date (the triple can be any valid triple for the architecture)
+1. `./build.sh`
+1. `python3 featuregen.py`
+1. Copy the output into `src/build_settings.cpp`