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

["hubicka at ucw dot cz" <gcc-bugzilla@gcc.gnu.org>]

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