Index: compiler/optcse.pas
===================================================================
--- compiler/optcse.pas	(revision 47639)
+++ compiler/optcse.pas	(working copy)
@@ -64,6 +64,15 @@
 
     function searchsubdomain(var n:tnode; arg: pointer) : foreachnoderesult;
       begin
+        { When writing to an array, the index can be uninitialised if a
+          conditional value that depends on the index is assigned to it }
+        if (n.nodetype = vecn) and (nf_callunique in n.flags) then
+          begin
+            pboolean(arg)^:=false;
+            result:=fen_norecurse_true;
+            Exit;
+          end;
+
         if (n.nodetype in cseinvariant) or
           ((n.nodetype=inlinen) and
            (tinlinenode(n).inlinenumber in [in_length_x,in_assigned_x,in_sqr_real,in_sqrt_real,in_sin_real,in_cos_real,in_abs_long,
Index: tests/webtbs/tw38129.pp
===================================================================
--- tests/webtbs/tw38129.pp	(nonexistent)
+++ tests/webtbs/tw38129.pp	(working copy)
@@ -0,0 +1,43 @@
+{ %OPT=-O2 }
+program tw38129;
+{$mode objfpc}
+{$h+}
+{$WARN 5094 off : Function result variable of a managed type does not seem to be initialized}
+
+function WriteBits(const Val: Single; Divs: Integer): ansistring;
+  const
+    ProgChars: array[Boolean] of Char = (#192, #219);
+  var
+    I: Integer;
+  begin
+    SetLength(Result, Divs);
+    for I := 0 to Divs - 1 do
+      PAnsiChar(Result)[I] := ProgChars[(Val >= (0.75 + I) / Divs) or (I = Divs - 1) and (Val >= 1)];
+  end;
+
+const
+  Expected: array[0..4] of ansistring = (
+    #192#192#192#192,
+    #219#192#192#192,
+    #219#219#192#192,
+    #219#219#219#192,
+    #219#219#219#219
+  );
+
+var
+  X, Y: Integer;
+  CurrentBits: ansistring;
+begin
+  for X := Low(Expected) to High(Expected) do
+  begin
+    CurrentBits := WriteBits(X * 0.25, 4);
+    for Y := 1 to Length(CurrentBits) do
+      if CurrentBits[Y] <> Expected[X][Y] then
+      begin
+        WriteLn('FAIL on X = ', X, ', Y = ', Y, '; expected ', Expected[X][Y], ', got ', CurrentBits[X]);
+        Halt(1);
+      end;
+  end;
+  WriteLn('ok');
+end.
+