[llvm-bugs] [Bug 30837] New: logf function call not auto-vectorizing

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30837

            Bug ID: 30837
           Summary: logf function call not auto-vectorizing
           Product: libraries
           Version: 3.9
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: eric at ericmart.in
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 17513
  --> https://llvm.org/bugs/attachment.cgi?id=17513&action=edit
C test case, sh command to invoke clang, and example faulty IR

I've written 2 test functions in C that take in a float array x of size n and
output float array f(x), where f is either fabsf or logf. The LLVM 3.9
auto-vectorization docs claim that both functions will be vectorized:
http://llvm.org/releases/3.9.0/docs/Vectorizers.html#vectorization-of-function-calls

When running with "clang -O3 -march=x86-64 -mavx2 -ffast-math test.c -S
-emit-llvm", the function calling fabsf is vectorized while the function
calling logf is not.  This is with clang 3.9, but I've also confirmed the bug
exists back to at least clang 3.7. I've also observed that logf calls break
vectorization of more complex loops, and I provide the comparison with fabsf as
a reduced test case.

I've attached the C program, a sh script to invoke clang with correct
arguments, and an example LLVM IR file I produced by running on my system.

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