A few days back Intel's Clear Linux updated their LLVM Clang compiler from 3.9.1 to the recent 4.0.0 release, following Beignet getting LLVM 4.0 support. Here are some before/after benchmarks as well as fresh GCC benchmarks.
Clear Linux ships both LLVM Clang and GCC and switches between the default compiler when building its packages/bundles depending upon which compiler is known to do the best for the particular workload. As far as the default compiler exposed, GCC is the default. I ran some tests of Clear Linux 14620 that shipped with GCC 6.3 and and LLVM Clang 3.9.1 followed by upgrading to Clear Linux 14640 to get the distribution's LLVM Clang 4.0 build. (Note there was also a small update to its Linux 4.10 kernel with that build too.)
I ran this fresh Clear Linux compiler comparison on an Intel Core i7 6800K Broadwell-E box. The CFLAGS/CXXFLAGS were maintained the same throughout testing of "-O3 -march=native." All benchmarks were carried out in a fully-automated and reproducible manner using the open-source Phoronix Test Suite benchmarking software.