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Make base::StringPiece more constexpr
The following methods have been made constexpr: - the implicit conversion constructor base::StringPiece(const value_type*) - the comparison operators and base::StringPiece::compare() - base::StringPiece::starts_with() and base::StringPiece::ends_with() Since std::char_traits<> isn't constexpr-ified in C++14, this CL also implements base::CharTraits<> to provide constexpr versions for the base::StringPiece implementation. Finally, this also updates a few callers in Chrome to take advantage of the new constexpr functionality. Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_mojo Change-Id: Id3cf2be14bfb5adb53a5ca1bd74ec2a48e411f60 Reviewed-on: https://chromium-review.googlesource.com/1038746 Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org> Reviewed-by: Dmitry Gozman <dgozman@chromium.org> Reviewed-by: kylechar <kylechar@chromium.org> Reviewed-by: danakj <danakj@chromium.org> Commit-Queue: Daniel Cheng <dcheng@chromium.org> Cr-Commit-Position: refs/heads/master@{#556258}
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,90 @@ | ||
| // Copyright 2018 The Chromium Authors. All rights reserved. | ||
| // Use of this source code is governed by a BSD-style license that can be | ||
| // found in the LICENSE file. | ||
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| #ifndef BASE_STRINGS_CHAR_TRAITS_H_ | ||
| #define BASE_STRINGS_CHAR_TRAITS_H_ | ||
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| #include <stddef.h> | ||
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| namespace base { | ||
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| // constexpr version of http://en.cppreference.com/w/cpp/string/char_traits. | ||
| // This currently just implements the bits needed to support a (mostly) | ||
| // constexpr StringPiece. | ||
| // | ||
| // TODO(dcheng): Once we switch to C++17, most methods will become constexpr and | ||
| // we can switch over to using the one in the standard library. | ||
| template <typename T> | ||
| struct CharTraits { | ||
| // Performs a lexographical comparison of the first N characters of |s1| and | ||
| // |s2|. Returns 0 if equal, -1 if |s1| is less than |s2|, and 1 if |s1| is | ||
| // greater than |s2|. | ||
| static constexpr int compare(const T* s1, const T* s2, size_t n) noexcept; | ||
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| // Returns the length of |s|, assuming null termination (and not including the | ||
| // terminating null). | ||
| static constexpr size_t length(const T* s) noexcept; | ||
| }; | ||
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| template <typename T> | ||
| constexpr int CharTraits<T>::compare(const T* s1, | ||
| const T* s2, | ||
| size_t n) noexcept { | ||
| for (; n; --n, ++s1, ++s2) { | ||
| if (*s1 < *s2) | ||
| return -1; | ||
| if (*s1 > *s2) | ||
| return 1; | ||
| } | ||
| return 0; | ||
| } | ||
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| template <typename T> | ||
| constexpr size_t CharTraits<T>::length(const T* s) noexcept { | ||
| size_t i = 0; | ||
| for (; *s; ++s) | ||
| ++i; | ||
| return i; | ||
| } | ||
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| // char specialization of CharTraits that can use clang's constexpr instrinsics, | ||
| // where available. | ||
| template <> | ||
| struct CharTraits<char> { | ||
| static constexpr int compare(const char* s1, | ||
| const char* s2, | ||
| size_t n) noexcept; | ||
| static constexpr size_t length(const char* s) noexcept; | ||
| }; | ||
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| constexpr int CharTraits<char>::compare(const char* s1, | ||
| const char* s2, | ||
| size_t n) noexcept { | ||
| #if __has_feature(cxx_constexpr_string_builtins) | ||
| return __builtin_memcmp(s1, s2, n); | ||
| #else | ||
| for (; n; --n, ++s1, ++s2) { | ||
| if (*s1 < *s2) | ||
| return -1; | ||
| if (*s1 > *s2) | ||
| return 1; | ||
| } | ||
| return 0; | ||
| #endif | ||
| } | ||
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| constexpr size_t CharTraits<char>::length(const char* s) noexcept { | ||
| #if defined(__clang__) | ||
| return __builtin_strlen(s); | ||
| #else | ||
| size_t i = 0; | ||
| for (; *s; ++s) | ||
| ++i; | ||
| return i; | ||
| #endif | ||
| } | ||
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| } // namespace base | ||
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| #endif // BASE_STRINGS_CHAR_TRAITS_H_ |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,32 @@ | ||
| // Copyright 2018 The Chromium Authors. All rights reserved. | ||
| // Use of this source code is governed by a BSD-style license that can be | ||
| // found in the LICENSE file. | ||
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| #include "base/strings/char_traits.h" | ||
| #include "base/strings/string16.h" | ||
| #include "testing/gtest/include/gtest/gtest.h" | ||
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| namespace base { | ||
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| TEST(CharTraitsTest, CharCompare) { | ||
| static_assert(CharTraits<char>::compare("abc", "def", 3) == -1, ""); | ||
| static_assert(CharTraits<char>::compare("def", "def", 3) == 0, ""); | ||
| static_assert(CharTraits<char>::compare("ghi", "def", 3) == 1, ""); | ||
| } | ||
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| TEST(CharTraitsTest, CharLength) { | ||
| static_assert(CharTraits<char>::length("") == 0, ""); | ||
| static_assert(CharTraits<char>::length("abc") == 3, ""); | ||
| } | ||
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| TEST(CharTraitsTest, Char16TCompare) { | ||
| static_assert(CharTraits<char16_t>::compare(u"abc", u"def", 3) == -1, ""); | ||
| static_assert(CharTraits<char16_t>::compare(u"def", u"def", 3) == 0, ""); | ||
| static_assert(CharTraits<char16_t>::compare(u"ghi", u"def", 3) == 1, ""); | ||
| } | ||
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| TEST(CharTraitsTest, Char16TLength) { | ||
| static_assert(CharTraits<char16_t>::length(u"abc") == 3, ""); | ||
| } | ||
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| } // namespace base |
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