[cfe-dev] C++ Expression Template Benchmarks for GCC/Clang/Intel/PGI/MSVC

Douglas Gregor dgregor at apple.com
Fri Jun 15 10:23:10 PDT 2012

On Jun 15, 2012, at 10:15 AM, John McCall wrote:

> On Jun 15, 2012, at 9:16 AM, Douglas Gregor wrote:
>> On Jun 14, 2012, at 3:54 PM, Walter Landry wrote:
>>> Hello Everyone,
>>> I thought you might be interested in some C++ expression template
>>> benchmarks I have done.
>>> http://www.wlandry.net/Projects/FTensor#Benchmarks
>>> Clang's performance was mixed.  It optimized the expression template
>>> code just as well as the code that unrolled the expressions by hand,
>>> but that may be because it only did a mediocre job of optimizing the
>>> unrolled versions.  GCC had similar performance issues until I used
>>> -Ofast.  I could not find a similar option for Clang, partly because I
>>> could not find a complete list of Clang compiler options.  You can see
>>> a list of all of the compiler options that I used at
>> -Ofast enables unsafe optimizations that can change the results produced by floating-point operations, so it doesn't make sense to compare the code generated by one compiler using -Ofast (which gets to break the rules of floating-point math) against the code generated by another compiler that hasn't been allowed to break those rules. It's very possible that -Ofast doesn't even make sense for your library, unless you don't care about the accuracy of your results.
>> IIRC, Clang doesn't actually do anything with -ffast-math, either. So, an apples-to-apples comparison would not use -Ofast or -ffast-math for either. Of course, it's completely fair criticism to say that, for people who don't require exact FP math, -Ofast gives a very nice performance boost in GCC that Clang can't match.
> IIRC, fast-math does get propagated to LLVM, which does honor it.

You're right; I see it now.

	- Doug

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