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From: "martin@mpa-garching.mpg.de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99728] code pessimization when using wrapper classes around
 SIMD types
Date: Tue, 23 Mar 2021 14:28:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99728

--- Comment #5 from Martin Reinecke <martin@mpa-garching.mpg.de> ---
(In reply to Matthias Kretz (Vir) from comment #4)
> FWIW, using std::experimental::native_simd<double> also does not hoist the
> stores out of the loop. However, if you pass d by value and return d, the
> issue goes away. So I guess this is an aliasing pessimization.

This is an interesting data point ... In my first test case (attached to
https://gcc.gnu.org/pipermail/gcc-help/2021-March/139976.html), I explicitly
make a local copy of d and copy back at the end of the function, and this
didn't help. Strange.

> Even though
> you added __restrict__. In any case __m256 has the problem that it is
> declared with the may_alias attribute. I recommend to just never use __m2=
56
> unless you have no other choice.

I guess I need it for unaligned loads/stores, correct? Otherwise __v4df sho=
uld
work everywhere.=