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[Ada] Tree inconsistency between -O0 and -O1

Message ID 20210429080348.GA134064@adacore.com
State New
Headers show
Series [Ada] Tree inconsistency between -O0 and -O1 | expand

Commit Message

Pierre-Marie de Rodat April 29, 2021, 8:03 a.m. UTC 
The expansion performed in Expand_N_If_Statement only at -O1 and above
can create inconsistencies in the tree that can cause trouble when
compiling different files with different optimization levels. This
is visible in particular in GNAT LLVM when generating C code on ACATS
test cc50a01.

Fixed by only doing the transformation for internally generated nodes,
but systematically. This way we preserve the original control flow for
user code and always simplify internally generated trees which was the
original motivation of this change (simplify some discriminant checks).

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch5.adb (Expand_N_If_Statement): Only perform the
	simplification on return True/False for internal nodes when
	-fpreserve-control-flow is not set.
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Patch

diff --git a/gcc/ada/exp_ch5.adb b/gcc/ada/exp_ch5.adb
--- a/gcc/ada/exp_ch5.adb
+++ b/gcc/ada/exp_ch5.adb
@@ -3788,62 +3788,58 @@  package body Exp_Ch5 is
 
       --     return not (expression);
 
-      --  Only do these optimizations if we are at least at -O1 level and
-      --  do not do them if control flow optimizations are suppressed.
+      --  Do these optimizations only for internally generated code and only
+      --  when -fpreserve-control-flow isn't set, to preserve the original
+      --  source control flow.
 
-      if Optimization_Level > 0
+      if not Comes_From_Source (N)
         and then not Opt.Suppress_Control_Flow_Optimizations
+        and then Nkind (N) = N_If_Statement
+        and then No (Elsif_Parts (N))
+        and then Present (Else_Statements (N))
+        and then List_Length (Then_Statements (N)) = 1
+        and then List_Length (Else_Statements (N)) = 1
       then
-         if Nkind (N) = N_If_Statement
-           and then No (Elsif_Parts (N))
-           and then Present (Else_Statements (N))
-           and then List_Length (Then_Statements (N)) = 1
-           and then List_Length (Else_Statements (N)) = 1
-         then
-            declare
-               Then_Stm : constant Node_Id := First (Then_Statements (N));
-               Else_Stm : constant Node_Id := First (Else_Statements (N));
+         declare
+            Then_Stm : constant Node_Id := First (Then_Statements (N));
+            Else_Stm : constant Node_Id := First (Else_Statements (N));
 
-            begin
-               if Nkind (Then_Stm) = N_Simple_Return_Statement
+            Then_Expr : Node_Id;
+            Else_Expr : Node_Id;
+
+         begin
+            if Nkind (Then_Stm) = N_Simple_Return_Statement
+                 and then
+               Nkind (Else_Stm) = N_Simple_Return_Statement
+            then
+               Then_Expr := Expression (Then_Stm);
+               Else_Expr := Expression (Else_Stm);
+
+               if Nkind (Then_Expr) in N_Expanded_Name | N_Identifier
                     and then
-                  Nkind (Else_Stm) = N_Simple_Return_Statement
+                  Nkind (Else_Expr) in N_Expanded_Name | N_Identifier
                then
-                  declare
-                     Then_Expr : constant Node_Id := Expression (Then_Stm);
-                     Else_Expr : constant Node_Id := Expression (Else_Stm);
+                  if Entity (Then_Expr) = Standard_True
+                    and then Entity (Else_Expr) = Standard_False
+                  then
+                     Rewrite (N,
+                       Make_Simple_Return_Statement (Loc,
+                         Expression => Relocate_Node (Condition (N))));
+                     Analyze (N);
 
-                  begin
-                     if Nkind (Then_Expr) in N_Expanded_Name | N_Identifier
-                          and then
-                        Nkind (Else_Expr) in N_Expanded_Name | N_Identifier
-                     then
-                        if Entity (Then_Expr) = Standard_True
-                          and then Entity (Else_Expr) = Standard_False
-                        then
-                           Rewrite (N,
-                             Make_Simple_Return_Statement (Loc,
-                               Expression => Relocate_Node (Condition (N))));
-                           Analyze (N);
-                           return;
-
-                        elsif Entity (Then_Expr) = Standard_False
-                          and then Entity (Else_Expr) = Standard_True
-                        then
-                           Rewrite (N,
-                             Make_Simple_Return_Statement (Loc,
-                               Expression =>
-                                 Make_Op_Not (Loc,
-                                   Right_Opnd =>
-                                     Relocate_Node (Condition (N)))));
-                           Analyze (N);
-                           return;
-                        end if;
-                     end if;
-                  end;
+                  elsif Entity (Then_Expr) = Standard_False
+                    and then Entity (Else_Expr) = Standard_True
+                  then
+                     Rewrite (N,
+                       Make_Simple_Return_Statement (Loc,
+                         Expression =>
+                           Make_Op_Not (Loc,
+                             Right_Opnd => Relocate_Node (Condition (N)))));
+                     Analyze (N);
+                  end if;
                end if;
-            end;
-         end if;
+            end if;
+         end;
       end if;
    end Expand_N_If_Statement;