From 8d1d4d4f097f3e865d73f3c710269c3ec96e9cb7 Mon Sep 17 00:00:00 2001
From: Lenny Maiorani <lenny@serenityos.org>
Date: Sun, 13 Mar 2022 20:59:29 -0600
Subject: [PATCH] AK: Make static constexpr variables to avoid stack copy in
 Base64

Alphabet and lookup table are created and copied to the stack on each
call. Create them and store them in static memory.
---
 AK/Base64.cpp | 33 ++++++++++++++-------------------
 1 file changed, 14 insertions(+), 19 deletions(-)

diff --git a/AK/Base64.cpp b/AK/Base64.cpp
index 6517635485..ad12be9949 100644
--- a/AK/Base64.cpp
+++ b/AK/Base64.cpp
@@ -13,24 +13,19 @@
 
 namespace AK {
 
-static constexpr auto make_alphabet()
-{
-    Array alphabet = {
-        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
-        'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
-        'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
-        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
-        'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
-        'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
-        'w', 'x', 'y', 'z', '0', '1', '2', '3',
-        '4', '5', '6', '7', '8', '9', '+', '/'
-    };
-    return alphabet;
-}
+static constexpr Array alphabet = {
+    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
+    'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
+    'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
+    'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
+    'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
+    'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
+    'w', 'x', 'y', 'z', '0', '1', '2', '3',
+    '4', '5', '6', '7', '8', '9', '+', '/'
+};
 
-static constexpr auto make_lookup_table()
+static consteval auto make_lookup_table()
 {
-    constexpr auto alphabet = make_alphabet();
     Array<i16, 256> table;
     table.fill(-1);
     for (size_t i = 0; i < alphabet.size(); ++i) {
@@ -39,6 +34,8 @@ static constexpr auto make_lookup_table()
     return table;
 }
 
+static constexpr auto alphabet_lookup_table = make_lookup_table();
+
 size_t calculate_base64_decoded_length(StringView input)
 {
     return input.length() * 3 / 4;
@@ -52,7 +49,6 @@ size_t calculate_base64_encoded_length(ReadonlyBytes input)
 ErrorOr<ByteBuffer> decode_base64(StringView input)
 {
     auto get = [&](size_t& offset, bool* is_padding, bool& parsed_something) -> ErrorOr<u8> {
-        constexpr auto table = make_lookup_table();
         while (offset < input.length() && is_ascii_space(input[offset]))
             ++offset;
         if (offset >= input.length())
@@ -65,7 +61,7 @@ ErrorOr<ByteBuffer> decode_base64(StringView input)
             *is_padding = true;
             return 0;
         }
-        i16 result = table[ch];
+        i16 result = alphabet_lookup_table[ch];
         if (result < 0)
             return Error::from_string_literal("Invalid character in base64 data");
         VERIFY(result < 256);
@@ -106,7 +102,6 @@ ErrorOr<ByteBuffer> decode_base64(StringView input)
 
 String encode_base64(ReadonlyBytes input)
 {
-    constexpr auto alphabet = make_alphabet();
     StringBuilder output(calculate_base64_encoded_length(input));
 
     auto get = [&](const size_t offset, bool* need_padding = nullptr) -> u8 {