[llvm] r267440 - [LVI] Clarify comments describing the lattice values

[Philip Reames via llvm-commits]

Author: reames
Date: Mon Apr 25 13:48:43 2016
New Revision: 267440

URL: http://llvm.org/viewvc/llvm-project?rev=267440&view=rev
Log:
[LVI] Clarify comments describing the lattice values

There has been much recent confusion about the partition in the lattice between constant and non-constant values.  Hopefully, documenting this will prevent confusion going forward.


Modified:
    llvm/trunk/lib/Analysis/LazyValueInfo.cpp

Modified: llvm/trunk/lib/Analysis/LazyValueInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/LazyValueInfo.cpp?rev=267440&r1=267439&r2=267440&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/LazyValueInfo.cpp (original)
+++ llvm/trunk/lib/Analysis/LazyValueInfo.cpp Mon Apr 25 13:48:43 2016
@@ -63,19 +63,24 @@ namespace llvm {
 namespace {
 class LVILatticeVal {
   enum LatticeValueTy {
-    /// This Value has no known value yet.
+    /// This Value has no known value yet.  As a result, this implies the
+    /// producing instruction is dead.  Caution: We use this as the starting
+    /// state in our local meet rules.  In this usage, it's taken to mean
+    /// "nothing known yet".  
     undefined,
 
-    /// This Value has a specific constant value.
+    /// This Value has a specific constant value.  (For integers, constantrange
+    /// is used instead.)
     constant,
 
-    /// This Value is known to not have the specified value.
+    /// This Value is known to not have the specified value.  (For integers,
+    /// constantrange is used instead.)
     notconstant,
 
-    /// The Value falls within this range.
+    /// The Value falls within this range. (Used only for integer typed values.)
     constantrange,
 
-    /// This value is not known to be constant, and we know that it has a value.
+    /// We can not precisely model the dynamic values this value might take.
     overdefined
   };