------- Comment #6 from victork at gcc dot gnu dot org 2008-03-23 13:33 ------- Here is AN even more reduced example which demonstrates the problem: int main() { char buf[256]; char *dest; int i;
dest = &buf[2]; for (i = 0; i < 32; i++) { *(unsigned *)dest = 0; dest += 4; } return buf[2]; } gcc -O3 t.c && ./a.out Segmentation fault The problem is that vectorizer is assuming that the access was aligned to the the size of the element before vectorization. A loop peeling only handles misalignment in multiples of element size. In this case, vectorizer could check the actual alignment in compile time and prevent vectorization. (Though the assignment of a constant still can be vectorized if value of constant is adjusted accordingly). In general case, when alignment of the non-vectorized access is unknown, a fix of this bug would require an addition of a run-time test. -- victork at gcc dot gnu dot org changed: What |Removed |Added ---------------------------------------------------------------------------- AssignedTo|unassigned at gcc dot gnu |victork at gcc dot gnu dot |dot org |org Status|NEW |ASSIGNED Last reconfirmed|2008-03-21 17:47:58 |2008-03-23 13:33:08 date| | http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35653