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Add -Zannotate-moves for profiler visibility of move/copy operations (codegen) **Note:** this is an alternative implementation of https://github.com/rust-lang/rust/pull/147206; rather than being a MIR transform, it adds the annotations closer to codegen. It's functionally the same but the implementation is lower impact and it could be more correct. --- This implements a new unstable compiler flag `-Zannotate-moves` that makes move and copy operations visible in profilers by creating synthetic debug information. This is achieved with zero runtime cost by manipulating debug info scopes to make moves/copies appear as calls to `compiler_move<T, SIZE>` and `compiler_copy<T, SIZE>` marker functions in profiling tools. This allows developers to identify expensive move/copy operations in their code using standard profiling tools, without requiring specialized tooling or runtime instrumentation. The implementation works at codegen time. When processing MIR operands (`Operand::Move` and `Operand::Copy`), the codegen creates an `OperandRef` with an optional `move_annotation` field containing an `Instance` of the appropriate profiling marker function. When storing the operand, `store_with_annotation()` wraps the store operation in a synthetic debug scope that makes it appear inlined from the marker. Two marker functions (`compiler_move` and `compiler_copy`) are defined in `library/core/src/profiling.rs`. These are never actually called - they exist solely as debug info anchors. Operations are only annotated if: - We're generating debug info and the feature is enabled. - Meets the size threshold (default: 65 bytes, configurable via `-Zannotate-moves=SIZE`), and is non-zero - Has a memory representation This has a very small size impact on object file size. With the default limit it's well under 0.1%, and even with a very small limit of 8 bytes it's still ~1.5%. This could be enabled by default.
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.