> On Thu, Nov 11, 2021 at 2:41 PM Jan Hubicka via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: > > > > Hi, > > We make self recursive functions as looping of fear of endless recursion. > > This is done correctly for local pure/const and for non-trivial SCCs in > > callgraph, but for trivial SCCs we miss the flag. > > > > I think it is bad decision since infinite recursion will run out of stack, > > Note it might not always in case we can eliminate the tail-recursion or avoid > stack use by the recursion by other means. So I think it is conservatively > correct.
I don't know. If function is pure and has infinite recursion in it it means that it can only run forever without side effects if it gets lucky and we tail-recurse it. There are no other means avoid the stack use from growing. First i think code relying on tail-recurse optimization to not run out of stack is not strictly valid in C/C++ other languages we care. Also in C++ there is the forced progression which makes even the tail optiimzed code invalid. I think in high level code such recursive accessors used for no good reason are not that infrequent. Also we had this bug in tree probably forever since LOOPING_PURE_CONST was added and no one complained ;) Relaxing this rule breaks some testcases, but odd ones - they are infinitely self-recursive builtin implementations where we then both prove function as noreturn & later optimize builtin to constant so the assembly matching does not see expected thing. Honza
