[Bug rtl-optimization/123017] New: [15/16 Regression] Very aggressive ifcvt results in performance loss since g:b219cbeda72d23b7ad6ff12cd159784b7ef00667

[tnfchris at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123017

            Bug ID: 123017
           Summary: [15/16 Regression] Very aggressive ifcvt results in
                    performance loss since
                    g:b219cbeda72d23b7ad6ff12cd159784b7ef00667
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: tnfchris at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64*

Since g:b219cbeda72d23b7ad6ff12cd159784b7ef00667

The following

void f(const int *restrict in,
   int *restrict out,
   int n, int threshold)
{
  for (int i = 0; i < n; ++i) {
    int v = in[i];
    if (v > threshold) {
      int t = v * 3;
      t += 7;
      t ^= 0x55;
      t *= 0x55;
      t -= 0x5;
      t &= 0xFE;
      t ^= 0x55;
      out[i] = t;
    } else {
      out[i] = v;
    }
  }
}

compiled at -O2

results in aggressive if-conversion which increases the number of dynamic
instructions and the latency of the loop as it has to wait for t to be
calculated now in all cases.

This has led to big performance losses in packages like zstd [1] which in turns
affects packaging and LTO speed.

[1] https://github.com/facebook/zstd/pull/4418#issuecomment-3004606000