middle-end: Teach ranger about POLY_INT_CST ranges for VRP
Improves optimization of SVE-based VLA branches by enabling range analysis for vector length constants.
This patch enhances GCC’s ranger pass to better optimize branches guarded by variable-length array (VLA) constants, particularly for SVE. By introducing a poly_int_indeterminate_bound target hook, GCC can now understand that SVE vector length queries (svcntb, svcntw) have defined bounds. This allows the compiler to fold away known true/false comparisons, simplifying code generation for loop pre-headers and matching LLVM’s optimization capabilities.
In Details
This commit introduces the poly_int_indeterminate_bound target hook in target.def and utilizes it in value-query.cc's range_query::get_tree_range. This hook allows targets to specify the minimum and maximum bounds for POLY_INT_CST values. For SVE, this means the ranger pass can now correctly establish that svcntb() and svcntw() results are within architectural limits, enabling the folding of comparisons that are always true or false, thus improving VRP (Value Range Propagation) optimizations.
- VLA
- Variable Length Array, a C99 feature allowing array sizes to be determined at runtime. In this context, it refers to code structures involving runtime-determined vector lengths.
- ranger
- A GCC compiler pass responsible for determining the range of values that variables can take, used for various optimizations.
- VRP
- Value Range Propagation, an optimization pass that determines the possible range of values for variables to enable further optimizations.
- POLY_INT_CST
- A GCC internal representation for an integer constant that may vary, often used in symbolic computations or with variable vector lengths.
- SVE
- Scalable Vector Extension, an ARM architecture feature that allows vector length to be determined at runtime.