Delay SLP discovery failure in BB vectorization
Vectorization pass defers potential failures in SLP discovery.
This commit modifies the basic block (BB) vectorization pass in GCC to delay the failure of SLP (Simultaneous Linear Patterns) discovery. Previously, failures could occur early in the process. Now, the failure is delayed until after analyzing definitions that might provide better splitting points for the pattern group. This change aims to improve the success rate of vectorization for simple cases without significantly impacting compile times.
In Details
The SLP vectorization strategy in GCC's tree-vectorizer sometimes requires unrolling to discover vectorizable patterns. In the context of BB vectorization, vect_build_slp_tree_1 could 'soft-fail' prematurely. This commit moves the soft-failing logic to vect_build_slp_tree_2, placing it after vect_get_and_check_slp_defs. This ensures that potential defs are analyzed first, potentially revealing better ways to split SLP groups. Additionally, vect_get_and_check_slp_defs is updated to not fall back to vect_external_def on mismatches when soft-failing, preventing extraneous analysis.
- tree-optimization
- A phase in the compiler where the code is represented as a tree structure to perform various high-level transformations and optimizations.
- vectorization
- An optimization technique where the compiler transforms code that operates on single data elements into code that operates on multiple data elements simultaneously, often using SIMD instructions.
- SLP
- Simultaneous Linear Patterns. A vectorization technique that identifies sequences of identical scalar operations on different data elements and transforms them into a single vector operation.
- basic block (BB)
- A sequence of consecutive instructions in a program that has only one entry point and one exit point.
- unrolling
- A loop optimization technique where the loop body is duplicated multiple times to reduce loop overhead and potentially expose more parallelism.