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When -polly-annotate-metadata-vectorize is active, Polly marks its generated loops with llvm.loop.vectorize.enable=true. This is harmful for loops with a loop-carried dependence of distance 1 that involve floating-point operations: the Loop Vectorizer reorders FP operations (e.g. scalar reduction like q = factor*q), producing results that differ from the sequential scalar reference and causing correctness failures. Two changes are made: 1. IslAst.cpp: add PollyVectorizeMetadata to the PerformParallelTest gate so that dependence-distance computation is performed whenever -polly-annotate-metadata-vectorize is passed, not only when -polly-parallel or a vectorizer is active. 2. IslNodeBuilder.cpp / LoopGenerators.cpp: when a loop has a dist=1 dependence involving FP operations, omit the vectorize.enable annotation entirely. This lets the Loop Vectorizer apply its own cost model and legality checks decide. |
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Polly - Polyhedral optimizations for LLVM ----------------------------------------- http://polly.llvm.org/ Polly uses a mathematical representation, the polyhedral model, to represent and transform loops and other control flow structures. Using an abstract representation it is possible to reason about transformations in a more general way and to use highly optimized linear programming libraries to figure out the optimal loop structure. These transformations can be used to do constant propagation through arrays, remove dead loop iterations, optimize loops for cache locality, optimize arrays, apply advanced automatic parallelization, drive vectorization, or they can be used to do software pipelining.