Enable compare costs by default for x86 vectorization.
GCC now enables vector cost comparison by default for x86, improving vectorization decisions.
The default behavior of the ix86-vect-compare-costs parameter has been flipped, enabling vector cost comparison by default for x86. This change, driven by discussions during stage3/4, aims to improve vectorization decisions by comparing the costs of different vectorization strategies. Several test cases were updated and adjusted to reflect the new default, ensuring that existing functionality remains intact while leveraging the benefits of cost comparison.
In Details
This flips the default for --param ix86-vect-compare-costs, which impacts ix86_vector_costs and its interactions with the vectorizer's cost model. The change influences decisions within the vectorization epilogue selection process. Test adjustments required consideration of in-order reduction costs, and the addition of -fno-vect-cost-model indicates a dependency on more advanced cost modeling features. See also PR120398, PR123603.
For Context
Vectorization is a compiler optimization that replaces scalar operations with vector operations, processing multiple data elements simultaneously. The decision to vectorize and the choice of vectorization strategy involves cost modeling. GCC's cost model estimates the performance impact of different vectorization options. This commit changes the default behavior of a parameter that enables a comparison of these costs for x86 architectures, potentially leading to better code generation. The vectorizer lives between the high-level tree-SSA IR and the instruction selector.