From b481cd519e07b3ad2bd3e81c89b0dd8efd68d6bc Mon Sep 17 00:00:00 2001
From: Christopher Tetreault <ctetreau@quicinc.com>
Date: Tue, 2 Feb 2021 09:14:55 -0800
Subject: [PATCH] Ensure that InstructionCost actually implements a total
 ordering

Previously, operator== would consider the actual equality of the pairs
(lhs.Value, lhs.State) == (rhs.Value, rhs.State). However, if an invalid
cost was involved in a call to operator<, only the state would be
compared. Thus, it was not the case that ({2, Invalid} < {3, Invalid} ||
{2, Invalid} > {3, Invalid} || {2, Invalid} == {3, Invalid}).

This patch implements a true total ordering, where cost state is
considered first, then value. While it's not really imporant that
{2, Invalid} be considered to be less than {3, Invalid}, it's not a
problem either. This patch also implements operator== in terms of
operator<, so the two definitions will be kept in sync.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D95803
---
 llvm/include/llvm/Support/InstructionCost.h    | 29 +++++++++++++-------------
 llvm/unittests/Support/InstructionCostTest.cpp |  4 ++++
 2 files changed, 18 insertions(+), 15 deletions(-)

diff --git a/llvm/include/llvm/Support/InstructionCost.h b/llvm/include/llvm/Support/InstructionCost.h
index fbc898b..7101ed1 100644
--- a/llvm/include/llvm/Support/InstructionCost.h
+++ b/llvm/include/llvm/Support/InstructionCost.h
@@ -146,31 +146,30 @@ public:
     return Copy;
   }
 
+  /// For the comparison operators we have chosen to use lexicographical
+  /// ordering where valid costs are always considered to be less than invalid
+  /// costs. This avoids having to add asserts to the comparison operators that
+  /// the states are valid and users can test for validity of the cost
+  /// explicitly.
+  bool operator<(const InstructionCost &RHS) const {
+    return State < RHS.State || Value < RHS.Value;
+  }
+
+  // Implement in terms of operator< to ensure that the two comparisons stay in
+  // sync
   bool operator==(const InstructionCost &RHS) const {
-    return State == RHS.State && Value == RHS.Value;
+    return !(*this < RHS) && !(RHS < *this);
   }
 
   bool operator!=(const InstructionCost &RHS) const { return !(*this == RHS); }
 
   bool operator==(const CostType RHS) const {
-    return State == Valid && Value == RHS;
+    InstructionCost RHS2(RHS);
+    return *this == RHS2;
   }
 
   bool operator!=(const CostType RHS) const { return !(*this == RHS); }
 
-  /// For the comparison operators we have chosen to use total ordering with
-  /// the following rules:
-  ///  1. If either of the states != Valid then a lexicographical order is
-  ///     applied based upon the state.
-  ///  2. If both states are valid then order based upon value.
-  /// This avoids having to add asserts the comparison operators that the states
-  /// are valid and users can test for validity of the cost explicitly.
-  bool operator<(const InstructionCost &RHS) const {
-    if (State != Valid || RHS.State != Valid)
-      return State < RHS.State;
-    return Value < RHS.Value;
-  }
-
   bool operator>(const InstructionCost &RHS) const { return RHS < *this; }
 
   bool operator<=(const InstructionCost &RHS) const { return !(RHS < *this); }
diff --git a/llvm/unittests/Support/InstructionCostTest.cpp b/llvm/unittests/Support/InstructionCostTest.cpp
index 8ba9f99..2a881a7 100644
--- a/llvm/unittests/Support/InstructionCostTest.cpp
+++ b/llvm/unittests/Support/InstructionCostTest.cpp
@@ -25,6 +25,7 @@ TEST_F(CostTest, Operators) {
   InstructionCost VSix = 6;
   InstructionCost IThreeA = InstructionCost::getInvalid(3);
   InstructionCost IThreeB = InstructionCost::getInvalid(3);
+  InstructionCost ITwo = InstructionCost::getInvalid(2);
   InstructionCost TmpCost;
 
   EXPECT_NE(VThree, VNegTwo);
@@ -37,6 +38,9 @@ TEST_F(CostTest, Operators) {
   EXPECT_EQ(VThree - VNegTwo, 5);
   EXPECT_EQ(VThree * VNegTwo, -6);
   EXPECT_EQ(VSix / VThree, 2);
+  EXPECT_NE(IThreeA, ITwo);
+  EXPECT_LT(ITwo, IThreeA);
+  EXPECT_GT(IThreeA, ITwo);
 
   EXPECT_FALSE(IThreeA.isValid());
   EXPECT_EQ(IThreeA.getState(), InstructionCost::Invalid);
-- 
2.7.4