From bf3f49ac797216b8ebc72d5ece95a8c95743f418 Mon Sep 17 00:00:00 2001
From: Walter Bright <walter@walterbright.com>
Date: Wed, 18 Dec 2024 16:51:23 -0800
Subject: [PATCH] optimize s=s where s is a struct

---
 compiler/src/dmd/backend/cgelem.d |  7 +++++++
 compiler/test/runnable/mars1.d    | 21 +++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/compiler/src/dmd/backend/cgelem.d b/compiler/src/dmd/backend/cgelem.d
index 9576733e120..9cb055ae44d 100644
--- a/compiler/src/dmd/backend/cgelem.d
+++ b/compiler/src/dmd/backend/cgelem.d
@@ -3521,6 +3521,13 @@ elem * elstruct(elem *e, Goal goal)
         return optelem(e, goal);
     }
 
+    // Replace (e = e) with (e, e)
+    if (e.Eoper == OPstreq && el_match(e.E1, e.E2))
+    {
+        e.Eoper = OPcomma;
+        return optelem(e, goal);
+    }
+
     if (!e.ET)
         return e;
     //printf("\tnumbytes = %d\n", cast(int)type_size(e.ET));
diff --git a/compiler/test/runnable/mars1.d b/compiler/test/runnable/mars1.d
index 8352f4d4fa9..7de4372237f 100644
--- a/compiler/test/runnable/mars1.d
+++ b/compiler/test/runnable/mars1.d
@@ -2507,6 +2507,26 @@ void test20574()
 
 ////////////////////////////////////////////////////////////////////////
 
+struct S8
+{
+    int x,y,z;
+}
+
+int test8x(S8 s)
+{
+    s = s;
+    return s.y;
+}
+
+void test8()
+{
+    S8 s;
+    s.y = 2;
+    assert(test8x(s) == 2);
+}
+
+////////////////////////////////////////////////////////////////////////
+
 int main()
 {
     // All the various integer divide tests
@@ -2607,6 +2627,7 @@ int main()
     test21835();
     testDoWhileContinue();
     test20574();
+    test8();
 
     printf("Success\n");
     return 0;