By jhenning on Jun 09, 2009
Contributed by: John Henning and Karsten Guthridge
The GNU C Compiler, GCC, is popular, widely available, and an exemplary collaborative effort.
But how does it do for performance -- for example, on Intel's latest hot "Nehalem" processor family? How does it compare to the freely available Sun Studio compiler?
Using the SPEC CPU benchmarks, we take a look at this question. These benchmarks depend primarily on performance of the chip, the memory hierarchy, and the compiler. By holding the first two of these constant, it is possible to focus in on compiler contributions to performance.
Current Record Holder
The current SPEC CPU2006 floating point speed record holder is the Sun Blade X6270 server module. Using 2x Intel Xeon X5570 processor chips and 24 GB of DDR3-1333 memory, it delivers a result of 45.0 SPECfp_base2006 and 50.4 SPECfp2006. 
We used this same blade system to compare GCC vs. Studio. On separate, but same-model disks, this software was installed:
- SuSE Linux Enterprise Server 11.0 (x86_64) and GCC V4.4.0 built with gmp-4.3.1 and mpfr-2.4.1
- OpenSolaris2008.11 and Sun Studio 12 Update 1
Tie One Hand Behind Studio's Back
In order to make the comparison more fair to GCC, we took several steps.
We simplified the tuning for the OpenSolaris/Sun Studio configuration. This was done in order to counter the criticism that one sometimes hears that SPEC benchmarks have overly aggressive tuning. Benchmarks were optimized with a reasonably short tuning string:
For all: -fast -xipo=2 -m64 -xvector=simd -xautopar For C++, add: -library=stlport4
Recall that SPEC CPU2006 allows two kinds of tuning: "base", and "peak". The base metrics require that all benchmarks of a given language use the same tuning. The peak metrics allow individual benchmarks to have differing tuning, and more aggressive optimizations, such as compiler feedback. The simplified Studio configuration used only the less aggressive base tuning.
Both of the above changes limited the performance of Sun Studio. Several measures were used to increase the performance of GCC:
We tested the latest released version of GCC, 4.4.0, which was announced on 21 April 2009. In our testing, GCC 4.4.0 provides about 10% better overall floating point performance than V4.3.2. Note that GCC 4.4.0 is more recent than the compiler that is included with recent Linux distributions such as SuSE 11, which includes 4.3.2; or Ubuntu 8.10, which updates to 4.3.2 when one does "apt-get install gcc". It was installed with the math libraries mpfr 2.4.1 and gmp 4.3.1, which are labeled as the latest releases as of 1 June 2009.
A tuning effort was undertaken with GCC, including testing of -O2 -O3 -fprefetch-loop-arrays -funroll-all-loops -ffast-math -fno-strict-aliasing -ftree-loop-distribution -fwhole-program -combine and -fipa-struct-reorg
Eventually, we settled on this tuning string for GCC base:
For all: -O3 -m64 -mtune=core2 -msse4.2 -march=core2
For C++, add: -ffast-math
The reason that only the C++ benchmarks used the fast math library was that 435.gromacs, which uses C and Fortran, fails validation with this flag. (Note: we verified that the benchmarks successfully obtained 2MB pages.)
Studio wins by 2x, even with one hand tied behind its back
At this point, a fair base-to-base comparison can be made, and Sun Studio/OpenSolaris finishes the race while GCC/Linux is still looking for its glasses: 44.8 vs. 21.1 (see Table 1). Notice that Sun Studio provides more than 2x the performance of GCC.
Table 1: Initial Comparisons, SPECfp2006,
Sun Studio/Solaris vs. GCC/Linux
|Industry FP Record
Sun Studio 12 Update 1
|Studio/OpenSolaris: simplify above (less tuned)||44.8|
|GCC V4.4 / SuSE Linux 11||21.1|
|Notes: All results reported are from rule-compliant, "reportable" runs of the SPEC CPU2006 floating point suite, CFP2006. "Base" indicates the metric SPECfp_base2006. "Peak" indicates SPECfp2006. Peak uses the same benchmarks and workloads as base, but allows more aggressive tuning. A base result, may, optionally, be quoted as peak, but the converse is not allowed. For details, see SPEC's Readme1st.|
Fair? Did you say "Fair"?
Wait, wait, the reader may protest - this is all very unfair to GCC, because the Studio result used all 8 cores on this 2-chip system, whereas GCC used only one core! You're using trickery!
To this plaintive whine, we respond that:
Compiler auto-parallelization technology is not a trick. Rather, it is an essential technology in order to get the best performance from today's multi-core systems. Nearly all contemporary CPU chips provide support for multiple cores. Compilers should do everything possible to make it easy to take advantage of these resources.
We tried to use more than one core for GCC, via the -ftree-parallelize-loops=n flag. GCC's autoparallelization appears to be in a much earlier development stage than Studio's, since we did not observe any improvements for all values of "n" that we tested. From the GCC wiki, it appears that a new autoparallelization effort is under development, which may improve its results at a later time.
But, all right, if you insist, we will make things even harder for Studio, and see how it does.
Tie Another Hand Behind Studio's Back
The earlier section mentioned various ways in which the performance comparison had been made easier for GCC. Continuing the paragraph numbering from above, we took these additional measures:
Removed the autoparallelization from Studio, substituting instead a request for 2MB pagesizes (which the GCC tuning already had).
Added "peak" tuning to GCC: for benchmarks that benefit, add -ffast-math, and compiler profile-driven feedback
At this point, Studio base beats GCC base by 38%, and Studio base beats GCC peak by more than 25% (see table 2).
Table 2: Additional Comparisons, SPECfp2006,
Sun Studio/Solaris vs. GCC/Linux
|Sun Studio/OpenSolaris: base only, noautopar||29.1|
|GCC V4.4 / SuSE Linux 11||21.1||23.1|
|The notes from Table 1 apply here as well.|
The freely available Sun Studio 12 Update 1 compiler on OpenSolaris provides more than double the performance of GCC V4.4 on SuSE Linux, as measured by SPECfp_base2006.
If compilation is restricted to avoid using autoparallelization, Sun Studio still wins by 38% (base to base), or by more than 25% (Studio base vs. GCC peak).
YMMVYour mileage may vary. It is certain that both GCC and Studio could be improved with additional tuning efforts. Both provide dozens of compiler flags, which can keep the tester delightfully engaged for an unbounded number of days. We feel that the tuning presented here is reasonable, and that additional tuning effort, if applied to both compilers, would not radically alter the conclusions.
The results disclosed in this article are from "reportable" runs of the SPECfp2006 benchmarks, which have been submitted to SPEC.
 SPEC and SPECfp are registered trademarks of the Standard Performance Evaluation Corporation. Competitive comparisons are based on data published at www.spec.org as of 1 June 2009. The X6270 result can be found at http://www.spec.org/cpu2006/results/res2009q2/cpu2006-20090413-07019.html.