[llvm-branch-commits] [libcxx] [libc++][format] Improves escaping performance. (PR #88533)

[Louis Dionne via llvm-branch-commits]

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@@ -0,0 +1,100 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
+
+// <format>
+
+// Tests the properties of the Unicode escaped output table.
+// The libc++ algorithm has size and speed optimizations based on the properties
+// of Unicode. This means updating the Unicode tables has a likilihood of
+// breaking test. This is an assert; it requires validating whether the
+// assumptions of the size and speed optimizations are still valid.
+
+#include <algorithm>
+#include <numeric>
+#include <format>
+#include <cassert>
+
+// Contains the entries for [format.string.escaped]/2.2.1.2.1
+//   CE is a Unicode encoding and C corresponds to a UCS scalar value whose
+//   Unicode property General_Category has a value in the groups Separator (Z)
+//   or Other (C), as described by table 12 of UAX #44
+//
+// Separator (Z) consists of General_Category
+// - Zs Space_Separator,
+// - Zl Line_Separator,
+// - Zp Paragraph_Separator.
+//
+// Other (C) consists of General_Category
+// - Cc Control,
+// - Cf Format,
+// - Cs Surrogate,
+// - Co Private_Use,
+// - Cn Unassigned.
+inline constexpr int Zs = 17;
+inline constexpr int Zl = 1;
+inline constexpr int Zp = 1;
+inline constexpr int Z  = Zs + Zl + Zp;
+
+inline constexpr int Cc = 65;
+inline constexpr int Cf = 170;
+inline constexpr int Cs = 2'048;
+inline constexpr int Co = 137'468;
+inline constexpr int Cn = 824'718;
+inline constexpr int C  = Cc + Cf + Cs + Co + Cn;
+
+// This is the final part of the Unicode properties table:
+//
+// 31350..323AF  ; Lo # [4192] CJK UNIFIED IDEOGRAPH-31350..CJK UNIFIED IDEOGRAPH-323AF
+// 323B0..E0000  ; Cn # [711761] <reserved-323B0>..<reserved-E0000>
+// E0001         ; Cf #       LANGUAGE TAG
+// E0002..E001F  ; Cn #  [30] <reserved-E0002>..<reserved-E001F>
+// E0020..E007F  ; Cf #  [96] TAG SPACE..CANCEL TAG
+// E0080..E00FF  ; Cn # [128] <reserved-E0080>..<reserved-E00FF>
+// E0100..E01EF  ; Mn # [240] VARIATION SELECTOR-17..VARIATION SELECTOR-256
+// E01F0..EFFFF  ; Cn # [65040] <reserved-E01F0>..<noncharacter-EFFFF>
+// F0000..FFFFD  ; Co # [65534] <private-use-F0000>..<private-use-FFFFD>
+// FFFFE..FFFFF  ; Cn #   [2] <noncharacter-FFFFE>..<noncharacter-FFFFF>
+// 100000..10FFFD; Co # [65534] <private-use-100000>..<private-use-10FFFD>
+// 10FFFE..10FFFF; Cn #   [2] <noncharacter-10FFFE>..<noncharacter-10FFFF>
+//
+// It can be observed all entries in the range 323B0..10FFFF are in the
+// categories Cf, Co, Cn, except a small range with the property Mn.
+// In order to reduce the size of the table only the entires in the range
+// [0000, 323B0) are stored in the table. The entries in the range
+// [323B0, 10FFFF] use a hand-crafted algorithm.
+//
+// This means a number of entries are omitted
+inline constexpr int excluded = ((0x10FFFF - 0x323B0) + 1) - 240;
+
+inline constexpr int entries = Z + C - excluded;
+
+static constexpr int count_entries() {
+  return std::transform_reduce(
+      std::begin(std::__escaped_output_table::__entries),
+      std::end(std::__escaped_output_table::__entries),
+      0,
+      std::plus{},
+      [](auto entry) { return 1 + static_cast<int>(entry & 0x3fffu); });
+}
+static_assert(count_entries() == entries);
+
+int main() {
----------------
ldionne wrote:

```suggestion
int main(int, char**) {
```

and `return 0;`

https://github.com/llvm/llvm-project/pull/88533