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Commit 5d06f796 by Abseil Team Committed by Copybara-Service
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Rollback of "Speed up StrAppend by up to 4x." 

PiperOrigin-RevId: 553158292
Change-Id: I28350321550accd72da2f9f6f5992af311fe4b00
parent e945c8d9
Showing with 196 additions and 134 deletions
absl/strings/str_cat.cc
......@@ -14,9 +14,17 @@
#include "absl/strings/str_cat.h"
#include <assert.h>
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <string>
#include "absl/strings/ascii.h"
#include "absl/strings/internal/resize_uninitialized.h"
#include "absl/strings/numbers.h"
#include "absl/strings/string_view.h"
namespace absl {
......@@ -29,26 +37,170 @@ ABSL_NAMESPACE_BEGIN
// of a mix of raw C strings, string_views, strings, and integer values.
// ----------------------------------------------------------------------
// Append is merely a version of memcpy that returns the address of the byte
// after the area just overwritten.
static char* Append(char* out, const AlphaNum& x) {
// memcpy is allowed to overwrite arbitrary memory, so doing this after the
// call would force an extra fetch of x.size().
char* after = out + x.size();
if (x.size() != 0) {
memcpy(out, x.data(), x.size());
}
return after;
}
std::string StrCat(const AlphaNum& a, const AlphaNum& b) {
std::string result;
absl::strings_internal::STLStringResizeUninitialized(&result,
a.size() + b.size());
char* const begin = &result[0];
char* out = begin;
out = Append(out, a);
out = Append(out, b);
assert(out == begin + result.size());
return result;
}
std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c) {
std::string result;
strings_internal::STLStringResizeUninitialized(
&result, a.size() + b.size() + c.size());
char* const begin = &result[0];
char* out = begin;
out = Append(out, a);
out = Append(out, b);
out = Append(out, c);
assert(out == begin + result.size());
return result;
}
std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c,
const AlphaNum& d) {
std::string result;
strings_internal::STLStringResizeUninitialized(
&result, a.size() + b.size() + c.size() + d.size());
char* const begin = &result[0];
char* out = begin;
out = Append(out, a);
out = Append(out, b);
out = Append(out, c);
out = Append(out, d);
assert(out == begin + result.size());
return result;
}
namespace strings_internal {
bool HaveOverlap(const std::string& x, absl::string_view y) {
if (y.empty()) return false;
// TODO(b/290623057): Re-evaluate the check below: it detects when buffers
// overlap (which is good) but it also introduces undefined behaviour when
// buffers don't overlap (substracting pointers that do not belong to the same
// array is UB [expr.add]). In other words, if compiler assumes that a program
// never has UB, then it can replace `assert(HaveOverlap(x, y))` with
// `assert(false)`.
return (uintptr_t(y.data() - x.data()) <= uintptr_t(x.size()));
// Do not call directly - these are not part of the public API.
std::string CatPieces(std::initializer_list<absl::string_view> pieces) {
std::string result;
size_t total_size = 0;
for (absl::string_view piece : pieces) total_size += piece.size();
strings_internal::STLStringResizeUninitialized(&result, total_size);
char* const begin = &result[0];
char* out = begin;
for (absl::string_view piece : pieces) {
const size_t this_size = piece.size();
if (this_size != 0) {
memcpy(out, piece.data(), this_size);
out += this_size;
}
}
assert(out == begin + result.size());
return result;
}
#if defined(__GNUC__) && !defined(__clang__)
char* AppendAlphaNum(char* dst, const AlphaNum& a) {
return std::copy_n(a.data(), a.size(), dst);
// It's possible to call StrAppend with an absl::string_view that is itself a
// fragment of the string we're appending to. However the results of this are
// random. Therefore, check for this in debug mode. Use unsigned math so we
// only have to do one comparison. Note, there's an exception case: appending an
// empty string is always allowed.
