[llvm] r253921 - [Support] Add optional argument to SaturatingAdd() and SaturatingMultiply() to indicate that overflow occurred
Xinliang David Li via llvm-commits
llvm-commits at lists.llvm.org
Mon Nov 23 15:30:08 PST 2015
One reason I can see is that using default arguments will resulting in
bigger code -- as all the updates need to check nullness.
David
On Mon, Nov 23, 2015 at 3:20 PM, Sean Silva via llvm-commits <
llvm-commits at lists.llvm.org> wrote:
> Is there a reason you used overloading instead of default arguments? I.e.
> instead of just declaring the ResultOverflowed variable as `bool
> *ResultOverflowed = nullptr`.
>
> -- Sean Silva
>
> On Mon, Nov 23, 2015 at 1:54 PM, Nathan Slingerland via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: slingn
>> Date: Mon Nov 23 15:54:22 2015
>> New Revision: 253921
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=253921&view=rev
>> Log:
>> [Support] Add optional argument to SaturatingAdd() and
>> SaturatingMultiply() to indicate that overflow occurred
>>
>> Summary: Adds the ability for callers to detect when saturation occurred
>> on the result of saturating addition/multiplication.
>>
>> Reviewers: davidxl, silvas, rsmith
>>
>> Subscribers: llvm-commits
>>
>> Differential Revision: http://reviews.llvm.org/D14931
>>
>> Modified:
>> llvm/trunk/include/llvm/Support/MathExtras.h
>> llvm/trunk/unittests/Support/MathExtrasTest.cpp
>>
>> Modified: llvm/trunk/include/llvm/Support/MathExtras.h
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/MathExtras.h?rev=253921&r1=253920&r2=253921&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/include/llvm/Support/MathExtras.h (original)
>> +++ llvm/trunk/include/llvm/Support/MathExtras.h Mon Nov 23 15:54:22 2015
>> @@ -655,46 +655,79 @@ inline int64_t SignExtend64(uint64_t X,
>>
>> /// \brief Add two unsigned integers, X and Y, of type T.
>> /// 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
>> -SaturatingAdd(T X, T Y) {
>> +SaturatingAdd(T X, T Y, bool &ResultOverflowed) {
>> // Hacker's Delight, p. 29
>> T Z = X + Y;
>> - if (Z < X || Z < Y)
>> + ResultOverflowed = (Z < X || Z < Y);
>> + if (ResultOverflowed)
>> return std::numeric_limits<T>::max();
>> else
>> return Z;
>> }
>>
>> +/// \brief Add two unsigned integers, X and Y, of type T.
>> +/// Clamp the result to the maximum representable value of T on overflow.
>> +template <typename T>
>> +typename std::enable_if<std::is_unsigned<T>::value, T>::type
>> +SaturatingAdd(T X, T Y) {
>> + bool ResultOverflowed;
>> + return SaturatingAdd(X, Y, ResultOverflowed);
>> +}
>> +
>> /// \brief Multiply two unsigned integers, X and Y, of type T.
>> /// 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
>> -SaturatingMultiply(T X, T Y) {
>> +SaturatingMultiply(T X, T Y, bool &ResultOverflowed) {
>> // Hacker's Delight, p. 30 has a different algorithm, but we don't use
>> that
>> // because it fails for uint16_t (where multiplication can have
>> undefined
>> // behavior due to promotion to int), and requires a division in
>> addition
>> // to the multiplication.
>>
>> + ResultOverflowed = false;
>> +
>> // Log2(Z) would be either Log2Z or Log2Z + 1.
>> // Special case: if X or Y is 0, Log2_64 gives -1, and Log2Z
>> // will necessarily be less than Log2Max as desired.
>> int Log2Z = Log2_64(X) + Log2_64(Y);
>> const T Max = std::numeric_limits<T>::max();
>> int Log2Max = Log2_64(Max);
>> - if (Log2Z < Log2Max)
>> + if (Log2Z < Log2Max) {
>> return X * Y;
>> - if (Log2Z > Log2Max)
>> + }
>> + if (Log2Z > Log2Max) {
>> + ResultOverflowed = true;
>> return Max;
>> + }
>>
>> // We're going to use the top bit, and maybe overflow one
>> // bit past it. Multiply all but the bottom bit then add
>> // that on at the end.
>> T Z = (X >> 1) * Y;
>> - if (Z & ~(Max >> 1))
>> + if (Z & ~(Max >> 1)) {
>> + ResultOverflowed = true;
>> return Max;
>> + }
>> Z <<= 1;
>> - return (X & 1) ? SaturatingAdd(Z, Y) : Z;
>> + if (X & 1)
>> + return SaturatingAdd(Z, Y, ResultOverflowed);
>> +
>> + return Z;
>> +}
>> +
>> +/// \brief Multiply two unsigned integers, X and Y, of type T.
