14fbc3c005
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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The Rust Programming Language
This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.
Note: this README is for users rather than contributors. If you wish to contribute to the compiler, you should read CONTRIBUTING.md instead.
Table of Contents
Quick Start
Read "Installation" from The Book.
Installing from Source
If you really want to install from source (though this is not recommended), see INSTALL.md.
Getting Help
See https://www.rust-lang.org/community for a list of chat platforms and forums.
Contributing
See CONTRIBUTING.md.
License
Rust is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0), with portions covered by various BSD-like licenses.
See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.
Trademark
The Rust Foundation owns and protects the Rust and Cargo trademarks and logos (the "Rust Trademarks").
If you want to use these names or brands, please read the media guide.
Third-party logos may be subject to third-party copyrights and trademarks. See Licenses for details.