#define ASSERT_NO_OVERLAP(dest, src) \
assert(((src).size() == 0) || \
(uintptr_t((src).data() - (dest).data()) > uintptr_t((dest).size())))
void AppendPieces(std::string* dest,
std::initializer_list<absl::string_view> pieces) {
size_t old_size = dest->size();
size_t total_size = old_size;
for (absl::string_view piece : pieces) {
ASSERT_NO_OVERLAP(*dest, piece);
total_size += piece.size();
}
strings_internal::STLStringResizeUninitializedAmortized(dest, total_size);
char* const begin = &(*dest)[0];
char* out = begin + old_size;
for (absl::string_view piece : pieces) {
const size_t this_size = piece.size();
if (this_size != 0) {
memcpy(out, piece.data(), this_size);
out += this_size;
}
}
assert(out == begin + dest->size());
}
#endif
} // namespace strings_internal
void StrAppend(std::string* dest, const AlphaNum& a) {
ASSERT_NO_OVERLAP(*dest, a);
std::string::size_type old_size = dest->size();
strings_internal::STLStringResizeUninitializedAmortized(dest,
old_size + a.size());
char* const begin = &(*dest)[0];
char* out = begin + old_size;
out = Append(out, a);
assert(out == begin + dest->size());
}
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b) {
ASSERT_NO_OVERLAP(*dest, a);
ASSERT_NO_OVERLAP(*dest, b);
std::string::size_type old_size = dest->size();
strings_internal::STLStringResizeUninitializedAmortized(
dest, old_size + a.size() + b.size());
char* const begin = &(*dest)[0];
char* out = begin + old_size;
out = Append(out, a);
out = Append(out, b);
assert(out == begin + dest->size());
}
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c) {
ASSERT_NO_OVERLAP(*dest, a);
ASSERT_NO_OVERLAP(*dest, b);
ASSERT_NO_OVERLAP(*dest, c);
std::string::size_type old_size = dest->size();
strings_internal::STLStringResizeUninitializedAmortized(
dest, old_size + a.size() + b.size() + c.size());
char* const begin = &(*dest)[0];
char* out = begin + old_size;
out = Append(out, a);
out = Append(out, b);
out = Append(out, c);
assert(out == begin + dest->size());
}
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c, const AlphaNum& d) {
ASSERT_NO_OVERLAP(*dest, a);
ASSERT_NO_OVERLAP(*dest, b);
ASSERT_NO_OVERLAP(*dest, c);
ASSERT_NO_OVERLAP(*dest, d);
std::string::size_type old_size = dest->size();
strings_internal::STLStringResizeUninitializedAmortized(
dest, old_size + a.size() + b.size() + c.size() + d.size());
char* const begin = &(*dest)[0];
char* out = begin + old_size;
out = Append(out, a);
out = Append(out, b);
out = Append(out, c);
out = Append(out, d);
assert(out == begin + dest->size());
}
ABSL_NAMESPACE_END
} // namespace absl
absl/strings/str_cat.h
......@@ -91,9 +91,7 @@
#include <array>
#include <cstdint>
#include <cstring>
#include <functional>
#include <string>
#include <tuple>
#include <type_traits>
#include <utility>
#include <vector>
......@@ -101,7 +99,6 @@
#include "absl/base/attributes.h"
#include "absl/base/port.h"
#include "absl/strings/internal/has_absl_stringify.h"
#include "absl/strings/internal/resize_uninitialized.h"
#include "absl/strings/internal/stringify_sink.h"
#include "absl/strings/numbers.h"
#include "absl/strings/string_view.h"
......@@ -412,93 +409,6 @@ class AlphaNum {
char digits_[numbers_internal::kFastToBufferSize];
};
namespace strings_internal {
// Inlining this function improves performance. However, on GCC this triggers
// [-Werror=stringop-overflow=], and in that case we split declaration and
// definition as a workaround.
#if defined(__GNUC__) && !defined(__clang__)
char* AppendAlphaNum(char* dst, const AlphaNum& a);
#else
inline char* AppendAlphaNum(char* dst, const AlphaNum& a) {
return std::copy_n(a.data(), a.size(), dst);
}
#endif
// It's possible to call StrAppend with an absl::string_view that is itself a
// fragment of the string we're appending to. However the results of this are
// UB. Therefore, check for this in debug mode. Use unsigned math so we
// only have to do one comparison. Note, there's an exception case: appending an
// empty string is always allowed.
bool HaveOverlap(const std::string& x, absl::string_view y);
// C++14 compatible alternative to left
// https://en.cppreference.com/w/cpp/language/fold expression.
// TODO(b/290784225): Remove `FoldLeft` once Abseil drops C++14 support.
template <typename F, typename Acc>
auto FoldLeft(const F&, Acc&& acc) {
return std::forward<Acc>(acc);
}
template <typename F, typename Acc, typename First, typename... Rest>
auto FoldLeft(const F& f, Acc&& acc, First&& first, Rest&&... rest) {
return FoldLeft(f, f(std::forward<Acc>(acc), std::forward<First>(first)),
std::forward<Rest>(rest)...);
}
template <std::size_t... Indices, typename Tuple>
inline void StrAppendTemplate(std::index_sequence<Indices...>,
std::string* dest, Tuple args) {
// Note on `Is` and `Tuple`:
//
// The goal is to pass N arguments to `StrAppend` and retain `N` as a
// compile-time constant. Thus, we cannot use dynamically sized collections
// such as `absl::Span` or `std::initialized_list`: `std::tuple` comes to
// the rescue.
// `Tuple` is a fixed-sized collection of N elements of `const AlphaNum&`.
