llvm/lib/BinaryFormat/COFF.cpp

   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 }