[Bug libgomp/58482] gomp4: user defined reduction produce wrong result

["vincenzo.innocente at cern dot ch" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58482

--- Comment #2 from vincenzo Innocente <vincenzo.innocente at cern dot ch> ---
Thanks Jakub for the clear answer.
The reduction operator should be strictly commutative!
and I now understand the meaning of 
omp declare reduction (I hope)
so I modified it as you can see below
results ok.
but omp does not vectorize while good old -Ofast doesâ
shall I prepare a simple example with corresponding assembler and submit?

c++ -std=c++11  ured_omp4.cpp -O2 -ftree-vectorizer-verbose=4 -fopenmp

Analyzing loop at ured_omp4.cpp:39
ured_omp4.cpp:39:22: note: ===== analyze_loop_nest =====
ured_omp4.cpp:39:22: note: === vect_analyze_loop_form ===
ured_omp4.cpp:39:22: note: === get_loop_niters ===
ured_omp4.cpp:39:22: note: ==> get_loop_niters:(unsigned int) NN_15(D)
ured_omp4.cpp:39:22: note: Symbolic number of iterations is (unsigned int)
NN_15(D)
ured_omp4.cpp:39:22: note: === vect_analyze_data_refs ===
ured_omp4.cpp:39:22: note: got vectype for stmt: _20 = *_19;
const vector(4) float
ured_omp4.cpp:39:22: note: got vectype for stmt: _30 = MEM[(struct TwoInt
*)&D.63343][_16].a;
vector(4) float
ured_omp4.cpp:39:22: note: not vectorized: not suitable for gather load _30 =
MEM[(struct TwoInt *)&D.63343][_16].a;

ured_omp4.cpp:39:22: note: bad data references.
ured_omp4.cpp:35:8: note: vectorized 0 loops in function.
ured_omp4.cpp:35:8: note: loop turned into non-loop; it never loops
ured_omp4.cpp:35:8: note: loop turned into non-loop; it never loops

I spare you the 4 pages of dump in case of -Ofast


pb-d-128-141-131-94:vectorize innocent$  c++ -std=c++11  ured_omp4.cpp -O2
-ftree-vectorizer-verbose=1 -fopenmp; ./a.out
ured_omp4.cpp:35:8: note: loop turned into non-loop; it never loops
ured_omp4.cpp:35:8: note: loop turned into non-loop; it never loops
523776,-523776
523776,-523776
pb-d-128-141-131-94:vectorize innocent$  c++ -std=c++11  ured_omp4.cpp -Ofast
-ftree-vectorizer-verbose=1; ./a.out
ured_omp4.cpp:38:3: note: loop vectorized
ured_omp4.cpp:38:3: note: loop peeled for vectorization to enhance alignment
ured_omp4.cpp:38:3: note: loop with 3 iterations completely unrolled
ured_omp4.cpp:35:8: note: loop with 6 iterations completely unrolled
ured_omp4.cpp:46:13: note: loop vectorized
ured_omp4.cpp:45:8: note: loop with 2 iterations completely unrolled
ured_omp4.cpp:38:3: note: loop vectorized
ured_omp4.cpp:63:3: note: loop vectorized
ured_omp4.cpp:63:3: note: loop with 4 iterations completely unrolled
523776,-523776
523776,-523776



cat ured_omp4.cpp 
#include<algorithm>
#define Type float

struct TwoInt {
  Type a=0;
  Type b=0;

#pragma omp declare simd
  TwoInt & operator+=(TwoInt rh) {
    a+=rh.a;
    b-=rh.b;
  }

#pragma omp declare simd
  TwoInt & add(TwoInt rh) {
    a+=rh.a;
    b-=rh.b;
    return *this;
  }


#pragma omp declare simd
  TwoInt & reduce(TwoInt rh) {
    a+=rh.a;
    b+=rh.b;
    return *this;
  }


};

#pragma omp declare reduction (foo:struct TwoInt: omp_out.reduce(omp_in))


TwoInt sum(Type const * q, int NN) {
  TwoInt s;
#pragma omp simd  aligned(q: 16) reduction(foo:s)
  for (int i=0;i<NN;++i) {
    TwoInt l; l.a=q[i]; l.b = q[i];
    s.add(l);
  }
  return s;
}

TwoInt sum4(Type const * q, int NN) {
  TwoInt s[4];
  for (int i=0;i<NN;i+=4) {
    for (int j=0;j<std::min(4,NN-i);++j) {
      TwoInt l; l.a=q[i+j]; l.b = q[i+j];
      s[j].add(l);
    }
  }
  s[0].reduce(s[1]); s[3].reduce(s[2]); s[3].reduce(s[0]);
  return s[3];
}


#include<iostream>
int main() {
  constexpr int NN=1024;
  Type q[NN];
  Type a=0;
  for (auto & e: q) e=a++;

  auto s = sum(q,NN);
  std::cout << s.a << "," << s.b << std::endl;
  s = sum4(q,NN);
  std::cout << s.a << "," << s.b << std::endl;

  return 0;
}