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Bug 1765 - Regression with MSVC 19.16 and later in C++17 mode
Summary: Regression with MSVC 19.16 and later in C++17 mode
Alias: None
Product: Eigen
Classification: Unclassified
Component: Core - general (show other bugs)
Version: 3.3 (current stable)
Hardware: All Windows
: Normal Performance Problem
Assignee: Nobody
Depends on:
Blocks: 3.x
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Reported: 2019-10-31 10:57 UTC by Christoph Hertzberg
Modified: 2019-12-04 18:52 UTC (History)
4 users (show)


Description Christoph Hertzberg 2019-10-31 10:57:24 UTC
Reported via mail:

> In some of our code using Eigen, we got a 50% increase in runtime 
> when updating from C++14 to C++17.
> We are using Eigen 3.3.7 and MSVC 19.23.
> Below is a small example, which gives different assembly on,
> when compiled with
>    *   MSVC 19.22 /std:c++14 /O2 /Ob2 /GS-
>    *   MSVC 19.22 /std:c++17 /O2 /Ob2 /GS-
> It seems that with C++17, the constructor call is not optimized so the
> runtime is much larger. Do you know what is going on?
> #include <Eigen/Core>
> int main()
> {
>   for (size_t i = 0; i < 0.3e9; ++i)
>   {
>     Eigen::Matrix2cd R;
>   }
>   return 0;
> }

I confirm this, even with /DNDEBUG, but it seems to be a regression from MSVC 19.15 to 19.16 (the constructor of std::complex does not get optimized away):

Possible workaround: Add more force-inlines at some places.
Comment 2 Christoph Hertzberg 2019-11-01 09:33:42 UTC
Even more reduced example:
I.e., just a struct with four std::complex<double> does not get optimized away either when compiled in C++17 mode. 
I don't see any possible workaround, except storing `std::complex<double>[size]` as `double[2*size]` or trying to provide our own `Eigen::complex` type. Both sound like a lot of effort just for a workaround.

We may document this as "known issue" (not sure if anyone actually looks here):
Comment 3 Gael Guennebaud 2019-11-14 16:52:53 UTC
I've added an entry to the FAQ. We cannot do better, so closing.
Comment 4 Nobody 2019-12-04 18:52:20 UTC
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