bbb6f68e28
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. |
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