[PATCH] D15385: [Support] Add saturating multiply-add support function

[Nathan Slingerland via llvm-commits]

slingn created this revision.
slingn added reviewers: davidxl, silvas.
slingn added a subscriber: llvm-commits.

Add SaturatingMultiplyAdd convenience function template since A + (X * Y) comes up frequently when doing weighted arithmetic.

http://reviews.llvm.org/D15385

Files:
  include/llvm/Support/MathExtras.h
  unittests/Support/MathExtrasTest.cpp

Index: unittests/Support/MathExtrasTest.cpp
===================================================================
--- unittests/Support/MathExtrasTest.cpp
+++ unittests/Support/MathExtrasTest.cpp
@@ -304,4 +304,66 @@
   SaturatingMultiplyTestHelper<uint64_t>();
 }
 
+template<typename T>
+void SaturatingMultiplyAddTestHelper()
+{
+  const T Max = std::numeric_limits<T>::max();
+  bool ResultOverflowed;
+
+  // Test basic multiply-add.
+  EXPECT_EQ(T(16), SaturatingMultiplyAdd(T(2), T(3), T(10)));
+  EXPECT_EQ(T(16), SaturatingMultiplyAdd(T(2), T(3), T(10), &ResultOverflowed));
+  EXPECT_FALSE(ResultOverflowed);
+
+  // Test multiply overflows, add doesn't overflow
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(Max, Max, T(0), &ResultOverflowed));
+  EXPECT_TRUE(ResultOverflowed);
+
+  // Test multiply doesn't overflow, add overflows
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(T(1), T(1), Max, &ResultOverflowed));
+  EXPECT_TRUE(ResultOverflowed);
+
+  // Test multiply-add with Max as operand
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(T(1), T(1), Max));
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(T(1), T(1), Max, &ResultOverflowed));
+  EXPECT_TRUE(ResultOverflowed);
+
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(T(1), Max, T(1)));
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(T(1), Max, T(1), &ResultOverflowed));
+  EXPECT_TRUE(ResultOverflowed);
+
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(Max, Max, T(1)));
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(Max, Max, T(1), &ResultOverflowed));
+  EXPECT_TRUE(ResultOverflowed);
+
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(Max, Max, Max));
+  EXPECT_EQ(Max, SaturatingMultiplyAdd(Max, Max, Max, &ResultOverflowed));
+  EXPECT_TRUE(ResultOverflowed);
+
+  // Test multiply-add with 0 as operand
+  EXPECT_EQ(T(1), SaturatingMultiplyAdd(T(1), T(1), T(0)));
+  EXPECT_EQ(T(1), SaturatingMultiplyAdd(T(1), T(1), T(0), &ResultOverflowed));
+  EXPECT_FALSE(ResultOverflowed);
+
+  EXPECT_EQ(T(1), SaturatingMultiplyAdd(T(1), T(0), T(1)));
+  EXPECT_EQ(T(1), SaturatingMultiplyAdd(T(1), T(0), T(1), &ResultOverflowed));
+  EXPECT_FALSE(ResultOverflowed);
+
+  EXPECT_EQ(T(1), SaturatingMultiplyAdd(T(0), T(0), T(1)));
+  EXPECT_EQ(T(1), SaturatingMultiplyAdd(T(0), T(0), T(1), &ResultOverflowed));
+  EXPECT_FALSE(ResultOverflowed);
+
+  EXPECT_EQ(T(0), SaturatingMultiplyAdd(T(0), T(0), T(0)));
+  EXPECT_EQ(T(0), SaturatingMultiplyAdd(T(0), T(0), T(0), &ResultOverflowed));
+  EXPECT_FALSE(ResultOverflowed);
+
+}
+
+TEST(MathExtras, SaturatingMultiplyAdd) {
+  SaturatingMultiplyAddTestHelper<uint8_t>();
+  SaturatingMultiplyAddTestHelper<uint16_t>();
+  SaturatingMultiplyAddTestHelper<uint32_t>();
+  SaturatingMultiplyAddTestHelper<uint64_t>();
+}
+
 }
Index: include/llvm/Support/MathExtras.h
===================================================================
--- include/llvm/Support/MathExtras.h
+++ include/llvm/Support/MathExtras.h
@@ -717,6 +717,24 @@
   return Z;
 }
 
+/// \brief Multiply two unsigned integers, X and Y, of type T. Add the
+/// unsigned integer, A, also of type T, to the product.
+/// Clamp the result to the maximum representable value of T on overflow.
+/// ResultOverflowed indicates if the result is larger than the maximum
+/// representable value of type T.
+template <typename T>
+typename std::enable_if<std::is_unsigned<T>::value, T>::type
+SaturatingMultiplyAdd(T X, T Y, T A, bool *ResultOverflowed = nullptr) {
+  bool Dummy;
+  bool &Overflowed = ResultOverflowed ? *ResultOverflowed : Dummy;
+
+  T Product = SaturatingMultiply(X, Y, &Overflowed);
+  if (Overflowed)
+    return Product;
+
+  return SaturatingAdd(A, Product, &Overflowed);
+}
+
 extern const float huge_valf;
 } // End llvm namespace
 


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