forked from OSchip/llvm-project
2eed29f3c8
This provides an initial implementation of getUnrollingPreferences for x86. getUnrollingPreferences is used by the generic (concatenation) unroller, which is distinct from the unrolling done by the loop vectorizer. Many modern x86 cores have some kind of uop cache and loop-stream detector (LSD) used to efficiently dispatch small loops, and taking full advantage of this requires unrolling small loops (small here means 10s of uops). These caches also have limits on the number of taken branches in the loop, and so we also cap the loop unrolling factor based on the maximum "depth" of the loop. This is currently calculated with a partial DFS traversal (partial because it will stop early if the path length grows too much). This is still an approximation, and one that is both conservative (because it does not account for branches eliminated via block placement) and optimistic (because it is only recording the maximum depth over minimum paths). Nevertheless, because the loops that fit in these uop caches are so small, it is not clear how much the details matter. The original set of patches posted for review produced the following test-suite performance results (from the TSVC benchmark) at that time: ControlLoops-dbl - 13% speedup ControlLoops-flt - 15% speedup Reductions-dbl - 7.5% speedup llvm-svn: 205348 |
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| clang | ||
| clang-tools-extra | ||
| compiler-rt | ||
| debuginfo-tests | ||
| libclc | ||
| libcxx | ||
| libcxxabi | ||
| lld | ||
| lldb | ||
| llvm | ||
| openmp | ||
| polly | ||