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.
log_autovec_test.tgz (2.75 KB)