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| 1 | //===-- include/flang/Common/visit.h ----------------------------*- C++ -*-===// |
|---|---|
| 2 | // |
| 3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| 4 | // See https://llvm.org/LICENSE.txt for license information. |
| 5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| 6 | // |
| 7 | //===----------------------------------------------------------------------===// |
| 8 | |
| 9 | // common::visit() is a drop-in replacement for std::visit() that reduces both |
| 10 | // compiler build time and compiler execution time modestly, and reduces |
| 11 | // compiler build memory requirements significantly (overall & maximum). |
| 12 | // It does not require redefinition of std::variant<>. |
| 13 | // |
| 14 | // The C++ standard mandates that std::visit be O(1), but most variants are |
| 15 | // small and O(logN) is faster in practice to compile and execute, avoiding |
| 16 | // the need to build a dispatch table. |
| 17 | // |
| 18 | // Define FLANG_USE_STD_VISIT to avoid this code and make common::visit() an |
| 19 | // alias for ::std::visit(). |
| 20 | |
| 21 | #ifndef FORTRAN_COMMON_VISIT_H_ |
| 22 | #define FORTRAN_COMMON_VISIT_H_ |
| 23 | |
| 24 | #include "variant.h" |
| 25 | #include "flang/Common/api-attrs.h" |
| 26 | #include <type_traits> |
| 27 | |
| 28 | namespace Fortran::common { |
| 29 | namespace log2visit { |
| 30 | |
| 31 | template <std::size_t LOW, std::size_t HIGH, typename RESULT, typename VISITOR, |
| 32 | typename... VARIANT> |
| 33 | inline RT_API_ATTRS RESULT Log2VisitHelper( |
| 34 | VISITOR &&visitor, std::size_t which, VARIANT &&...u) { |
| 35 | if constexpr (LOW + 7 >= HIGH) { |
| 36 | switch (which - LOW) { |
| 37 | #define VISIT_CASE_N(N) \ |
| 38 | case N: \ |
| 39 | if constexpr (LOW + N <= HIGH) { \ |
| 40 | return visitor(std::get<(LOW + N)>(std::forward<VARIANT>(u))...); \ |
| 41 | } |
| 42 | VISIT_CASE_N(1) |
| 43 | VISIT_CASE_N(2) |
| 44 | VISIT_CASE_N(3) |
| 45 | VISIT_CASE_N(4) |
| 46 | VISIT_CASE_N(5) |
| 47 | VISIT_CASE_N(6) |
| 48 | VISIT_CASE_N(7) |
| 49 | #undef VISIT_CASE_N |
| 50 | } |
| 51 | return visitor(std::get<LOW>(std::forward<VARIANT>(u))...); |
| 52 | } else { |
| 53 | static constexpr std::size_t mid{(HIGH + LOW) / 2}; |
| 54 | if (which <= mid) { |
| 55 | return Log2VisitHelper<LOW, mid, RESULT>( |
| 56 | std::forward<VISITOR>(visitor), which, std::forward<VARIANT>(u)...); |
| 57 | } else { |
| 58 | return Log2VisitHelper<(mid + 1), HIGH, RESULT>( |
| 59 | std::forward<VISITOR>(visitor), which, std::forward<VARIANT>(u)...); |
| 60 | } |
| 61 | } |
| 62 | } |
| 63 | |
| 64 | template <typename VISITOR, typename... VARIANT> |
| 65 | inline RT_API_ATTRS auto visit(VISITOR &&visitor, VARIANT &&...u) |
| 66 | -> decltype(visitor(std::get<0>(std::forward<VARIANT>(u))...)) { |
| 67 | using Result = decltype(visitor(std::get<0>(std::forward<VARIANT>(u))...)); |
| 68 | if constexpr (sizeof...(u) == 1) { |
| 69 | static constexpr std::size_t high{ |
| 70 | (std::variant_size_v<std::decay_t<decltype(u)>> * ...) - 1}; |
| 71 | return Log2VisitHelper<0, high, Result>(std::forward<VISITOR>(visitor), |
| 72 | u.index()..., std::forward<VARIANT>(u)...); |
| 73 | } else { |
| 74 | // TODO: figure out how to do multiple variant arguments |
| 75 | return ::std::visit( |
| 76 | std::forward<VISITOR>(visitor), std::forward<VARIANT>(u)...); |
| 77 | } |
| 78 | } |
| 79 | |
| 80 | } // namespace log2visit |
| 81 | |
| 82 | // Some versions of clang have bugs that cause compilation to hang |
| 83 | // on these templates. MSVC and older GCC versions may work but are |
| 84 | // not well tested. So enable only for GCC 9 and better. |
| 85 | #if __GNUC__ < 9 |
| 86 | #define FLANG_USE_STD_VISIT |
| 87 | #endif |
| 88 | |
| 89 | #ifdef FLANG_USE_STD_VISIT |
| 90 | using ::std::visit; |
| 91 | #else |
| 92 | using Fortran::common::log2visit::visit; |
| 93 | #endif |
| 94 | |
| 95 | } // namespace Fortran::common |
| 96 | #endif // FORTRAN_COMMON_VISIT_H_ |
| 97 |
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