[gcc r15-396] ada: Refactor GNAT.Directory_Operations.Read to minimise runtime checks

[gcc r15-396] ada: Refactor GNAT.Directory_Operations.Read to minimise runtime checks

[Marc Poulhi?s]

https://gcc.gnu.org/g:105bba84b10625848ca39a26a42deaa1b2783d71

commit r15-396-g105bba84b10625848ca39a26a42deaa1b2783d71
Author: Piotr Trojanek <trojanek@adacore.com>
Date:   Tue Jun 14 13:08:38 2022 +0200

    ada: Refactor GNAT.Directory_Operations.Read to minimise runtime checks
    
    Array assignments are likely more efficient than element-by-element
    copying; in particular, they avoid constraints checks in every iteration
    of a loop (when the runtime is compiled with checks enabled).
    
    A cleanup and improvement opportunity spotted while working on improved
    detection of uninitialised local scalar objects.
    
    gcc/ada/
    
            * libgnat/g-dirope.adb (Read): Use null-excluding,
            access-to-constant type; replace element-by-element copy with
            array assignments.

Diff:
---
 gcc/ada/libgnat/g-dirope.adb | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/gcc/ada/libgnat/g-dirope.adb b/gcc/ada/libgnat/g-dirope.adb
index c23aa68b700f..428d27d9e8de 100644
--- a/gcc/ada/libgnat/g-dirope.adb
+++ b/gcc/ada/libgnat/g-dirope.adb
@@ -676,13 +676,9 @@ package body GNAT.Directory_Operations is
          return;
       end if;
 
-      Last :=
-        (if Str'Length > Filename_Len then Str'First + Filename_Len - 1
-         else Str'Last);
-
       declare
          subtype Path_String is String (1 .. Filename_Len);
-         type    Path_String_Access is access Path_String;
+         type    Path_String_Access is not null access constant Path_String;
 
          function Address_To_Access is new
            Ada.Unchecked_Conversion
@@ -693,9 +689,13 @@ package body GNAT.Directory_Operations is
                          Address_To_Access (Filename_Addr);
 
       begin
-         for J in Str'First .. Last loop
-            Str (J) := Path_Access (J - Str'First + 1);
-         end loop;
+         if Str'Length > Filename_Len then
+            Last := Str'First + Filename_Len - 1;
+            Str (Str'First .. Last) := Path_Access.all;
+         else
+            Last := Str'Last;
+            Str := Path_Access (1 .. Str'Length);
+         end if;
       end;
    end Read;