[gcc r11-5705] Fix PR middle-end/98082

[Eric Botcazou <ebotcazou@gcc.gnu.org>]

https://gcc.gnu.org/g:5d103763b9bc6a998dd4a453861663b229d7a24a

commit r11-5705-g5d103763b9bc6a998dd4a453861663b229d7a24a
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Thu Dec 3 15:52:15 2020 +0100

    Fix PR middle-end/98082
    
    this fixes an ICE introduced by the fix for PR middle-end/97078 where
    use_register_for_decl was changed to return true at -O0 for a parameter
    of a thunk.  It turns out that we need to do the same for a result in
    this case.
    
    gcc/ChangeLog:
            PR middle-end/98082
            * function.c (use_register_for_decl): Also return true for a result
            if cfun->tail_call_marked is true.
    
    gcc/testsuite/ChangeLog:
            * g++.dg/cpp2a/pr98082.C: New test.

Diff:
---
 gcc/function.c                       |  6 ++++--
 gcc/testsuite/g++.dg/cpp2a/pr98082.C | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/gcc/function.c b/gcc/function.c
index 59fd72b0e82..af9618e18df 100644
--- a/gcc/function.c
+++ b/gcc/function.c
@@ -2206,13 +2206,15 @@ use_register_for_decl (const_tree decl)
       /* Otherwise, if RESULT_DECL is DECL_BY_REFERENCE, it will take
 	 the function_result_decl's assignment.  Since it's a pointer,
 	 we can short-circuit a number of the tests below, and we must
-	 duplicat e them because we don't have the
-	 function_result_decl to test.  */
+	 duplicate them because we don't have the function_result_decl
+	 to test.  */
       if (!targetm.calls.allocate_stack_slots_for_args ())
 	return true;
       /* We don't set DECL_IGNORED_P for the function_result_decl.  */
       if (optimize)
 	return true;
+      if (cfun->tail_call_marked)
+	return true;
       /* We don't set DECL_REGISTER for the function_result_decl.  */
       return false;
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/pr98082.C b/gcc/testsuite/g++.dg/cpp2a/pr98082.C
new file mode 100644
index 00000000000..b2caacb55df
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/pr98082.C
@@ -0,0 +1,14 @@
+/* PR middle-end/98082 */
+/* Reported by Martin Liska <marxin@gcc.gnu.org> */
+
+/* { dg-do compile { target c++20 } } */
+/* { dg-options "-fipa-icf" } */
+
+class GoodIter {
+public:
+  GoodIter();
+  GoodIter(GoodIter &);
+};
+
+GoodIter operator-(int, GoodIter) { return GoodIter(); }
+GoodIter operator+(int, GoodIter) { return GoodIter(); }