cfe07cd42a
During incremental ThinLTO compilation, we attempt to re-use the optimized (post-ThinLTO) bitcode file for a module if it is 'safe' to do so. Up until now, 'safe' has meant that the set of modules that our current modules imports from/exports to is unchanged from the previous compilation session. See PR #67020 and PR #71131 for more details. However, this turns out be insufficient to guarantee that it's safe to reuse the post-LTO module (i.e. that optimizing the pre-LTO module would produce the same result). When LLVM optimizes a module during ThinLTO, it may look at other information from the 'module index', such as whether a (non-imported!) global variable is used. If this information changes between compilation runs, we may end up re-using an optimized module that (for example) had dead-code elimination run on a function that is now used by another module. Fortunately, LLVM implements its own ThinLTO module cache, which is used when ThinLTO is performed by a linker plugin (e.g. when clang is used to compile a C proect). Using this cache directly would require extensive refactoring of our code - but fortunately for us, LLVM provides a function that does exactly what we need. The function `llvm::computeLTOCacheKey` is used to compute a SHA-1 hash from all data that might influence the result of ThinLTO on a module. In addition to the module imports/exports that we manually track, it also hashes information about global variables (e.g. their liveness) which might be used during optimization. By using this function, we shouldn't have to worry about new LLVM passes breaking our module re-use behavior. In LLVM, the output of this function forms part of the filename used to store the post-ThinLTO module. To keep our current filename structure intact, this PR just writes out the mapping 'CGU name -> Hash' to a file. To determine if a post-LTO module should be reused, we compare hashes from the previous session. This should unblock PR #75199 - by sheer chance, it seems to have hit this issue due to the particular CGU partitioning and optimization decisions that end up getting made. |
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| Cargo.toml | ||
| README.md | ||
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.