[Bug c++/69564] [5/6 Regression] lto and/or C++ make scimark2 LU slower

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69564

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Looks like since GCC 5 we wrap all loops in a while (1).  Eventually this just
confuses GCCs static prediction machinery but who knows.

Old (not the problematic function!):

{
    int i;
    int j;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (i = 0) >>>>>;
  goto <D.3610>;
  <D.3609>:;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (j = 0) >>>>>;
  goto <D.3608>;
  <D.3607>:;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (*(*(lu + (sizetype) ((long unsigned int) i * 8)) + (sizetype) ((long
unsigned int) j * 8)) = *(*(A + (sizetype) ((long unsigned int) i * 8)) +
(sizetype) ((long unsigned int) j * 8))) >>>>>;
  <<cleanup_point (void) j++ >>;
  <D.3608>:;
  if (j < N) goto <D.3607>; else goto <D.3605>;
  <D.3605>:;
  <<cleanup_point (void) i++ >>;
  <D.3610>:;
  if (i < M) goto <D.3609>; else goto <D.3603>;
  <D.3603>:;
}

New:

{
    int i;
    int j;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (i = 0) >>>>>;
  while (1)
    {
      if (i >= M) goto <D.3704>;
      <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (j = 0) >>>>>;
      while (1)
        {
          if (j >= N) goto <D.3706>;
          <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (*(*(lu + (sizetype) ((long unsigned int) i * 8)) + (sizetype) ((long
unsigned int) j * 8)) = *(*(A + (sizetype) ((long unsigned int) i * 8)) +
(sizetype) ((long unsigned int) j * 8))) >>>>>;
          <<cleanup_point (void) j++ >>;
        }
      <D.3706>:;
      <<cleanup_point (void) i++ >>;
    }
  <D.3704>:;
}

Or more likely it's the increment after exit check and thus no longer
do-while style loops.