Rollback of "Speed up StrAppend by up to 4x."

PiperOrigin-RevId: 553158292
Change-Id: I28350321550accd72da2f9f6f5992af311fe4b00

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