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Bug 1905399 - Introduce SaturatedCast<>. r=nbp

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Differential Revision: https://phabricator.services.mozilla.com/D215779
This commit is contained in:
Paul Adenot
2024-07-12 07:58:26 +00:00
parent 9e9327bba0
commit bc06a41410
2 changed files with 191 additions and 0 deletions
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53
mfbt/Casting.h
View File

@@ -249,6 +249,59 @@ inline To ReleaseAssertedCast(const From aFrom) {
return static_cast<To>(aFrom);
}
/**
* Cast from type From to type To, clamping to minimum and maximum value of the
* destination type if needed.
*/
template <typename To, typename From>
inline To SaturatingCast(const From aFrom) {
static_assert(std::is_arithmetic_v<To> && std::is_arithmetic_v<From>);
// This implementation works up to 64-bits integers.
static_assert(sizeof(From) <= 8 && sizeof(To) <= 8);
constexpr bool fromFloat = std::is_floating_point_v<From>;
constexpr bool toFloat = std::is_floating_point_v<To>;
// It's not clear what the caller wants here, it could be round, truncate,
// closest value, etc.
static_assert((fromFloat && !toFloat) || (!fromFloat && !toFloat),
"Handle manually depending on desired behaviour");
// If the source is floating point and the destination isn't, it can be that
// casting changes the value unexpectedly. Casting to double and clamping to
// the max of the destination type is correct, this also handles infinity.
if constexpr (fromFloat) {
if (aFrom > static_cast<double>(std::numeric_limits<To>::max())) {
return std::numeric_limits<To>::max();
}
if (aFrom < static_cast<double>(std::numeric_limits<To>::lowest())) {
return std::numeric_limits<To>::lowest();
}
return static_cast<To>(aFrom);
}
// Source and destination are of opposite signedness
if constexpr (std::is_signed_v<From> != std::is_signed_v<To>) {
// Input is negative, output is unsigned, return 0
if (std::is_signed_v<From> && aFrom < 0) {
return 0;
}
// At this point the input is positive, cast everything to uint64_t for
// simplicity and compare
uint64_t inflated = AssertedCast<uint64_t>(aFrom);
if (inflated > static_cast<uint64_t>(std::numeric_limits<To>::max())) {
return std::numeric_limits<To>::max();
}
return static_cast<To>(aFrom);
}
// Regular case: clamp to destination type range
if (aFrom > std::numeric_limits<To>::max()) {
return std::numeric_limits<To>::max();
}
if (aFrom < std::numeric_limits<To>::lowest()) {
return std::numeric_limits<To>::lowest();
}
return static_cast<To>(aFrom);
}
namespace detail {
template <typename From>

