[llvm-bugs] [Bug 34125] New: Clang punts on vectorization and issues incorrect diagnostic

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Aug 8 13:57:46 PDT 2017


            Bug ID: 34125
           Summary: Clang punts on vectorization and issues incorrect
           Product: clang
           Version: 4.0
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jeff.science at gmail.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 18948
  --> https://bugs.llvm.org/attachment.cgi?id=18948&action=edit
make command output

Clang claims the reason it does not vectorize star[45] is because the trip
count cannot be determined, but the trip count determination is identical in
star[123], which are vectorized.

./stencil_tbb.hpp:63:7: remark: loop not vectorized: could not determine number
of loop iterations [-Rpass-analysis]
      for (auto j=r.cols().begin(); j!=r.cols().end(); ++j ) {

As best I can tell, the issue is that the loop bodies for star[45] are too
large.  Functions with 12 or fewer terms are vectorized whereas those with 16
or more are not.

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