[Bug tree-optimization/102512] New: Redudant max/min operation for vector reduction

[crazylht at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102512

            Bug ID: 102512
           Summary: Redudant max/min operation for vector reduction
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: crazylht at gmail dot com
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-*-* i?86-*-*

cat test.c

#define MAX(a, b) ((a) > (b) ? (a) : (b))

short
foo1 (short* p)
{
  short max = p[0];
  for (int i = 0; i != 8; i++)
    max = MAX(max, p[i]);
  return max;
}

short
foo2 (short* p)
{
  short max = p[0];
  for (int i = 1; i != 8; i++)
    max = MAX(max, p[i]);
  return max;
}

gcc -O3 -mavx2 -S 



in foo1 the first MAX_EXPR <_10, vect__4.7_13> is redundant since it's
contained by the latter .REDUC_MAX.
in foo2 vectorizer failed to recognize .REDUC_MAX pattern. 

;; Function foo1 (foo1, funcdef_no=0, decl_uid=2991, cgraph_uid=1,
symbol_order=0)

.248t.optimized
short int foo1 (short int * p)
{
  vector(8) short int vect_max_11.8;
  vector(8) short int vect__4.7;
  short int max;
  vector(8) short int _10;
  short int _20;

  <bb 2> [local count: 119292720]:
  max_9 = *p_8(D);
  _10 = {max_9, max_9, max_9, max_9, max_9, max_9, max_9, max_9};
  vect__4.7_13 = MEM <vector(8) short int> [(short int *)p_8(D)];
  vect_max_11.8_14 = MAX_EXPR <_10, vect__4.7_13>;
  _20 = .REDUC_MAX (vect_max_11.8_14); [tail call]
  return _20;

}



;; Function foo2 (foo2, funcdef_no=1, decl_uid=3000, cgraph_uid=2,
symbol_order=1)

short int foo2 (short int * p)
{
  short int stmp_max_11.21;
  vector(4) short int vect_max_11.20;
  vector(4) short int vect__4.19;
  short int max;
  short int _4;
  short int _25;
  vector(4) short int _30;
  short int _34;
  vector(4) short int _38;
  vector(4) short int _39;
  vector(4) short int _40;
  vector(4) short int _41;
  short int _44;
  short int _46;

  <bb 2> [local count: 268435454]:
  max_9 = *p_8(D);
  _30 = {max_9, max_9, max_9, max_9};
  vect__4.19_35 = MEM <vector(4) short int> [(short int *)p_8(D) + 2B];
  vect_max_11.20_36 = MAX_EXPR <_30, vect__4.19_35>;
  _38 = VEC_PERM_EXPR <vect_max_11.20_36, { 0, 0, 0, 0 }, { 2, 3, 4, 5 }>;
  _39 = MAX_EXPR <vect_max_11.20_36, _38>;
  _40 = VEC_PERM_EXPR <_39, { 0, 0, 0, 0 }, { 1, 2, 3, 4 }>;
  _41 = MAX_EXPR <_39, _40>;
  stmp_max_11.21_42 = BIT_FIELD_REF <_41, 16, 0>;
  _4 = MEM[(short int *)p_8(D) + 10B];
  _46 = MEM[(short int *)p_8(D) + 12B];
  _34 = MAX_EXPR <_4, _46>;
  _25 = MEM[(short int *)p_8(D) + 14B];
  _44 = MAX_EXPR <_25, stmp_max_11.21_42>;
  max_26 = MAX_EXPR <_34, _44>;
  return max_26;

}