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Air: direct representation of ranges in switch cases
This commit modifies the representation of the AIR `switch_br` instruction to represent ranges in cases. Previously, Sema emitted different AIR in the case of a range, where the `else` branch of the `switch_br` contained a simple `cond_br` for each such case which did a simple range check (`x > a and x < b`). Not only does this add complexity to Sema, which -- as our secondary bottleneck -- we would like to keep as small as possible, but it also gets in the way of the implementation of ziglang#8220. This proposal turns certain `switch` statements into a looping construct, and for optimization purposes, we want to lower this to AIR fairly directly (i.e. without involving a `loop` instruction). That means we would ideally like a single instruction to represent the entire `switch` statement, so that we can dispatch back to it with a different operand as in ziglang#8220. This is not really possible to do correctly under the status quo system. For now, the actual lowering of `switch` is identical for the LLVM and C backends. This commit contains a TODO which temporarily regresseses all remaining self-hosted backends in the presence of switch case ranges. This functionality will be restored for at least the x86_64 backend before merge of this branch.
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