[llvm-bugs] [Bug 40961] New: loop vectorizer unable to reason with trip count bounds

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40961

            Bug ID: 40961
           Summary: loop vectorizer unable to reason with trip count
                    bounds
           Product: libraries
           Version: trunk
          Hardware: Other
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: oiskuu at gmail.com
                CC: llvm-bugs at lists.llvm.org

I see bugs 38280, 37423, but this one is simpler still. Consider the following:

    void topup(int a[], unsigned long i)
    {
        for (; i < 16; i++) {
            a[i] = 1;
        }
    }

The result, compiled with -O -march=haswell

- loop is vectorized despite the small absolute limit on trip count
- vectorized part has loop step of 256 (!) and is actually unreachable

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