f77c624c03
Filter out short-lived LLVM diagnostics before they reach the rustc handler During profiling I saw remark passes being unconditionally enabled: for example `Machine Optimization Remark Emitter`. The diagnostic remarks enabled by default are [from missed optimizations and opt analyses](https://github.com/rust-lang/rust/pull/113339#discussion_r1259480303). They are created by LLVM, passed to the diagnostic handler on the C++ side, emitted to rust, where they are unpacked, C++ strings are converted to rust, etc. Then they are discarded in the vast majority of the time (i.e. unless some kind of `-Cremark` has enabled some of these passes' output to be printed). These unneeded allocations are very short-lived, basically only lasting between the LLVM pass emitting them and the rust handler where they are discarded. So it doesn't hugely impact max-rss, and is only a slight reduction in instruction count (cachegrind reports a reduction between 0.3% and 0.5%) _on linux_. It's possible that targets without `jemalloc` or with a worse allocator, may optimize these less. It is however significant in the aggregate, looking at the total number of allocated bytes: - it's the biggest source of allocations according to dhat, on the benchmarks I've tried e.g. `syn` or `cargo` - allocations on `syn` are reduced by 440MB, 17% (from 2440722647 bytes total, to 2030461328 bytes) - allocations on `cargo` are reduced by 6.6GB, 19% (from 35371886402 bytes total, to 28723987743 bytes) Some of these diagnostics objects [are allocated in LLVM](https://github.com/rust-lang/rust/pull/113339#discussion_r1252387484) *before* they're emitted to our diagnostic handler, where they'll be filtered out. So we could remove those in the future, but that will require changing a few LLVM call-sites upstream, so I left a FIXME. |
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| .. | ||
| .editorconfig | ||
| ArchiveWrapper.cpp | ||
| CoverageMappingWrapper.cpp | ||
| Linker.cpp | ||
| LLVMWrapper.h | ||
| PassWrapper.cpp | ||
| README | ||
| RustWrapper.cpp | ||
| SymbolWrapper.cpp | ||
This directory currently contains some LLVM support code. This will generally be sent upstream to LLVM in time; for now it lives here. NOTE: the LLVM C++ ABI is subject to between-version breakage and must *never* be exposed to Rust. To allow for easy auditing of that, all Rust-exposed types must be typedef-ed as "LLVMXyz", or "LLVMRustXyz" if they were defined here. Functions that return a failure status and leave the error in the LLVM last error should return an LLVMRustResult rather than an int or anything to avoid confusion. When translating enums, add a single `Other` variant as the first one to allow for new variants to be added. It should abort when used as an input. All other types must not be typedef-ed as such.