Re: ipa-inline & what TARGET_CAN_INLINE_P can assume

[Andrew Pinski <pinskia@gmail.com>]

On Mon, Sep 25, 2023 at 10:16 AM Richard Sandiford via Gcc
<gcc@gcc.gnu.org> wrote:
>
> Hi,
>
> I have a couple of questions about what TARGET_CAN_INLINE_P is
> alllowed to assume when called from ipa-inline.  (Callers from the
> front-end don't matter for the moment.)
>
> I'm working on an extension where a function F1 without attribute A
> can't be inlined into a function F2 with attribute A.  That part is
> easy and standard.
>
> But it's expected that many functions won't have attribute A,
> even if they could.  So we'd like to detect automatically whether
> F1's implementation is compatible with attribute A.  This is something
> we can do by scanning the gimple code.
>
> However, even if we detect that F1's code is compatible with attribute A,
> we don't want to add attribute A to F1 itself because (a) it would change
> F1's ABI and (b) it would restrict the optimisation of any non-inlined
> copy of F1.  So this is a test for inlining only.
>
> TARGET_CAN_INLINE_P (F2, F1) can check whether F1's current code
> is compatible with attribute A.  But:
>
> (a) Is it safe to assume (going forward) that F1 won't change before
>     it is inlined into F2?  Specifically, is it safe to assume that
>     nothing will be inlined into F1 between the call to TARGET_CAN_INLINE_P
>     and the inlining of F1 into F2?
>
> (b) For compile-time reasons, I'd like to cache the result in
>     machine_function.  The cache would be a three-state:
>
>     - not tested
>     - compatible with A
>     - incompatible with A
>
>     The cache would be reset to "not tested" whenever TARGET_CAN_INLINE_P
>     is called with F1 as the *caller* rather than the callee.  The idea
>     is to handle cases where something is inlined into F1 after F1 has
>     been inlined into F2.  (This would include calls from the main
>     inlining pass, after the early pass has finished.)
>
>     Is resetting the cache in this way sufficient?  Or should we have a
>     new interface for this?
>
> Sorry for the long question :)  I have something that seems to work,
> but I'm not sure whether it's misusing the interface.


The rs6000 backend has a similar issue and defined the following
target hooks which seems exactly what you need in this case
TARGET_NEED_IPA_FN_TARGET_INFO
TARGET_UPDATE_IPA_FN_TARGET_INFO

And then use that information in can_inline_p target hook to mask off
the ISA bits:
      unsigned int info = ipa_fn_summaries->get (callee_node)->target_info;
      if ((info & RS6000_FN_TARGET_INFO_HTM) == 0)
        {
          callee_isa &= ~OPTION_MASK_HTM;
          explicit_isa &= ~OPTION_MASK_HTM;
        }


Thanks,
Andrew Pinski


>
> Thanks,
> Richard