[llvm-bugs] [Bug 28363] New: BFNZ not generated for simple loops

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https://llvm.org/bugs/show_bug.cgi?id=28363

            Bug ID: 28363
           Summary: BFNZ not generated for simple loops
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: PowerPC
          Assignee: unassignedbugs at nondot.org
          Reporter: amehsan at ca.ibm.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

For many loops, sometimes very simple ones like:

     for (i = 0; i < m; i++)
        sum += b[i]*a[i];

Where sum, a and b are single precision float (arrays), we do not generate
bdnz, and instead use, three instructions in the loop. (add, compare,
branch-conditional).

I looked at one of the loops like this and Scalar Evolution was not able to
compute loop iteration count. When the loop is not unrolled (compile with
-fno-unroll-loops) this is not an issue. Also for the residue loops of an
unrolled loop we generate bdnz.

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