[Support] Fix SaturatingMultiply<T>() to be correct (and fast), Re-enable Unit Tests
Summary:
This change fixes the SaturatingMultiply<T>() function template to not cause undefined behavior with T=uint16_t.
Thanks to Richard Smith's contribution, it also no longer requires an integer division.
Patch by Richard Smith.
Reviewers: silvas, davidxl
Subscribers: rsmith, davidxl, llvm-commits
Differential Revision: http://reviews.llvm.org/D14845
llvm-svn: 253870
diff --git a/llvm/unittests/Support/MathExtrasTest.cpp b/llvm/unittests/Support/MathExtrasTest.cpp
index 0f38546..fa12f75 100644
--- a/llvm/unittests/Support/MathExtrasTest.cpp
+++ b/llvm/unittests/Support/MathExtrasTest.cpp
@@ -207,4 +207,52 @@
SaturatingAddTestHelper<uint64_t>();
}
+template<typename T>
+void SaturatingMultiplyTestHelper()
+{
+ const T Max = std::numeric_limits<T>::max();
+
+ // Test basic multiplication.
+ EXPECT_EQ(T(6), SaturatingMultiply(T(2), T(3)));
+ EXPECT_EQ(T(6), SaturatingMultiply(T(3), T(2)));
+
+ // Test multiplication by zero.
+ EXPECT_EQ(T(0), SaturatingMultiply(T(0), T(0)));
+ EXPECT_EQ(T(0), SaturatingMultiply(T(1), T(0)));
+ EXPECT_EQ(T(0), SaturatingMultiply(T(0), T(1)));
+ EXPECT_EQ(T(0), SaturatingMultiply(Max, T(0)));
+ EXPECT_EQ(T(0), SaturatingMultiply(T(0), Max));
+
+ // Test multiplication by maximum value.
+ EXPECT_EQ(Max, SaturatingMultiply(Max, T(2)));
+ EXPECT_EQ(Max, SaturatingMultiply(T(2),Max));
+ EXPECT_EQ(Max, SaturatingMultiply(Max, Max));
+
+ // Test interesting boundary conditions for algorithm -
+ // ((1 << A) - 1) * ((1 << B) + K) for K in [-1, 0, 1]
+ // and A + B == std::numeric_limits<T>::digits.
+ // We expect overflow iff A > B and K = 1.
+ const int Digits = std::numeric_limits<T>::digits;
+ for (int A = 1, B = Digits - 1; B >= 1; ++A, --B) {
+ for (int K = -1; K <= 1; ++K) {
+ T X = (T(1) << A) - T(1);
+ T Y = (T(1) << B) + K;
+ bool OverflowExpected = A > B && K == 1;
+
+ if(OverflowExpected) {
+ EXPECT_EQ(Max, SaturatingMultiply(X, Y));
+ } else {
+ EXPECT_EQ(X * Y, SaturatingMultiply(X, Y));
+ }
+ }
+ }
+}
+
+TEST(MathExtras, SaturatingMultiply) {
+ SaturatingMultiplyTestHelper<uint8_t>();
+ SaturatingMultiplyTestHelper<uint16_t>();
+ SaturatingMultiplyTestHelper<uint32_t>();
+ SaturatingMultiplyTestHelper<uint64_t>();
+}
+
}