Bug with auto-vectorization of logf

I intended to file this bug on Bugzilla, but I’ve received no response from llvm-admin in the 10 days since asking for a Bugzilla account.

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.

If someone makes a Bugzilla account for me, I can refile this bug there.

Eric Martin

log_autovec_test.tgz (2.75 KB)

+Tanya for the account issue.


I apologize for any delay or confusion. From my records/list archives, I saw that Anton had created an account for you on Oct 17th and responded to your email to llvm-admin. I am not sure what happened after that point as I thought the account was done.

I just confirmed there is an account for you in bugzilla, so you should be good to go if you reset your password.


Thank you Tanya.
The first email I received from Anton was on Oct 28th, but I’m able to access my Bugzilla account now so all confusion is resolved.

The bug mentioned in this thread has been filed here: https://llvm.org/bugs/show_bug.cgi?id=30837