71 lines · cpp
1// RUN: %clangxx_nsan -O0 -mllvm -nsan-shadow-type-mapping=dqq -g -DSUM=NaiveSum -DFLT=float %s -o %t2// RUN: env NSAN_OPTIONS=halt_on_error=1,log2_max_relative_error=19 not %run %t 2>&1 | FileCheck %s3 4// RUN: %clangxx_nsan -O3 -mllvm -nsan-shadow-type-mapping=dqq -g -DSUM=NaiveSum -DFLT=float %s -o %t5// RUN: env NSAN_OPTIONS=halt_on_error=1,log2_max_relative_error=19 not %run %t 2>&1 | FileCheck %s6 7// RUN: %clangxx_nsan -O0 -mllvm -nsan-shadow-type-mapping=dqq -g -DSUM=KahanSum -DFLT=float %s -o %t8// RUN: env NSAN_OPTIONS=halt_on_error=1,log2_max_relative_error=19 %run %t9 10// RUN: %clangxx_nsan -O3 -mllvm -nsan-shadow-type-mapping=dqq -g -DSUM=KahanSum -DFLT=float %s -o %t11// RUN: env NSAN_OPTIONS=halt_on_error=1,log2_max_relative_error=19 %run %t12 13#include <chrono>14#include <iostream>15#include <random>16#include <vector>17 18// A naive, unstable summation.19template <typename T>20__attribute__((noinline)) // To check call stack reporting.21T NaiveSum(const std::vector<T>& values) {22 T sum = 0;23 for (T v : values) {24 sum += v;25 }26 return sum;27 // CHECK: WARNING: NumericalStabilitySanitizer: inconsistent shadow results while checking return28 // CHECK: float{{ *}}precision (native):29 // CHECK: double{{ *}}precision (shadow):30 // CHECK: {{#0 .*in .* NaiveSum}}31}32 33// Kahan's summation is a numerically stable sum.34// https://en.wikipedia.org/wiki/Kahan_summation_algorithm35template <typename T>36__attribute__((noinline)) T KahanSum(const std::vector<T> &values) {37 T sum = 0;38 T c = 0;39 for (T v : values) {40 T y = v - c;41 T t = sum + y;42 c = (t - sum) - y;43 sum = t;44 }45 return sum;46}47 48int main() {49 std::vector<FLT> values;50 constexpr int kNumValues = 1000000;51 values.reserve(kNumValues);52 // Using a seed to avoid flakiness.53 constexpr uint32_t kSeed = 0x123456;54 std::mt19937 gen(kSeed);55 std::uniform_real_distribution<FLT> dis(0.0f, 1000.0f);56 for (int i = 0; i < kNumValues; ++i) {57 values.push_back(dis(gen));58 }59 60 const auto t1 = std::chrono::high_resolution_clock::now();61 const auto sum = SUM(values);62 const auto t2 = std::chrono::high_resolution_clock::now();63 printf("sum: %.8f\n", sum);64 std::cout << "runtime: "65 << std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1)66 .count() /67 1000.068 << "ms\n";69 return 0;70}71