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Replace the default branch with an unreachable branch If it is the last variant Fixes #119520. Fixes #110097. LLVM currently has limited ability to eliminate dead branches in switches, even with the patch of https://github.com/llvm/llvm-project/issues/73446. The main reasons are as follows: - Additional costs are required to calculate the range of values, and there exist many scenarios that cannot be analyzed accurately. - Matching values by bitwise calculation cannot handle odd branches, nor can it handle values like `-1, 0, 1`. See [SimplifyCFG.cpp#L5424](https://github.com/llvm/llvm-project/blob/llvmorg-17.0.6/llvm/lib/Transforms/Utils/SimplifyCFG.cpp#L5424) and https://llvm.godbolt.org/z/qYMqhvMa8 - The current range information is continuous, even if the metadata for the range is submitted. See [ConstantRange.cpp#L1869-L1870](https://github.com/llvm/llvm-project/blob/llvmorg-17.0.6/llvm/lib/IR/ConstantRange.cpp#L1869-L1870). - The metadata of the range may be lost in passes such as SROA. See https://rust.godbolt.org/z/e7f87vKMK. Although we can make improvements, I think it would be more appropriate to put this issue to rustc first. After all, we can easily know the possible values. Note that we've currently found a slow compilation problem in the presence of unreachable branches. See https://github.com/llvm/llvm-project/issues/78578. r? compiler |
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