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fixedpoint: avoid signed-overflow UB in scalar Neg #219

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fixedpoint: avoid signed-overflow UB in scalar Neg #219

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12 changes: 10 additions & 2 deletions fixedpoint/fixedpoint.h
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,7 @@
#include <cmath>
#include <cstdint>
#include <limits>
#include <type_traits>

#include "../internal/detect_platform.h"

Expand Down Expand Up @@ -107,10 +108,17 @@ tIntegerType Sub(tIntegerType a, tIntegerType b) {
return a - b;
}

// Integer unary negative. Not saturating. Overflow is undefined behavior.
// Integer unary negative. Not saturating. In case of overflow (negating the
// most negative representable value), no Undefined Behavior: the result wraps
// around (implementation-defined, in practice back to that same most negative
// value). This mirrors the SIMD implementations of Neg (which negate in
// hardware without UB) and the UB-avoidance policy documented on ShiftLeft
// below. The negation is done through the unsigned counterpart type, where it
// cannot overflow, then converted back.
template <typename tIntegerType>
tIntegerType Neg(tIntegerType a) {
return -a;
typedef typename std::make_unsigned<tIntegerType>::type UnsignedType;
return static_cast<tIntegerType>(-static_cast<UnsignedType>(a));
}

// Integer arithmetic left-shift, equivalent to multiplying with a power of two.
Expand Down