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Jon Ross-Perkins
3a489fd7b4
Right now, ConstantValueStore defaults to having unknown values use NotConstant. This generally works for the current IR, but with imports we're expecting sparse entries which are generally unknown -- and distinguishing between NotConstant and simply unset would be helpful. As a consequence, add Invalid. We discussed whether to simply have ConstantValueStore default to Invalid going forward, or to make the default flexible. The upside to the former is consistency, the upside to the latter is that it should result in fewer Sets when operating on the current IR (which will more frequently have known non-constant values). This PR offers both approaches in separate commits, but I somewhat lean towards the latter for fewer array resizes. Note, a totally different approach would be to use a different class (not ConstantValueStore) for imported IRs -- then the default of Invalid versus NotConstant would be type-dependent. However, I expect we're going to want to do at least somewhat consistent lookups, and using the same ConstantValueStore for both cases allows avoiding a virtual interface or templating. Also, I'm hoping to only maintain the ConstantValueStore for an imported IR as part of Context (not File), which would mean the SmallVector storage overhead is ephemeral, mitigating one of the potential advantages of using a different type for imported IRs.