[Bug ipa/110334] [13/14 Regresssion] unused functions not eliminated before LTO streaming

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110334

--- Comment #12 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 26 Jun 2023, hubicka at ucw dot cz wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110334
> 
> --- Comment #11 from Jan Hubicka <hubicka at ucw dot cz> ---
> Hi,
> what about this. It should make at least quite basic inlining to happen
> to always_inline. I do not think many critical always_inlines have
> indirect calls in them.  The test for lto is quite bad and I can
> work on solving this incrementally (it would be nice to have this
> tested and possibly backport it).
> 
> diff --git a/gcc/ipa-inline.cc b/gcc/ipa-inline.cc
> index efc8df7d4e0..dcec07e49e1 100644
> --- a/gcc/ipa-inline.cc
> +++ b/gcc/ipa-inline.cc
> @@ -702,6 +702,38 @@ can_early_inline_edge_p (struct cgraph_edge *e)
>    if (!can_inline_edge_p (e, true, true)
>        || !can_inline_edge_by_limits_p (e, true, false, true))
>      return false;
> +  /* When inlining regular functions into always-inline functions
> +     during early inlining watch for possible inline cycles.  */
> +  if (DECL_DISREGARD_INLINE_LIMITS (caller->decl)
> +      && lookup_attribute ("always_inline", DECL_ATTRIBUTES (caller->decl))
> +      && (!DECL_DISREGARD_INLINE_LIMITS (callee->decl)
> +         || !lookup_attribute ("always_inline", DECL_ATTRIBUTES
> (callee->decl))))
> +    {
> +      /* If there are indirect calls, inlining may produce direct call.
> +        TODO: We may lift this restriction if we avoid errors on formely
> +        indirect calls to always_inline functions.  Taking address
> +        of always_inline function is generally bad idea and should
> +        have been declared as undefined, but sadly we allow this.  */
> +      if (caller->indirect_calls || e->callee->indirect_calls)

why disallow caller->indirect_calls?

> +       return false;
> +      for (cgraph_edge *e2 = callee->callees; e2; e2 = e2->next_callee)

I don't think this flys - it looks quadratic.  Can we compute this
in the inline summary once instead?

As for indirect calls, can we maybe mark initial direct GIMPLE call
stmts as "always-inline" and only look at that marking, thus an
indirect call will never become "always-inline"?  Iff cgraph edges
prevail during all early inlining we could mark call edges for
this purpose?

> +       {
> +         struct cgraph_node *callee2 = e2->callee->ultimate_alias_target ();
> +         /* As early inliner runs in RPO order, we will see uninlined
> +            always_inline calls only in the case of cyclic graphs.  */
> +         if (DECL_DISREGARD_INLINE_LIMITS (callee2->decl)
> +             || lookup_attribute ("always_inline", callee2->decl))
> +           return false;
> +         /* With LTO watch for case where function is later replaced
> +            by always_inline definition.
> +            TODO: We may either stop treating noninlined cross-module always
> +            inlines as errors, or we can extend decl merging to produce
> +            syntacic alias and honor always inline only in units it has
> +            been declared as such.  */
> +         if (flag_lto && callee2->externally_visible)
> +           return false;
> +       }
> +    }
>    return true;
>  }
> 
> @@ -3034,18 +3066,7 @@ early_inliner (function *fun)
> 
>    if (!optimize
>        || flag_no_inline
> -      || !flag_early_inlining
> -      /* Never inline regular functions into always-inline functions
> -        during incremental inlining.  This sucks as functions calling
> -        always inline functions will get less optimized, but at the
> -        same time inlining of functions calling always inline
> -        function into an always inline function might introduce
> -        cycles of edges to be always inlined in the callgraph.
> -
> -        We might want to be smarter and just avoid this type of inlining.  */
> -      || (DECL_DISREGARD_INLINE_LIMITS (node->decl)
> -         && lookup_attribute ("always_inline",
> -                              DECL_ATTRIBUTES (node->decl))))
> +      || !flag_early_inlining)
>      ;
>    else if (lookup_attribute ("flatten",
>                              DECL_ATTRIBUTES (node->decl)) != NULL)
> 
>