https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117123
--- Comment #5 from Filip Kastl <pheeck at gcc dot gnu.org> --- PRE is the pass that should be able to optimize most of the code in the testcase away. It doesn't remove the code if sccopy1 has run. I looked at gimple dumps before PRE for compiler runs with sccopy1 and without sccopy1. I think that the only difference is here with sccopy --- 43 <bb 3> [local count: 19861441]: 44 _36 = a_24(D) & 1; 45 if (_36 == 0) 46 goto <bb 4>; [66.00%] 47 else 48 goto <bb 5>; [34.00%] 49 50 <bb 4> [local count: 13108551]: 51 if (a_24(D) != 10) 52 goto <bb 6>; [66.00%] 53 else 54 goto <bb 5>; [34.00%] 55 56 <bb 5> [local count: 21441448]: 57 # spud$size_22 = PHI <10(4), a_24(D)(2), a_24(D)(3)> 58 _5 = a_24(D) == -100; 59 goto <bb 9>; [100.00%] --- without sccopy --- 43 <bb 3> [local count: 21441448]: 44 # spud$size_55 = PHI <a_31(D)(2), a_31(D)(4), 10(5)> 45 _7 = a_31(D) == -100; 46 goto <bb 9>; [100.00%] 47 48 <bb 4> [local count: 19861441]: 49 _44 = a_31(D) & 1; 50 if (_44 == 0) 51 goto <bb 5>; [66.00%] 52 else 53 goto <bb 3>; [34.00%] 54 55 <bb 5> [local count: 13108551]: 56 if (a_31(D) != 10) 57 goto <bb 6>; [66.00%] 58 else 59 goto <bb 3>; [34.00%] --- If we ignore ssa name version numbers, these snippets are basically the same. The only difference is the order of basic blocks. For some reason, this difference is important to PRE. Note that I have only a basic idea about what PRE does and how it does it.
