1 //===- WasmWriter.cpp -----------------------------------------------------===//
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 "WasmWriter.h"
10 #include "llvm/BinaryFormat/Wasm.h"
11 #include "llvm/Support/Endian.h"
12 #include "llvm/Support/Errc.h"
13 #include "llvm/Support/LEB128.h"
14 #include "llvm/Support/raw_ostream.h"
15 
16 namespace llvm {
17 namespace objcopy {
18 namespace wasm {
19 
20 using namespace object;
21 using namespace llvm::wasm;
22 
23 Writer::SectionHeader Writer::createSectionHeader(const Section &S,
24                                                   size_t &SectionSize) {
25   SectionHeader Header;
26   raw_svector_ostream OS(Header);
27   OS << S.SectionType;
28   bool HasName = S.SectionType == WASM_SEC_CUSTOM;
29   SectionSize = S.Contents.size();
30   if (HasName)
31     SectionSize += getULEB128Size(S.Name.size()) + S.Name.size();
32   // If we read this section from an object file, use its original size for the
33   // padding of the LEB value to avoid changing the file size. Otherwise, pad
34   // out to 5 bytes to make it predictable, and match the behavior of clang.
35   unsigned HeaderSecSizeEncodingLen =
36       S.HeaderSecSizeEncodingLen ? *S.HeaderSecSizeEncodingLen : 5;
37   encodeULEB128(SectionSize, OS, HeaderSecSizeEncodingLen);
38   if (HasName) {
39     encodeULEB128(S.Name.size(), OS);
40     OS << S.Name;
41   }
42   // Total section size is the content size plus 1 for the section type and
43   // the LEB-encoded size.
44   SectionSize = SectionSize + 1 + HeaderSecSizeEncodingLen;
45   return Header;
46 }
47 
48 size_t Writer::finalize() {
49   size_t ObjectSize = sizeof(WasmMagic) + sizeof(WasmVersion);
50   SectionHeaders.reserve(Obj.Sections.size());
51   // Finalize the headers of each section so we know the total size.
52   for (const Section &S : Obj.Sections) {
53     size_t SectionSize;
54     SectionHeaders.push_back(createSectionHeader(S, SectionSize));
55     ObjectSize += SectionSize;
56   }
57   return ObjectSize;
58 }
59 
60 Error Writer::write() {
61   size_t TotalSize = finalize();
62   Out.reserveExtraSpace(TotalSize);
63 
64   // Write the header.
65   Out.write(Obj.Header.Magic.data(), Obj.Header.Magic.size());
66   uint32_t Version;
67   support::endian::write32le(&Version, Obj.Header.Version);
68   Out.write(reinterpret_cast<const char *>(&Version), sizeof(Version));
69 
70   // Write each section.
71   for (size_t I = 0, S = SectionHeaders.size(); I < S; ++I) {
72     Out.write(SectionHeaders[I].data(), SectionHeaders[I].size());
73     Out.write(reinterpret_cast<const char *>(Obj.Sections[I].Contents.data()),
74               Obj.Sections[I].Contents.size());
75   }
76 
77   return Error::success();
78 }
79 
80 } // end namespace wasm
81 } // end namespace objcopy
82 } // end namespace llvm
83