// `Indices` is deduced from `std::make_index_sequence<N>` and is used as a
// helper to writing Fold expressions on `Tuple`.
assert(FoldLeft(std::logical_and<>{},
!HaveOverlap(*dest, std::get<Indices>(args).Piece())...));
size_t old_size = dest->size();
size_t new_size =
FoldLeft(std::plus<>{}, old_size, std::get<Indices>(args).size()...);
strings_internal::STLStringResizeUninitializedAmortized(dest, new_size);
FoldLeft(AppendAlphaNum, &(*dest)[old_size], std::get<Indices>(args)...);
}
template <std::size_t... Indices, typename Tuple>
inline std::string StrCatTemplate(std::index_sequence<Indices...>, Tuple args) {
// See implementation comments in `StrAppendTemplate`.
std::string dest;
size_t size = FoldLeft(std::plus<size_t>{}, size_t{0},
std::get<Indices>(args).size()...);
strings_internal::STLStringResizeUninitializedAmortized(&dest, size);
FoldLeft(AppendAlphaNum, &dest[0], std::get<Indices>(args)...);
return dest;
}
// Template functions `StrAppendImpl` and `StrCatImpl` are not exported directly
// due to historical reasons: there are many existing dependencies that expect
// 0-4 argument overloads to be non-template functions. Hence implementations
// are wrapped to keep the compatibility (0-4 arguments only, 5+ arguments are
// templates).
template <typename... AV>
inline void StrAppendImpl(std::string* dest, const AV&... args) {
strings_internal::StrAppendTemplate(
std::make_index_sequence<sizeof...(AV)>{}, dest,
std::forward_as_tuple(static_cast<const AlphaNum&>(args)...));
}
template <typename... AV>
inline std::string StrCatImpl(const AV&... args) {
return strings_internal::StrCatTemplate(
std::make_index_sequence<sizeof...(AV)>{},
std::forward_as_tuple(static_cast<const AlphaNum&>(args)...));
}
} // namespace strings_internal
// -----------------------------------------------------------------------------
// StrCat()
// -----------------------------------------------------------------------------
......@@ -526,32 +436,36 @@ inline std::string StrCatImpl(const AV&... args) {
// quadratic time operation with O(n) dynamic allocations.
//
// See `StrAppend()` below for more information.
namespace strings_internal {
// Do not call directly - this is not part of the public API.
std::string CatPieces(std::initializer_list<absl::string_view> pieces);
void AppendPieces(std::string* dest,
std::initializer_list<absl::string_view> pieces);
} // namespace strings_internal
ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }
ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a) {
return std::string(a.data(), a.size());
}
ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a,
const AlphaNum& b) {
return strings_internal::StrCatImpl(a, b);
}
ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a,
const AlphaNum& b,
const AlphaNum& c) {
return strings_internal::StrCatImpl(a, b, c);
}
ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a,
const AlphaNum& b,
const AlphaNum& c,
const AlphaNum& d) {
return strings_internal::StrCatImpl(a, b, c, d);
}
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b);
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c);
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c, const AlphaNum& d);
// Support 5 or more arguments
template <typename... AV>
ABSL_MUST_USE_RESULT inline std::string StrCat(
const AlphaNum& a, const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,
const AlphaNum& e, const AV&... args) {
return strings_internal::StrCatImpl(a, b, c, d, e, args...);
return strings_internal::CatPieces(
{a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),
static_cast<const AlphaNum&>(args).Piece()...});
}
// -----------------------------------------------------------------------------
......@@ -580,27 +494,23 @@ ABSL_MUST_USE_RESULT inline std::string StrCat(
// std::string s = "foobar";
// absl::string_view p = s;
// StrAppend(&s, p);
inline void StrAppend(std::string*) {}
inline void StrAppend(std::string* dest, const AlphaNum& a) {
strings_internal::StrAppendImpl(dest, a);
}
inline void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b) {
strings_internal::StrAppendImpl(dest, a, b);
}
inline void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c) {
strings_internal::StrAppendImpl(dest, a, b, c);
}
inline void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c, const AlphaNum& d) {
strings_internal::StrAppendImpl(dest, a, b, c, d);
}
void StrAppend(std::string* dest, const AlphaNum& a);
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b);
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c);
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c, const AlphaNum& d);
// Support 5 or more arguments
template <typename... AV>
inline void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
const AlphaNum& c, const AlphaNum& d, const AlphaNum& e,
const AV&... args) {
strings_internal::StrAppendImpl(dest, a, b, c, d, e, args...);
strings_internal::AppendPieces(
dest, {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),
static_cast<const AlphaNum&>(args).Piece()...});
}
// Helper function for the future StrCat default floating-point format, %.6g
......
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