Simplify identify_magic.

This patch defines a memcmp-ish helper function to simplify identify_magic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286928 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Path.cpp

@@ -980,6 +980,11 @@ void directory_entry::replace_filename(const Twine &filename, file_status st) {
   Status = st;
 }
 
+template <size_t N>
+static bool startswith(StringRef Magic, const char (&S)[N]) {
+  return Magic.startswith(StringRef(S, N - 1));
+}
+
 /// @brief Identify the magic in magic.
 file_magic identify_magic(StringRef Magic) {
   if (Magic.size() < 4)
@@ -987,7 +992,7 @@ file_magic identify_magic(StringRef Magic) {
   switch ((unsigned char)Magic[0]) {
     case 0x00: {
       // COFF bigobj, CL.exe's LTO object file, or short import library file
-      if (memcmp(Magic.data() + 1, "\0\xFF\xFF", 3) == 0) {
+      if (startswith(Magic, "\0\0\xFF\xFF")) {
         size_t MinSize = offsetof(COFF::BigObjHeader, UUID) + sizeof(COFF::BigObjMagic);
         if (Magic.size() < MinSize)
           return file_magic::coff_import_library;
@@ -1000,9 +1005,7 @@ file_magic identify_magic(StringRef Magic) {
         return file_magic::coff_import_library;
       }
       // Windows resource file
-      const char Expected[] = { 0, 0, 0, 0, '\x20', 0, 0, 0, '\xff' };
-      if (Magic.size() >= sizeof(Expected) &&
-          memcmp(Magic.data(), Expected, sizeof(Expected)) == 0)
+      if (startswith(Magic, "\0\0\0\0\x20\0\0\0\xFF"))
         return file_magic::windows_resource;
       // 0x0000 = COFF unknown machine type
       if (Magic[1] == 0)
@@ -1010,28 +1013,24 @@ file_magic identify_magic(StringRef Magic) {
       break;
     }
     case 0xDE:  // 0x0B17C0DE = BC wraper
-      if (Magic[1] == (char)0xC0 && Magic[2] == (char)0x17 &&
-          Magic[3] == (char)0x0B)
+      if (startswith(Magic, "\xDE\xC0\x17\x0B"))
         return file_magic::bitcode;
       break;
     case 'B':
-      if (Magic[1] == 'C' && Magic[2] == (char)0xC0 && Magic[3] == (char)0xDE)
+      if (startswith(Magic, "BC\xC0\xDE"))
         return file_magic::bitcode;
       break;
     case '!':
-      if (Magic.size() >= 8)
-        if (memcmp(Magic.data(), "!<arch>\n", 8) == 0 ||
-            memcmp(Magic.data(), "!<thin>\n", 8) == 0)
-          return file_magic::archive;
+      if (startswith(Magic, "!<arch>\n") || startswith(Magic, "!<thin>\n"))
+        return file_magic::archive;
       break;
 
     case '\177':
-      if (Magic.size() >= 18 && Magic[1] == 'E' && Magic[2] == 'L' &&
-          Magic[3] == 'F') {
+      if (startswith(Magic, "\177ELF") && Magic.size() >= 18) {
         bool Data2MSB = Magic[5] == 2;
         unsigned high = Data2MSB ? 16 : 17;
         unsigned low  = Data2MSB ? 17 : 16;
-        if (Magic[high] == 0)
+        if (Magic[high] == 0) {
           switch (Magic[low]) {
             default: return file_magic::elf;
             case 1: return file_magic::elf_relocatable;
@@ -1039,15 +1038,15 @@ file_magic identify_magic(StringRef Magic) {
             case 3: return file_magic::elf_shared_object;
             case 4: return file_magic::elf_core;
           }
-        else
-          // It's still some type of ELF file.
-          return file_magic::elf;
+        }
+        // It's still some type of ELF file.
+        return file_magic::elf;
       }
       break;
 
     case 0xCA:
-      if (Magic[1] == char(0xFE) && Magic[2] == char(0xBA) &&
-          (Magic[3] == char(0xBE) || Magic[3] == char(0xBF))) {
+      if (startswith(Magic, "\xCA\xFE\xBA\xBE") ||
+          startswith(Magic, "\xCA\xFE\xBA\xBF")) {
         // This is complicated by an overlap with Java class files.
         // See the Mach-O section in /usr/share/file/magic for details.
         if (Magic.size() >= 8 && Magic[7] < 43)
@@ -1062,9 +1061,8 @@ file_magic identify_magic(StringRef Magic) {
     case 0xCE:
     case 0xCF: {
       uint16_t type = 0;
-      if (Magic[0] == char(0xFE) && Magic[1] == char(0xED) &&
-          Magic[2] == char(0xFA) &&
-          (Magic[3] == char(0xCE) || Magic[3] == char(0xCF))) {
+      if (startswith(Magic, "\xFE\xED\xFA\xCE") ||
+          startswith(Magic, "\xFE\xED\xFA\xCF")) {
         /* Native endian */
         size_t MinSize;
         if (Magic[3] == char(0xCE))
@@ -1073,9 +1071,8 @@ file_magic identify_magic(StringRef Magic) {
           MinSize = sizeof(MachO::mach_header_64);
         if (Magic.size() >= MinSize)
           type = Magic[12] << 24 | Magic[13] << 12 | Magic[14] << 8 | Magic[15];
-      } else if ((Magic[0] == char(0xCE) || Magic[0] == char(0xCF)) &&
-                 Magic[1] == char(0xFA) && Magic[2] == char(0xED) &&
-                 Magic[3] == char(0xFE)) {
+      } else if (startswith(Magic, "\xCE\xFA\xED\xFE") ||
+                 startswith(Magic, "\xCF\xFA\xED\xFE")) {
         /* Reverse endian */
         size_t MinSize;
         if (Magic[0] == char(0xCE))
@@ -1118,7 +1115,7 @@ file_magic identify_magic(StringRef Magic) {
       break;
 
     case 'M': // Possible MS-DOS stub on Windows PE file
-      if (Magic[1] == 'Z') {
+      if (startswith(Magic, "MZ")) {
         uint32_t off = read32le(Magic.data() + 0x3c);
         // PE/COFF file, either EXE or DLL.
         if (off < Magic.size() &&