138
mfbt/tests/TestCasting.cpp
View File

@@ -11,9 +11,13 @@
#include <cstdint>
#include <limits>
#include <type_traits>
#include <iostream>
#include <tuple>
#include <type_traits>
using mozilla::AssertedCast;
using mozilla::BitwiseCast;
using mozilla::SaturatingCast;
using mozilla::detail::IsInBounds;
static const uint8_t floatMantissaBitsPlusOne = 24;
@@ -243,6 +247,139 @@ void TestFloatConversion() {
!(IsInBounds<double, float>(-std::numeric_limits<double>::max())));
}
#define ASSERT_EQ(a, b) \
if ((a) != (b)) { \
std::cerr << __FILE__ << ":" << __LINE__ << " Actual: " << +(a) << ", " \
<< "expected: " << +(b) << std::endl; \
MOZ_CRASH(); \
}
#ifdef ENABLE_DEBUG_PRINT
# define DEBUG_PRINT(in, out) \
std::cout << "\tIn: " << +in << ", " << "out: " << +out << std::endl;
#else
# define DEBUG_PRINT(in, out)
#endif
template <typename In, typename Out>
void TestTypePairImpl() {
std::cout << __PRETTY_FUNCTION__ << std::endl;
std::cout << std::fixed;
// Test casting infinities to integer works
if constexpr (std::is_floating_point_v<In> &&
!std::is_floating_point_v<Out>) {
Out v = SaturatingCast<Out>(std::numeric_limits<In>::infinity());
ASSERT_EQ(v, std::numeric_limits<Out>::max());
v = SaturatingCast<Out>(-std::numeric_limits<In>::infinity());
ASSERT_EQ(v, std::numeric_limits<Out>::lowest());
}
// Saturation of a floating point value that is infinity is infinity
if constexpr (std::is_floating_point_v<Out> && std::is_floating_point_v<In>) {
In in = std::numeric_limits<In>::infinity();
Out v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, std::numeric_limits<Out>::infinity());
in = -std::numeric_limits<In>::infinity();
v = SaturatingCast<In>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, -std::numeric_limits<Out>::infinity());
return;
} else {
if constexpr (sizeof(In) > sizeof(Out) && std::is_integral_v<Out>) {
// Test with a value just outside the range of the output type
In in = static_cast<In>(std::numeric_limits<Out>::max()) + 1ull;
Out v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, std::numeric_limits<Out>::max());
if (std::is_signed_v<In>) {
// Test with a value just below the range of the output type
Out lowest = std::numeric_limits<Out>::lowest();
in = static_cast<In>(lowest) - 1;
v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
if constexpr (std::is_signed_v<In> && !std::is_signed_v<Out>) {
ASSERT_EQ(v, 0);
} else {
ASSERT_EQ(v, std::numeric_limits<Out>::lowest());
}
}
} else if constexpr (std::is_integral_v<In> && std::is_integral_v<Out> &&
sizeof(In) == sizeof(Out) && !std::is_signed_v<In> &&
std::is_signed_v<Out>) {
// Test that max uintXX_t saturates to max intXX_t
In in = static_cast<In>(std::numeric_limits<Out>::max()) + 1;
Out v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, std::numeric_limits<Out>::max());
}
// SaturatingCast of zero is zero
In in = static_cast<In>(0);
Out v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, 0);
if constexpr (sizeof(In) >= sizeof(Out) && std::is_signed_v<Out> &&
std::is_signed_v<In>) {
// Test with a value within the range of the output type
In in = static_cast<In>(std::numeric_limits<Out>::max() / 2);
Out v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, in);
// Test with a negative value within the range of the output type
in = static_cast<In>(std::numeric_limits<Out>::lowest() / 2);
v = SaturatingCast<Out>(in);
DEBUG_PRINT(in, v);
ASSERT_EQ(v, in);
}
}
}
template <typename In, typename Out>
void TestTypePair() {
constexpr bool fromFloat = std::is_floating_point_v<In>;
constexpr bool toFloat = std::is_floating_point_v<Out>;
// Don't test casting to the same type
if constexpr (!std::is_same_v<In, Out>) {
if constexpr ((fromFloat && !toFloat) || (!fromFloat && !toFloat)) {
TestTypePairImpl<In, Out>();
}
}
}
template <typename T, typename... Ts>
void for_each_type_pair(std::tuple<T, Ts...>) {
(TestTypePair<T, Ts>(), ...);
(TestTypePair<Ts, T>(), ...);
if constexpr (sizeof...(Ts) > 1) {
for_each_type_pair(std::tuple<Ts...>{});
}
}
template <typename... Args>
void TestSaturatingCastImpl() {
for_each_type_pair(std::tuple<Args...>{});
}
template <typename T, typename... Ts>
void TestFirstToOthers() {
(TestTypePair<T, Ts>(), ...);
}
void TestSaturatingCast() {
// Each integer type against every other
TestSaturatingCastImpl<short, int, long, int8_t, uint8_t, int16_t, uint16_t,
int32_t, uint32_t, int64_t, uint64_t>();
// Floating point types to every integer type
TestFirstToOthers<float, short, int, long, int8_t, uint8_t, int16_t, uint16_t,
int32_t, uint32_t, int64_t, uint64_t>();
TestFirstToOthers<double, short, int, long, int8_t, uint8_t, int16_t,
uint16_t, int32_t, uint32_t, int64_t, uint64_t>();
}
int main() {
TestBitwiseCast();
@@ -250,6 +387,7 @@ int main() {
TestToBiggerSize();
TestToSmallerSize();
TestFloatConversion();
TestSaturatingCast();
return 0;
}