Add -Zannotate-moves for profiler visibility of move/copy operations (codegen)

[jsgf]

Note: this is an alternative implementation of #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.

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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 the type:

• Meets the size threshold (default: 65 bytes, configurable via -Zannotate-moves=SIZE)
• Has a non-scalar backend representation (scalars use registers, not memcpy)

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.

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[rustbot]

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[jsgf]

MCP filed rust-lang/compiler-team#928

[saethlin]

Does this support cranelift? The advantage to doing such things in MIR is that it is much easier to support cranelift.

[jsgf]

Yeah, that's a good point which I mentioned in the MCP. I had assumed that cranelift and gcc used the same framework as llvm so supporting them would just be a matter of implementing with_move_annotation for them. But I guess that's not the case?

I didn't check whether the MIR transform approach did actually work for cranelift or gcc (does gcc support debuginfo in general?). But if it does, then I wonder if there's a middle ground between MIR and codegen to get the best of both?

[saethlin]

Ah, I'll respond on the MCP Zulip thread.