Speed up StrAppend by up to 4x.

PiperOrigin-RevId: 552802740
Change-Id: I662da1b03bfffb7939b44ea3850566d3c209c6cc

1 files changed, 13 insertions, 165 deletions

diff --git a/absl/strings/str_cat.cc b/absl/strings/str_cat.cc
index 2e49c31b..5ec02995 100644
--- a/absl/strings/str_cat.cc
+++ b/absl/strings/str_cat.cc
@@ -14,17 +14,9 @@
 
 #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 {
@@ -37,170 +29,26 @@ 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 {
 
-// 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;
+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()));
 }
 
-// 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());
+#if defined(__GNUC__) && !defined(__clang__)
+char* AppendAlphaNum(char* dst, const AlphaNum& a) {
+  return std::copy_n(a.data(), a.size(), dst);
 }
+#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