100 lines
3.2 KiB
C
100 lines
3.2 KiB
C
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/* Copyright 2015 The OpenXLA Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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==============================================================================*/
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#ifndef XLA_OVERFLOW_UTIL_H_
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#define XLA_OVERFLOW_UTIL_H_
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#include <cstdint>
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#include <optional>
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#include <type_traits>
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#include <utility>
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#include "absl/base/attributes.h"
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#include "absl/base/optimization.h"
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namespace xla {
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// Multiply two non-negative int64_t's, returning the two's complement result
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// and a bool which is true when overflow or negative inputs occurs and false
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// otherwise.
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ABSL_ATTRIBUTE_ALWAYS_INLINE inline std::pair<int64_t, bool>
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OverflowSafeMultiply(const int64_t x, const int64_t y) {
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#if ABSL_HAVE_BUILTIN(__builtin_mul_overflow)
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int64_t result;
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bool bad = __builtin_mul_overflow(x, y, &result);
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bad |= x < 0;
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bad |= y < 0;
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return std::make_pair(result, bad);
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#else
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// Multiply in uint64_t rather than int64_t since signed overflow is
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// undefined. Negative values will wrap around to large unsigned values in the
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// casts (see section 4.7 [conv.integral] of the C++14 standard).
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const uint64_t ux = x;
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const uint64_t uy = y;
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const uint64_t uxy = ux * uy;
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// Cast back to signed.
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int64_t result = static_cast<int64_t>(uxy);
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bool bad = result < 0;
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// Check if we overflow uint64_t, using a cheap check if both inputs are small
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if (ABSL_PREDICT_FALSE((ux | uy) >> 32 != 0)) {
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if (x < 0 || y < 0) {
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// Ensure nonnegativity. Note that negative numbers will appear "large"
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// to the unsigned comparisons above.
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bad = true;
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} else if (ux != 0 && uxy / ux != uy) {
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// Otherwise, detect overflow using a division
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bad = true;
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}
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}
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return std::make_pair(result, bad);
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#endif
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}
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// Computes x + y and returns nullopt if it overflows.
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//
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// x and y must be signed integers.
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template <typename T>
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ABSL_ATTRIBUTE_ALWAYS_INLINE inline std::optional<T> OverflowSafeAdd(T x, T y) {
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static_assert(std::is_signed<T>::value,
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"Only implemented for signed numbers T.");
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static_assert(std::is_integral<T>::value, "Only implemented for integers T.");
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#if ABSL_HAVE_BUILTIN(__builtin_add_overflow)
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T result;
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if (ABSL_PREDICT_FALSE(__builtin_add_overflow(x, y, &result))) {
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return std::nullopt;
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}
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return result;
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#else
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// "Signed integer overflow occurs on integer addition iff the operands have
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// the same sign and the sum has a sign opposite to that of the operands."
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// Hacker's Delight 2nd ed, p 28.
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using U = typename std::make_unsigned<T>::type;
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const U ux = x;
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const U uy = y;
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const U usum = ux + uy;
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const T sum = usum;
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if (x >= 0 == y >= 0 && sum >= 0 != x >= 0) {
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return std::nullopt;
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}
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return sum;
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#endif
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}
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} // namespace xla
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#endif // XLA_OVERFLOW_UTIL_H_
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