Tree optimizer uses reference parameter bounds in trap analysis.
Tree optimizer now uses object bounds for reference parameters, improving loop invariant load motion and reducing cycles.
The tree optimizer’s trap analysis now considers the size of objects referred to by REFERENCE_TYPE parameters, not just the this pointer. This allows the Load Invariant Motion (LIM) pass to move safe invariant loads across these parameters, enabling optimizations that were previously blocked. This change improves performance, with a reported 6.2% cycle reduction on a specific benchmark.
In Details
The tree_could_trap_p analysis in tree-eh.cc has been enhanced to utilize object size information for REFERENCE_TYPE parameters, analogous to how it already handles the this parameter. A new function whole_object_param_size consolidates the logic for querying parameter object bounds. This change allows LIM to move loads that were previously held back by the inability to prove the size of referenced objects, enabling optimizations like moving loads to loop preheaders. The object bound is applied only to rvalues to avoid issues with potential const_cast scenarios.
- tree-optimization
- An intermediate representation (IR) and optimization framework within GCC. It manipulates code in a tree-like structure to perform various optimizations before generating machine code.
- trap analysis
- Analysis performed by the compiler to determine if a piece of code could potentially cause a runtime trap or exception, such as division by zero or accessing null pointers. This information is used for optimization.
- REFERENCE_TYPE
- A GCC internal representation for a reference type, similar to C++ references or Rust references. It signifies a parameter that refers to an object.
- Load Invariant Motion (LIM)
- A compiler optimization pass that moves loop-invariant loads (memory accesses whose values do not change within the loop) to the preheader of the loop to reduce redundant computations.
- rvalue
- In C++, an rvalue represents a temporary value that can be assigned to a variable or used in an expression, but not typically modified directly. It's often the result of an expression or function call.