diff --git a/compiler/nmat.pas b/compiler/nmat.pas
index dce92dd60d..ade96bbbf6 100644
--- a/compiler/nmat.pas
+++ b/compiler/nmat.pas
@@ -959,6 +959,29 @@ implementation
              left:=nil;
              exit;
           end;
+        if is_real(left.resultdef) then
+          begin
+            {
+              -(left-right) => right-left
+
+              As this result in -(1.0-1.0)=0.0 instead of 0.0, this is only valid in fastmath mode
+            }
+            if (cs_opt_fastmath in current_settings.optimizerswitches) and (left.nodetype=subn) then
+              begin
+                result:=caddnode.create(subn,taddnode(left).right.getcopy,taddnode(left).left.getcopy);
+                exit;
+              end;
+
+            { --node => node
+              this operation is always valid as reals do not use a two's complement representation for negative
+              numbers, -real means just flip the sign bit
+            }
+            if left.nodetype=unaryminusn then
+              begin
+                result:=tunarynode(left).left.getcopy;
+                exit;
+              end;
+          end;
       end;
 
 
diff --git a/tests/tbs/tb0685.pp b/tests/tbs/tb0685.pp
index f44e307fff..1970538db0 100644
--- a/tests/tbs/tb0685.pp
+++ b/tests/tbs/tb0685.pp
@@ -31,5 +31,18 @@ begin
   if (d3<>1.0) or (TDoubleRec(d3).Sign) then
     halt(5);
 
+  d1:=1.0;
+  d2:=1.0;
+  d3:=-(d2-d1);
+  writeln(d3);
+  if (d3<>0.0) or not(TDoubleRec(d3).Sign) then
+    halt(6);
+
+  d1:=-0.0;
+  d3:=--d1;
+  writeln(d3);
+  if (d3<>0.0) or not(TDoubleRec(d3).Sign) then
+    halt(7);
+
   writeln('ok');
 end.