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1 //===-- runtime/time-intrinsic.cpp ----------------------------------------===//
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2 //
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3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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4 // See https://llvm.org/LICENSE.txt for license information.
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5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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6 //
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7 //===----------------------------------------------------------------------===//
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8
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9 // Implements time-related intrinsic subroutines.
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10
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11 #include "time-intrinsic.h"
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12
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13 #include <ctime>
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14
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15 // CPU_TIME (Fortran 2018 16.9.57)
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16 // We can use std::clock() from the <ctime> header as a fallback implementation
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17 // that should be available everywhere. This may not provide the best resolution
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18 // and is particularly troublesome on (some?) POSIX systems where CLOCKS_PER_SEC
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19 // is defined as 10^6 regardless of the actual precision of std::clock().
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20 // Therefore, we will usually prefer platform-specific alternatives when they
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21 // are available.
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22 //
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23 // We can use SFINAE to choose a platform-specific alternative. To do so, we
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24 // introduce a helper function template, whose overload set will contain only
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25 // implementations relying on interfaces which are actually available. Each
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26 // overload will have a dummy parameter whose type indicates whether or not it
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27 // should be preferred. Any other parameters required for SFINAE should have
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28 // default values provided.
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29 namespace {
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30 // Types for the dummy parameter indicating the priority of a given overload.
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31 // We will invoke our helper with an integer literal argument, so the overload
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32 // with the highest priority should have the type int.
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33 using fallback_implementation = double;
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34 using preferred_implementation = int;
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35
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36 // This is the fallback implementation, which should work everywhere.
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37 template <typename Unused = void> double getCpuTime(fallback_implementation) {
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38 std::clock_t timestamp{std::clock()};
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39 if (timestamp != std::clock_t{-1}) {
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40 return static_cast<double>(timestamp) / CLOCKS_PER_SEC;
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41 }
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42
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43 // Return some negative value to represent failure.
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44 return -1.0;
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45 }
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46
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47 // POSIX implementation using clock_gettime. This is only enabled if
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48 // clock_gettime is available.
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49 template <typename T = int, typename U = struct timespec>
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50 double getCpuTime(preferred_implementation,
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51 // We need some dummy parameters to pass to decltype(clock_gettime).
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52 T ClockId = 0, U *Timespec = nullptr,
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53 decltype(clock_gettime(ClockId, Timespec)) *Enabled = nullptr) {
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54 #if defined CLOCK_THREAD_CPUTIME_ID
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55 #define CLOCKID CLOCK_THREAD_CPUTIME_ID
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56 #elif defined CLOCK_PROCESS_CPUTIME_ID
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57 #define CLOCKID CLOCK_PROCESS_CPUTIME_ID
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58 #elif defined CLOCK_MONOTONIC
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59 #define CLOCKID CLOCK_MONOTONIC
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60 #else
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61 #define CLOCKID CLOCK_REALTIME
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62 #endif
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63 struct timespec tspec;
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64 if (clock_gettime(CLOCKID, &tspec) == 0) {
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65 return tspec.tv_nsec * 1.0e-9 + tspec.tv_sec;
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66 }
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67
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68 // Return some negative value to represent failure.
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69 return -1.0;
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70 }
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71 } // anonymous namespace
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72
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73 namespace Fortran::runtime {
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74 extern "C" {
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75
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76 double RTNAME(CpuTime)() { return getCpuTime(0); }
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77 } // extern "C"
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78 } // namespace Fortran::runtime
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