https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95669
Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Component|c++ |middle-end Target| |x86_64-*-* Last reconfirmed| |2020-06-15 Ever confirmed|0 |1 Status|UNCONFIRMED |NEW Keywords| |missed-optimization --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- with 'dummy' and the implicit zero initialization of it we retain <bb 2> [local count: 1073741824]: if (a_3(D) < b_4(D)) goto <bb 3>; [50.00%] else goto <bb 4>; [50.00%] <bb 3> [local count: 536870913]: D.2350 = {}; goto <bb 5>; [100.00%] <bb 4> [local count: 536870913]: _1 = a_3(D) * b_4(D); D.2350.val = _1; MEM <unsigned int> [(void *)&D.2350 + 4B] = 1; <bb 5> [local count: 1073741824]: return D.2350; wich generates straigt-forward code while with 'dummy' elided we manage to completely scalarize things and do <bb 2> [local count: 1073741824]: if (a_3(D) < b_4(D)) goto <bb 4>; [50.00%] else goto <bb 3>; [50.00%] <bb 3> [local count: 536870913]: _1 = a_3(D) * b_4(D); <bb 4> [local count: 1073741824]: # cstore_11 = PHI <_1(3), 0(2)> # cstore_10 = PHI <1(3), 0(2)> D.2349.ok = cstore_10; D.2349.val = cstore_11; return D.2349; which is basically two conditional moves we expand via strange bit shufflings because D.2349 (struct res) is assigned a register.