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. Preview: AMD's FX-9590 Eight-Core At Up To 5.0GHz On Linux
  2. Intel Launches The Core i7 5960X, Mighty Powerful Haswell-E CPUs
  3. AMD Radeon R9 290: Gallium3D vs. Catalyst Drivers
  4. AMD Radeon R9 290 Open-Source Driver Works, But Has A Ways To Go
Latest Linux Articles
  1. How Intel Graphics On Linux Compare To Open-Source AMD/NVIDIA Drivers
  2. The Fastest NVIDIA GPUs For Open-Source Nouveau With Steam Linux Gaming
  3. Testing For The Latest Linux Kernel Power Regression
  4. The Most Energy Efficient Radeon GPU For AMD Linux Gaming
Latest Linux News
  1. Nouveau X.Org Driver Released With DRI3+Present, Maxwell, GLAMOR
  2. Microsoft & AMD Release C++ AMP Compiler With Linux Support
  3. AMD, Wine & Valve Dominated August For Linux Users
  4. Linux 3.17-rc3 Kernel Released Back On Schedule
  5. Lennart Poettering Talks Up His New Linux Vision That Involves Btrfs
  6. Mesa 10.3 RC2 Arrives Via Its New Release Manager
  7. Ubuntu 14.10's Lack Of X.Org Server 1.16 Gets Blamed On AMD
  8. MSI Motherboard BIOS Updating Remains A Pain For Linux Users
  9. See How Your Linux System Performs Against The Latest Intel/AMD CPUs
  10. AMD Steppe Eagle Flys To Coreboot
Latest Forum Discussions
  1. Lennart Poettering Talks Up His New Linux Vision That Involves Btrfs
  2. Updated and Optimized Ubuntu Free Graphics Drivers
  3. AMD Releases UVD Video Decode Support For R600 GPUs
  4. SSD seems slow
  5. Is laptop with Intel CPU and AMD dGPU worth buying considering especially AMD Enduro?
  6. Radeon HD5670 and Ubuntu 14.04
  7. Btrfs Gets Talked Up, Googler Encourages You To Try Btrfs
  8. Updated graphics drivers for Ubuntu 12.04 Precise LTS