[llvm-bugs] [Bug 34364] New: 'hoisting conditional out of loop' missed optimization opportunity

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34364

            Bug ID: 34364
           Summary: 'hoisting conditional out of loop' missed optimization
                    opportunity
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

Old code:

int get();

void func(int y, int width, int* img) {
  __builtin_assume(width > 4);

  int vpred[2][2] = {{0, 0}, {0, 0}};
  int hpred[2];

  for (int x = 0; x < width; x++) {
    int diff = get();
    if (x < 2)
      hpred[x] = vpred[y & 1][x] += diff;
    else
      hpred[x & 1] += diff;
    img[x] = hpred[x & 1];
  }
}


Equivalent code without inner if:


int get();

void func(int y, int width, int* img) {
  __builtin_assume(width > 4);

  int vpred[2][2] = {{0, 0}, {0, 0}};
  int hpred[2];

  int x;
  int diff;

  x = 0;
  diff = get();
  hpred[x] = vpred[y & 1][x] += diff;
  img[x] = hpred[x & 1];

  x = 1;
  diff = get();
  hpred[x] = vpred[y & 1][x] += diff;
  img[x] = hpred[x & 1];

  for (; x < width; x++) {
    diff = get();
    hpred[x & 1] += diff;
    img[x] = hpred[x & 1];
  }
}


According to godbolt, clang trunk can not do that: https://godbolt.org/g/cS6RD5

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