[Bug tree-optimization/86062] New: Missed redundancy elimination with struct and array

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86062

            Bug ID: 86062
           Summary: Missed redundancy elimination with struct and array
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

#include <array>

struct I { double i,s; I(double d):i(d),s(d){} };
typedef std::array<double,3> P;
typedef std::array<I,3> AP;

static AP c(P const&p){
  return {p[0],p[1],p[2]};
}
template<class T> auto const& ac(T const&p, int n){return p[n];}
static double g(P const&p, int n)
{
  I res = ac(c(p),n);
  return res.s-res.i;
}

__attribute__((flatten)) double fff(P p){ return g(p,1); }


compiles at -O2 to the following

  double res$8;
  struct AP D.30336;
  struct I res;
  double _2;
  double _3;

  <bb 2> [local count: 1073741825]:
  _2 = MEM[(const value_type &)&p + 8];
  MEM[(const struct I &)&D.30336 + 16] = _2;
  MEM[(const struct I &)&D.30336 + 24] = _2;
  res = MEM[(const struct I &)&D.30336][1];
  res$8_13 = MEM[(struct I *)&res + 8B];
  D.30336 ={v} {CLOBBER};
  _3 = res$8_13 - _2;
  return _3;

Or with -O3

  double res$8;
  struct AP D.30336;
  struct I res;
  double _2;
  double _3;
  vector(2) double _9;

  <bb 2> [local count: 1073741825]:
  _2 = MEM[(const value_type &)&p + 8];
  _9 = {_2, _2};
  MEM[(const struct I &)&D.30336 + 16] = _9;
  res = MEM[(const struct I &)&D.30336][1];
  res$8_13 = MEM[(struct I *)&res + 8B];
  D.30336 ={v} {CLOBBER};
  _3 = res$8_13 - _2;
  return _3;

While we should be able to notice that res$8_13 and 2 are the same (and thus
simplify to 0 with -ffast-math).