Re: [boost] [multiprecision]: 15% slowdown using -std=c++11 compiler flag

Subject: Re: [boost] [multiprecision]: 15% slowdown using -std=c++11 compiler flag
From: John Maddock (jz.maddock_at_[hidden])
Date: 2017-11-19 13:54:09

On 19/11/2017 10:32, Kim Walisch via Boost wrote:
> I have been investigating a 15% performance regression in my C++
> primesum program (https://github.com/kimwalisch/primesum/tree/256-bit)
> over the last 2 days.
>
> By lots of benchmarking I was able to identify the boost
> multiprecision library together with -std=c++11 (or -std=gnu++11)
> as the culprit for the performance regression because I have also
> a version of the primesum program which does not use the boost
> multiprecision library and in this version there is no performance
> regression when compiling using -std=c++11.
>
> I have tested using multiple versions of the boost multiprecision
> library including the latest 1.65.1. The slowdown happens on
> both GCC (versions: 5.4, 6.4, 7.2) and Clang (version 3.8) on
> x86_64 Linux. I am only using the int256_t and uint256_t types
> (hence cpp_int backend) and I am doing only simple integer
> arithmetic: +, - and *.
>
> Is this a known issue and is there a known workaround e.g. special
> compiler flag? I could revert to C++98 but I really don't want to do
> that...

No, not known, and if anything C++11 should speed things up by enabling 
rvalue-references etc.  If you can narrow it down some more I'll 
certainly investigate.

Thanks for the heads up, John.

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