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From: "alalaw01 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/68165] New: Not constant-folding setting vector element
Date: Fri, 30 Oct 2015 17:41:00 -0000
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X-Bugzilla-Component: tree-optimization
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X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68165

            Bug ID: 68165
           Summary: Not constant-folding setting vector element
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alalaw01 at gcc dot gnu.org
  Target Milestone: ---

I believe these two C functions are equivalent:
  typedef float __attribute__((__vector_size__ (2 * sizeof(float))))
float32x2_t;

  float32x2_t
  test_cprop ()
  {
    float32x2_t vec = {0.0, 0.0};
    vec[0] = 3.14f;
    vec[1] = 2.71f;
    return vec * ((float32x2_t) { 1.5f, 4.5f });
  }

  float32x2_t
  test_cprop2 ()
  {
      float32x2_t vec = {3.14f, 2.71f};
      return vec * ((float32x2_t) { 1.5f, 4.5f });
  }

at -O3 -fdump-tree-optimized, on AArch64:
=====
;; Function test_cprop (test_cprop, funcdef_no=0, decl_uid=2603, cgraph_uid=0,
symbol_order=0)

test_cprop ()
{
  float32x2_t vec;
  vector(2) float vec.0_5;
  float32x2_t _6;

  <bb 2>:
  vec = { 0.0, 0.0 };
  BIT_FIELD_REF <vec, 32, 0> = 3.1400001049041748046875e+0;
  BIT_FIELD_REF <vec, 32, 32> = 2.71000003814697265625e+0;
  vec.0_5 = vec;
  _6 = vec.0_5 * { 1.5e+0, 4.5e+0 };
  vec ={v} {CLOBBER};
  return _6;

}



;; Function test_cprop2 (test_cprop2, funcdef_no=1, decl_uid=2607,
cgraph_uid=1, symbol_order=1)

test_cprop2 ()
{
  <bb 2>:
  return { 4.71000003814697265625e+0, 1.219499969482421875e+1 };

}
=====
x86 is identical for test_cprop2, worse in test_cprop:
=====
test_cprop ()
{
  float32x2_t vec;
  vector(2) float vec.0_5;
  float32x2_t _6;
  float _8;
  float _9;
  float _10;
  float _11;

  <bb 2>:
  vec = { 0.0, 0.0 };
  BIT_FIELD_REF <vec, 32, 0> = 3.1400001049041748046875e+0;
  BIT_FIELD_REF <vec, 32, 32> = 2.71000003814697265625e+0;
  vec.0_5 = vec;
  _8 = BIT_FIELD_REF <vec.0_5, 32, 0>;
  _9 = _8 * 1.5e+0;
  _10 = BIT_FIELD_REF <vec.0_5, 32, 32>;
  _11 = _10 * 4.5e+0;
  _6 = {_9, _11};
  vec ={v} {CLOBBER};
  return _6;

}
=====
i.e. we are not understanding the result of assigning to the BIT_FIELD_REF on
the whole vector, although we can resolve individual elements:
  float32x2_t
  test_cprop3 ()
  {
    float32x2_t vec = {0.0, 0.0};
    vec[0] = 3.14f;
    vec[1] = 2.71f;
    return (float32x2_t) {vec[0], vec[1]} * ((float32x2_t) { 1.5f, 4.5f });
  }

produces
=====
test_cprop3 ()
{
  <bb 2>:
  return { 4.71000003814697265625e+0, 1.219499969482421875e+1 };

}