[lld/mac] Extract a sha256() function

[thakis]

Summary

No behavior change.

[oontvoo]

LG

lld/MachO/SyntheticSections.cpp

 
 using namespace llvm;
 using namespace llvm::MachO;
 using namespace llvm::support;
 using namespace llvm::support::endian;
 using namespace lld;
 using namespace lld::macho;
 
+// Reads `len` bytes at data and writes the 32-byte SHA256 checksum to `output`.
+static void sha256(const uint8_t *data, size_t len, uint8_t *output) {
+  ArrayRef<uint8_t> block(data, len);
+  std::array<uint8_t, 32> hash = SHA256::hash(block);
+  assert(hash.size() == CodeSignatureSection::hashSize);
+  memcpy(output, hash.data(), hash.size());
+}
+
 InStruct macho::in;
 std::vector<SyntheticSection *> macho::syntheticSections;
 
 SyntheticSection::SyntheticSection(const char *segname, const char *name)
     : OutputSection(SyntheticKind, name) {
   std::tie(this->segname, this->name) = maybeRenameSection({segname, name});
   isec = makeSyntheticInputSection(segname, name);
   isec->parent = this;

 
 void CodeSignatureSection::writeHashes(uint8_t *buf) const {
   // NOTE: Changes to this functionality should be repeated in llvm-objcopy's
   // MachOWriter::writeSignatureData.
   uint8_t *code = buf;
   uint8_t *codeEnd = buf + fileOff;
   uint8_t *hashes = codeEnd + allHeadersSize;
   while (code < codeEnd) {
-    StringRef block(reinterpret_cast<char *>(code),
-                    std::min(codeEnd - code, static_cast<ssize_t>(blockSize)));
-    SHA256 hasher;
-    hasher.update(block);
-    std::array<uint8_t, 32> hash = hasher.final();
-    assert(hash.size() == hashSize);
-    memcpy(hashes, hash.data(), hashSize);
+    sha256(code, std::min(static_cast<size_t>(codeEnd - code), blockSize),
+           hashes);
     code += blockSize;
     hashes += hashSize;
   }
 #if defined(__APPLE__)
   // This is macOS-specific work-around and makes no sense for any
   // other host OS. See https://openradar.appspot.com/FB8914231
   //
   // The macOS kernel maintains a signature-verification cache to