>> +/// Clamp the result to the maximum representable value of T on overflow.
>> +template <typename T>
>> +typename std::enable_if<std::is_unsigned<T>::value, T>::type
>> +SaturatingMultiply(T X, T Y) {
>> + bool ResultOverflowed;
>> + return SaturatingMultiply(X, Y, ResultOverflowed);
>> }
>>
>> extern const float huge_valf;
>>
>> Modified: llvm/trunk/unittests/Support/MathExtrasTest.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/MathExtrasTest.cpp?rev=253921&r1=253920&r2=253921&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/unittests/Support/MathExtrasTest.cpp (original)
>> +++ llvm/trunk/unittests/Support/MathExtrasTest.cpp Mon Nov 23 15:54:22
>> 2015
>> @@ -194,10 +194,27 @@ template<typename T>
>> void SaturatingAddTestHelper()
>> {
>> const T Max = std::numeric_limits<T>::max();
>> + bool ResultOverflowed;
>> +
>> EXPECT_EQ(T(3), SaturatingAdd(T(1), T(2)));
>> + EXPECT_EQ(T(3), SaturatingAdd(T(1), T(2), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(Max, SaturatingAdd(Max, T(1)));
>> + EXPECT_EQ(Max, SaturatingAdd(Max, T(1), ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>> +
>> + EXPECT_EQ(Max, SaturatingAdd(T(1), T(Max - 1)));
>> + EXPECT_EQ(Max, SaturatingAdd(T(1), T(Max - 1), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(Max, SaturatingAdd(T(1), Max));
>> + EXPECT_EQ(Max, SaturatingAdd(T(1), Max, ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>> +
>> EXPECT_EQ(Max, SaturatingAdd(Max, Max));
>> + EXPECT_EQ(Max, SaturatingAdd(Max, Max, ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>> }
>>
>> TEST(MathExtras, SaturatingAdd) {
>> @@ -211,22 +228,50 @@ template<typename T>
>> void SaturatingMultiplyTestHelper()
>> {
>> const T Max = std::numeric_limits<T>::max();
>> + bool ResultOverflowed;
>>
>> // Test basic multiplication.
>> EXPECT_EQ(T(6), SaturatingMultiply(T(2), T(3)));
>> + EXPECT_EQ(T(6), SaturatingMultiply(T(2), T(3), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(T(6), SaturatingMultiply(T(3), T(2)));
>> + EXPECT_EQ(T(6), SaturatingMultiply(T(3), T(2), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>>
>> // Test multiplication by zero.
>> EXPECT_EQ(T(0), SaturatingMultiply(T(0), T(0)));
>> + EXPECT_EQ(T(0), SaturatingMultiply(T(0), T(0), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(T(0), SaturatingMultiply(T(1), T(0)));
>> + EXPECT_EQ(T(0), SaturatingMultiply(T(1), T(0), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(T(0), SaturatingMultiply(T(0), T(1)));
>> + EXPECT_EQ(T(0), SaturatingMultiply(T(0), T(1), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(T(0), SaturatingMultiply(Max, T(0)));
>> + EXPECT_EQ(T(0), SaturatingMultiply(Max, T(0), ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> +
>> EXPECT_EQ(T(0), SaturatingMultiply(T(0), Max));
>> + EXPECT_EQ(T(0), SaturatingMultiply(T(0), Max, ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>>
>> // Test multiplication by maximum value.
>> EXPECT_EQ(Max, SaturatingMultiply(Max, T(2)));
>> - EXPECT_EQ(Max, SaturatingMultiply(T(2),Max));
>> + EXPECT_EQ(Max, SaturatingMultiply(Max, T(2), ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>> +
>> + EXPECT_EQ(Max, SaturatingMultiply(T(2), Max));
>> + EXPECT_EQ(Max, SaturatingMultiply(T(2), Max, ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>> +
>> EXPECT_EQ(Max, SaturatingMultiply(Max, Max));
>> + EXPECT_EQ(Max, SaturatingMultiply(Max, Max, ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>>
>> // Test interesting boundary conditions for algorithm -
>> // ((1 << A) - 1) * ((1 << B) + K) for K in [-1, 0, 1]
>> @@ -241,8 +286,12 @@ void SaturatingMultiplyTestHelper()
>>
>> if(OverflowExpected) {
>> EXPECT_EQ(Max, SaturatingMultiply(X, Y));
>> + EXPECT_EQ(Max, SaturatingMultiply(X, Y, ResultOverflowed));
>> + EXPECT_TRUE(ResultOverflowed);
>> } else {
>> EXPECT_EQ(X * Y, SaturatingMultiply(X, Y));
>> + EXPECT_EQ(X * Y, SaturatingMultiply(X, Y, ResultOverflowed));
>> + EXPECT_FALSE(ResultOverflowed);
>> }
>> }
>> }
>>
>>
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>
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