http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59594
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Created attachment 31919 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31919&action=edit gcc49-pr59594.patch Untested patch for discussion. The reason why we (incorrectly) vectorize the testcase is that we ignore the data dependency, on the testcase both the b[a] read vs. b[a+1] store and b[a] store vs. b[a+1] store DDRs have dist 1 and DDR_REVERSED_P set and we ignore those. Now on say: int printf (const char *, ...); int a; static int b[1024]; int main () { for (a = 0; a <= 512; a++) { b[a - 1] = b[a]; b[a] = 1; } printf ("%d\n", b[1]); return 0; } only the b[a] read vs. b[a-1] store is dist 1 DDR_REVERSED_P and b[a] store vs. b[a-1] store is dist 1 !DDR_REVERSED_P, thus we don't vectorize it (correctly). Unfortunately not ignoring dist > 0 && DDR_REVERSED_P ddrs for negative step regresses the testcase I've attached, where there is a write after read ddr and it works properly with the current check. While the attached patch keeps that testcase (no-vfa-vect-depend*.c) working and fixes the test (pr59594.c), the conditions are piled completely randomly, I'm afraid I don't know why it is so, if for the DDR_REVERSED_P continue it matters whether step is positive or negative, or if that is irrelevant and all the write after write DDR_REVERSED_P ddrs need to be checked normally (abs (dist) >= *max_vf), or if say only write after read should be treated as the code treats it right now and even read after write is problematic. The DDR_REVERSED_P stuff has been added in 2007 for PR32377, see e.g. http://gcc.gnu.org/ml/gcc-patches/2007-09/msg01067.html Richard, any ideas?
