Re: Fix recursion discovery in ipa-pure-const
> On Thu, Nov 11, 2021 at 2:41 PM Jan Hubicka via Gcc-patches > 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
Re: Fix recursion discovery in ipa-pure-const
On Thu, Nov 11, 2021 at 2:41 PM Jan Hubicka via Gcc-patches
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.
Richard.
> but changing it upsets some testcases and should be done independently.
> So this patch is fixing current behaviour to be consistent.
>
> Bootstrapped/regtested x86_64-linux, comitted.
>
> gcc/ChangeLog:
>
> 2021-11-11 Jan Hubicka
>
> * ipa-pure-const.c (propagate_pure_const): Self recursion is
> a side effects.
>
> diff --git a/gcc/ipa-pure-const.c b/gcc/ipa-pure-const.c
> index 505ed4f8a3b..64777cd2d91 100644
> --- a/gcc/ipa-pure-const.c
> +++ b/gcc/ipa-pure-const.c
> @@ -1513,6 +1611,9 @@ propagate_pure_const (void)
> enum pure_const_state_e edge_state = IPA_CONST;
> bool edge_looping = false;
>
> + if (e->recursive_p ())
> + looping = true;
> +
> if (dump_file && (dump_flags & TDF_DETAILS))
> {
> fprintf (dump_file, "Call to %s",
