1 //===- llvm/BinaryFormat/COFF.cpp - The COFF format -----------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "llvm/BinaryFormat/COFF.h"
10 #include "llvm/ADT/SmallVector.h"
11 #include "llvm/ADT/Twine.h"
12 
13 // Maximum offsets for different string table entry encodings.
14 enum : unsigned { Max7DecimalOffset = 9999999U };
15 enum : uint64_t { MaxBase64Offset = 0xFFFFFFFFFULL }; // 64^6, including 0
16 
17 // Encode a string table entry offset in base 64, padded to 6 chars, and
18 // prefixed with a double slash: '//AAAAAA', '//AAAAAB', ...
19 // Buffer must be at least 8 bytes large. No terminating null appended.
20 static void encodeBase64StringEntry(char *Buffer, uint64_t Value) {
21   assert(Value > Max7DecimalOffset && Value <= MaxBase64Offset &&
22          "Illegal section name encoding for value");
23 
24   static const char Alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
25                                  "abcdefghijklmnopqrstuvwxyz"
26                                  "0123456789+/";
27 
28   Buffer[0] = '/';
29   Buffer[1] = '/';
30 
31   char *Ptr = Buffer + 7;
32   for (unsigned i = 0; i < 6; ++i) {
33     unsigned Rem = Value % 64;
34     Value /= 64;
35     *(Ptr--) = Alphabet[Rem];
36   }
37 }
38 
39 bool llvm::COFF::encodeSectionName(char *Out, uint64_t Offset) {
40   if (Offset <= Max7DecimalOffset) {
41     // Offsets of 7 digits or less are encoded in ASCII.
42     SmallVector<char, COFF::NameSize> Buffer;
43     Twine('/').concat(Twine(Offset)).toVector(Buffer);
44     assert(Buffer.size() <= COFF::NameSize && Buffer.size() >= 2);
45     std::memcpy(Out, Buffer.data(), Buffer.size());
46     return true;
47   }
48 
49   if (Offset <= MaxBase64Offset) {
50     // Starting with 10,000,000, offsets are encoded as base64.
51     encodeBase64StringEntry(Out, Offset);
52     return true;
53   }
54 
55   // The offset is too large to be encoded.
56   return false;
57 }
58