On Mon, Oct 26, 2020 at 10:59 AM Stefan Schulze Frielinghaus via Gcc <gcc@gcc.gnu.org> wrote: > > I'm trying to detect loops of the form > > while (*x != y) > ++x; > > which mimic the behaviour of function rawmemchr. Note, the size of *x is not > necessarily one byte. Thus ultimately I would like to detect such loops and > replace them with calls to builtins rawmemchr8, rawmemchr16, rawmemchr32 if > they are implemented in the backend: > > x = __builtin_rawmemchr16(x, y); > > I'm wondering whether there is a particular place in order to detect such loop > patterns. For example, in the loop distribution pass GCC recognizes loops > which mimic the behavior of memset, memcpy, memmove and replaces them with > calls to their corresponding builtins, respectively. The pass and in > particular partitioning of statements depends on whether a statement is used > outside of a partition or not. This works perfectly fine for loops which > implement the mentioned mem* operations since their result is typically > ignored. However, the result of a rawmemchr function is/should never be > ignored. Therefore, such loops are currently recognized as having a reduction > which makes an implementation into the loop distribution pass not straight > forward to me. > > Are there other places where you would detect such loops? Any comments?
loop distribution is the correct pass to look at. You're simply the first to recognize a reduction pattern. And yes, you'll likely need some generic adjustments to the code to handle this. Richard. > Cheers, > Stefan
