[gcc r13-5210] ada: Fix pessimization of some CW objects initialized with function call

[Marc Poulhi?s <dkm@gcc.gnu.org>]

https://gcc.gnu.org/g:3b4c6e67710b09beccb7d0acf7f7257564d8a6f5

commit r13-5210-g3b4c6e67710b09beccb7d0acf7f7257564d8a6f5
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Tue Jan 10 16:09:44 2023 +0100

    ada: Fix pessimization of some CW objects initialized with function call
    
    The recent removal of the unconditional call to Remove_Side_Effects on the
    expression of an object declaration or an allocator with a class-wide type
    has introduced a pessimization in the former case for function calls that
    return a specific tagged type, because the object ultimately created on the
    primary stack has changed from being of a specific tagged type to being of
    the class-wide type, the latter type always formally requiring finalization.
    
    With the current finalization machinery, this means that a dispatching call
    to the Deep_Finalize routine is generated, which is unnecessary.  Although
    this is a generic finalization issue with class-wide objects, this restores
    the previous behavior in this case to fix the pessimization for now.
    
    gcc/ada/
    
            * exp_ch3.adb (Expand_N_Object_Declaration): For a class-wide non-
            interface stand-alone object initialized by a function call, call
            Remove_Side_Effects on the expression to capture the result.

Diff:
---
 gcc/ada/exp_ch3.adb | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/gcc/ada/exp_ch3.adb b/gcc/ada/exp_ch3.adb
index 6bc76aec5d1..6886bde7bd1 100644
--- a/gcc/ada/exp_ch3.adb
+++ b/gcc/ada/exp_ch3.adb
@@ -7797,6 +7797,25 @@ package body Exp_Ch3 is
          --  Common case of explicit object initialization
 
          else
+            --  Small optimization: if the expression is a function call and
+            --  the object is stand-alone, not declared at library level and of
+            --  a class-wide type, then we capture the result of the call into
+            --  a temporary, with the benefit that, if the result's type does
+            --  not need finalization, nothing will be finalized and, if it
+            --  does, the temporary only will be finalized by means of a direct
+            --  call to the Finalize primitive if the result's type is not a
+            --  class-wide type; whereas, in both cases, the stand-alone object
+            --  itself would be finalized by means of a dispatching call to the
+            --  Deep_Finalize routine.
+
+            if Nkind (Expr_Q) = N_Function_Call
+              and then not Special_Ret_Obj
+              and then not Is_Library_Level_Entity (Def_Id)
+              and then Is_Class_Wide_Type (Typ)
+            then
+               Remove_Side_Effects (Expr_Q);
+            end if;
+
             --  In most cases, we must check that the initial value meets any
             --  constraint imposed by the declared type. However, there is one
             --  very important exception to this rule. If the entity has an