1. Computers
  2. Display Drivers
  3. Graphics Cards
  4. Memory
  5. Motherboards
  6. Processors
  7. Software
  8. Storage
  9. Operating Systems


Facebook RSS Twitter Twitter Google Plus


Phoronix Test Suite

OpenBenchmarking.org

Benchmarking LLVM's Automatic Loop Vectorizer

Compiler

Published on 27 October 2012 08:36 AM EDT
Written by Michael Larabel in Compiler
1 Comment

Yesterday I wrote about an automatic loop vectorizer having been committed to LLVM this week. I've now carried out some benchmarks of this automatic loop vectorizer in conjunction with the Clang compiler to see the performance impact of this still experimental but promising feature.

As mentioned in Friday's article, the LLVM auto loop vectorizer is still deemed experimental and "far from being ready", thus is currently not enabled by default. This feature will premiere with LLVM 3.2 in December and can be enabled by passing the -mllvm -vectorize flags through the Clang C/C++ compiler.

While ultimately the automatic loop vectorizer should be a performance win, one of the Apple engineers working on this LLVM code wrote, "At the moment the vectorizer will vectorize anything it can, because we do not have a 'cost-model' to estimate the profitability of vectorization. Implementing a cost model is a high-priority for us, and until this is ready you should expect to see slowdowns on many loops. Another area which we need to improve is the memory dependence check. At the moment we have a very basic memory legality check which can be improved. Additionally, there are a number of cases where we generate poor vector code or suffer from a phase-rdering problem. Once we solve these problems we can continue to implement additional features."

Using the latest LLVM/Clang SVN code as of Friday, I carried out some early benchmarks of the automatic loop vectorizer when running some open-source Phoronix Test Suite / OpenBenchmarking.org tests. The only difference between test runs was setting the CFLAGS/CXXFLAGS for -mllvm -vectorize.

For some of the benchmarks, there was no measurable change in performance when the automatic loop vectorizer for LLVM was enabled.

The most common case, however, was actually a performance drop when the LLVM auto loop vectorizer was enabled. As mentioned, there isn't yet any cost-model for LLVM to determine when to vectorize a loop or not, plus other performance tuning of this newly-committed code is still needed.

Of the benchmarks run for this initial testing, the only test profile showing a positive improvement out of the automatic loop vectorizer was PostgreSQL.

More benchmarks and other system information/logs from the system during this initial LLVM auto loop vectorizer benchmarking can be found on OpenBenchmarking.org via the 1210264-RA-LLVMLOOPV66 result file.

About The Author
Michael Larabel is the principal author of Phoronix.com and founded the web-site in 2004 with a focus on enriching the Linux hardware experience and being the largest web-site devoted to Linux hardware reviews, particularly for products relevant to Linux gamers and enthusiasts but also commonly reviewing servers/workstations and embedded Linux devices. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics hardware drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated testing software. He can be followed via and or contacted via .
Latest Linux Hardware Reviews
  1. Intel Xeon E5-1680 v3 & E5-2687W v3 Compared To The Core i7 5960X On Linux
  2. Intel 120GB 530 Series SSD Linux Performance
  3. Btrfs/EXT4/XFS/F2FS RAID 0/1/5/6/10 Linux Benchmarks On Four SSDs
  4. AMD's Windows Catalyst Driver Remains Largely Faster Than Linux Drivers
Latest Linux Articles
  1. NVIDIA vs. Nouveau Drivers With Linux 3.18 + Mesa 10.4-devel
  2. Is The Open-Source NVIDIA Driver Fast Enough For Steam On Linux Gaming?
  3. Linux 3.18 File-System Performance Minimally Changed But Possible Regressions
  4. AMD Radeon Gallium3D Is Catching Up & Sometimes Beating Catalyst On Linux
Latest Linux News
  1. There's New In-Fighting Over The Future Of Compiz
  2. GTK+ Inspector Gains More Features Ahead Of GNOME 3.16
  3. Clang 3.6 Will Hopefully Have OpenMP Support
  4. A Go Front-End Could Soon Be Landing In LLVM
  5. Linux 3.18-rc6 Released, A Worrisome Regression Remains
  6. HandBrake 0.10 Brings H.265 & VP8 Encoders
  7. Gngr: A New Web Browser Focused On Privacy
  8. Linux 3.18 Kernel: Not Much Change With Intel Haswell Performance
  9. More File-System Tests Of The Linux 3.18 Kernel
  10. Using NVIDIA's NVENC On Linux With FFmpeg
Latest Forum Discussions
  1. how to configure module phoromatic ?
  2. PulseAudio 6.0 Is Coming & Other Linux Audio Plans For The Future
  3. Debian Developer Resigns From The Systemd Maintainership Team
  4. Roadmap to Catalyst 14.10 ?
  5. Updated and Optimized Ubuntu Free Graphics Drivers
  6. Cant get working Kaveri APU - A10-7850k
  7. Script for Fan Speed Control
  8. Debian Init System Coupling Vote Results