From f7b66052098aa4584fb34bbe8eeba3e3988fc358 Mon Sep 17 00:00:00 2001 From: Jeremy Fitzhardinge Date: Sun, 12 Oct 2025 22:41:03 -0700 Subject: [PATCH] Add -Zannotate-moves for profiler visibility of move/copy operations 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` and `compiler_copy` 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. --- src/builder.rs | 2 +- src/debuginfo.rs | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) diff --git a/src/builder.rs b/src/builder.rs index 5657620879ca..86031a8a4634 100644 --- a/src/builder.rs +++ b/src/builder.rs @@ -1069,7 +1069,7 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> { OperandValue::Ref(place.val) }; - OperandRef { val, layout: place.layout } + OperandRef { val, layout: place.layout, move_annotation: None } } fn write_operand_repeatedly( diff --git a/src/debuginfo.rs b/src/debuginfo.rs index 0f015cc23f52..3979f62987f2 100644 --- a/src/debuginfo.rs +++ b/src/debuginfo.rs @@ -19,7 +19,7 @@ use crate::context::CodegenCx; pub(super) const UNKNOWN_LINE_NUMBER: u32 = 0; pub(super) const UNKNOWN_COLUMN_NUMBER: u32 = 0; -impl<'a, 'gcc, 'tcx> DebugInfoBuilderMethods for Builder<'a, 'gcc, 'tcx> { +impl<'a, 'gcc, 'tcx> DebugInfoBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> { // FIXME(eddyb) find a common convention for all of the debuginfo-related // names (choose between `dbg`, `debug`, `debuginfo`, `debug_info` etc.). fn dbg_var_addr(