[PATCH] fnsplit: Set musttail call during function splitting if there are musttail calls [PR119376]

[Jakub Jelinek]

Hi!

The just posted inliner patch can regress musttail calls if we perform
function splitting and then inline the outlined body back into the original
(or inline both the small function and outlined large body into something
else).
If there are any musttail calls, I think we need to call the outlined
body using a musttail call, so that the inliner will preserve musttail
attributes in the body.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2025-03-20  Jakub Jelinek  <ja...@redhat.com>

        PR ipa/119376
        * ipa-split.cc (split_function): Call gimple_call_set_must_tail
        on the call to outlined partition if has_musttail and
        !add_tsan_func_exit.

        * g++.dg/opt/musttail1.C: New test.

--- gcc/ipa-split.cc.jj 2025-01-02 11:23:02.827707968 +0100
+++ gcc/ipa-split.cc    2025-03-20 13:54:57.541594191 +0100
@@ -1473,6 +1473,8 @@ split_function (basic_block return_bb, c
        args_to_pass[i] = arg;
       }
   call = gimple_build_call_vec (node->decl, args_to_pass);
+  if (cur_node->get_fun ()->has_musttail && !add_tsan_func_exit)
+    gimple_call_set_must_tail (call, true);
   gimple_set_block (call, DECL_INITIAL (current_function_decl));
   args_to_pass.release ();
 
--- gcc/testsuite/g++.dg/opt/musttail1.C.jj     2025-03-20 15:07:38.236255098 
+0100
+++ gcc/testsuite/g++.dg/opt/musttail1.C        2025-03-20 15:08:48.625280661 
+0100
@@ -0,0 +1,35 @@
+// PR ipa/119376
+// { dg-do compile { target musttail } }
+// { dg-options "-O2 -fdump-tree-optimized" }
+// { dg-final { scan-tree-dump-times "  \[^\n\r]* = foo \\\(\[^\n\r]*\\\); 
\\\[tail call\\\] \\\[must tail call\\\]" 2 "optimized" } }
+// { dg-final { scan-tree-dump-times "  \[^\n\r]* = foo \\\(\[^\n\r]*\\\); 
\\\[tail call\\\]" 3 "optimized" } }
+
+int foo (int x);
+typedef int (*F) (int);
+int v;
+
+inline int
+bar (int x)
+{
+  if (__builtin_expect (x == 42, 1))
+    return 1;
+  [[gnu::musttail]] return foo (x + v * (x | v) * (x & v) * (x - v) * (x + v * 
v));
+}
+
+int
+baz (int x)
+{
+  [[gnu::musttail]] return bar (x);
+}
+
+int
+qux (int x)
+{
+  return bar (x + 1);
+}
+
+F
+corge ()
+{
+  return &bar;
+}

        Jakub