https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98365
Bug ID: 98365 Summary: Miss vectoization Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: crazylht at gmail dot com CC: hjl.tools at gmail dot com, wwwhhhyyy333 at gmail dot com Target Milestone: --- Host: x86_64-pc-linux-gnu cat test.c int foo (char a[64], char c[64]) { int i; char cnt=0; for (int i = 0;i != 64; i++) if (a[i] == c[i]) cnt++; return cnt; } with -Ofast -mavx2 gcc failed to vectorize the loop due to dump of loop body: ----------- # cnt_21 = PHI <cnt_9(7), 0(15)> # i_22 = PHI <i_17(7), 0(15)> # ivtmp_19 = PHI <ivtmp_18(7), 64(15)> _1 = (sizetype) i_22; _2 = a_14(D) + _1; _3 = *_2; _5 = c_15(D) + _1; _6 = *_5; cnt.1_7 = (unsigned char) cnt_21; _8 = cnt.1_7 + 1; cnt_16 = (char) _8; cnt_9 = _3 == _6 ? cnt_16 : cnt_21; i_17 = i_22 + 1; ivtmp_18 = ivtmp_19 - 1; ---------- -fopt-info --------- test.c:5:20: note: vec_stmt_relevant_p: stmt live but not relevant. test.c:5:20: note: mark relevant 1, live 1: cnt_9 = _3 == _6 ? cnt_16 : cnt_21; test.c:5:20: note: init: stmt relevant? i_17 = i_22 + 1; test.c:5:20: note: init: stmt relevant? ivtmp_18 = ivtmp_19 - 1; test.c:5:20: note: init: stmt relevant? if (ivtmp_18 != 0) test.c:5:20: note: worklist: examine stmt: cnt_9 = _3 == _6 ? cnt_16 : cnt_21; test.c:5:20: note: vect_is_simple_use: operand *_2, type of def: internal test.c:5:20: note: mark relevant 1, live 0: _3 = *_2; test.c:5:20: note: vect_is_simple_use: operand *_5, type of def: internal test.c:5:20: note: mark relevant 1, live 0: _6 = *_5; test.c:5:20: note: vect_is_simple_use: operand (char) _8, type of def: internal test.c:5:20: note: mark relevant 1, live 0: cnt_16 = (char) _8; test.c:5:20: note: vect_is_simple_use: operand cnt_21 = PHI <cnt_9(7), 0(15)>, type of def: unknown test.c:5:20: missed: Unsupported pattern. ---------------- Shouldn't cnt_21 = PHI <cnt_9(7), 0(15)>, stmt relevant? BTW: with extra -fwrapv, gcc successfully vectorized the loop.