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[COMMITTED] ada: Refactor GNAT.Directory_Operations.Read to minimise runtime checks

Message ID 20240513083543.164532-1-poulhies@adacore.com
State New
Headers show
Series [COMMITTED] ada: Refactor GNAT.Directory_Operations.Read to minimise runtime checks | expand

Commit Message

Marc Poulhiès May 13, 2024, 8:35 a.m. UTC
From: Piotr Trojanek <trojanek@adacore.com>

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.

Tested on x86_64-pc-linux-gnu, committed on master.

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

diff --git a/gcc/ada/libgnat/g-dirope.adb b/gcc/ada/libgnat/g-dirope.adb
index c23aa68b700..428d27d9e8d 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;