[Bug rtl-optimization/95859] New: Statically true asserts not recognized as such with -O2, but with -O1, -Og, -O3

[tobi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95859

            Bug ID: 95859
           Summary: Statically true asserts not recognized as such with
                    -O2, but with -O1, -Og, -O3
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tobi at gcc dot gnu.org
  Target Milestone: ---

With -O2 compiling for x86-64, the compiler doesn't recognize that a number of
assertions in the code are true and they end up as function calls in the
assembly output.  They are removed with -O1, -Og, -O3 but not with -O2 or -O2 +
"all the additional compiler flags for -O3 given in the documentation" (that's
a documentation bug on top of this misoptimization).

The asserts look something like this in the assembled form:
   template <N>
   assertion(int n) {
      assert(n == N);
   }
and are only called with compile-time constant arguments, i.e. assertion<3>(3)
and the like.

The final output from the tree passes contains the assertion calls at all
optimization levels, so the rtl optimizers appear to perform worst in -O2 in
this particular case.  I don't see why the calls couldn't be removed before
lowering to RTL.

I'm not sure how to extract preprocessed output form the compiler explorer, but
will provide it once I can put my hands on a recent gcc.

Source code reproduced below, the compiler explorer link is here:
https://godbolt.org/z/REJDhy

#include <Eigen/Geometry>

struct m34 {
    float m[3][4];
};
typedef Eigen::Transform<double, 3, Eigen::AffineCompact, Eigen::DontAlign>
unalignedTrafo3d;
using Trafo3d = unalignedTrafo3d;


Trafo3d func34(m34 mat)
{
    using mapType = Eigen::Map<Eigen::Matrix<float, 3, 4, Eigen::RowMajor>>;

    // should compile to a conversion of 12 floats to double.
    auto mR = Trafo3d{ mapType{ (float*)mat.m }.cast<double>() };
    return mR;    
}