[Mono-bugs] [Bug 54917][Wis] New - We miss some optimizations with callvirt

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Changed by bmaurer@users.sf.net.

http://bugzilla.ximian.com/show_bug.cgi?id=54917

--- shadow/54917	2004-02-26 17:52:29.000000000 -0500
+++ shadow/54917.tmp.29314	2004-02-26 17:52:29.000000000 -0500
@@ -0,0 +1,66 @@
+Bug#: 54917
+Product: Mono/Compilers
+Version: unspecified
+OS: 
+OS Details: 
+Status: NEW   
+Resolution: 
+Severity: 
+Priority: Wishlist
+Component: C#
+AssignedTo: mono-bugs@ximian.com                            
+ReportedBy: bmaurer@users.sf.net               
+QAContact: mono-bugs@ximian.com
+TargetMilestone: ---
+URL: 
+Cc: 
+Summary: We miss some optimizations with callvirt
+
+Callvirt not only does vtable lookups, but also does checking for null
+values. It is what generates NullReferenceExceptions. However, the checks
+obviously come at a cost. Right now, we do not avoid callvirt in some
+situations when we can.
+
+Take the code:
+
+class T {
+	static void Main () {
+		new T ().Foo ();
+	}
+	
+	void Foo () { Bar (); }
+	void Bar () {}
+}
+
+csc emits the following code:
+
+Main:
+        IL_0000:  newobj instance void class 'T'::.ctor()
+        IL_0005:  call instance void class 'T'::'Foo'()
+        IL_000a:  ret
+
+ 
+Foo:
+        IL_0000:  ldarg.0
+        IL_0001:  call instance void class 'T'::'Bar'()
+        IL_0006:  ret
+
+We emit callvirt for the two call instructions. the case for emitting call
+in Foo is pretty clear. the implicit this argument is never going to be
+null, so you dont need to do the null check.
+
+The Main case is a bit more fuzzy. I am guessing the rationale is that a
+ctor always generates a non-null value.
+
+If i use the code:
+	static void Main () {
+		T t = new T ();
+		t.Foo ();
+	}
+
+csc emits:
+        IL_0000:  newobj instance void class 'T'::.ctor()
+        IL_0005:  stloc.0
+        IL_0006:  ldloc.0
+        IL_0007:  callvirt instance void class 'T'::'Foo'()
+        IL_000c:  ret