Teach llvm-bcanalyzer to skip over the header we use on LLVM IR files.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68458 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/ReaderWriter.h

@@ -24,7 +24,7 @@ namespace llvm {
   class ModulePass;
   class BitstreamWriter;
   class raw_ostream;
-
+  
   /// getBitcodeModuleProvider - Read the header of the specified bitcode buffer
   /// and prepare for lazy deserialization of function bodies.  If successful,
   /// this takes ownership of 'buffer' and returns a non-null pointer.  On
@@ -57,6 +57,57 @@ namespace llvm {
   /// createBitcodeWriterPass - Create and return a pass that writes the module
   /// to the specified ostream.
   ModulePass *createBitcodeWriterPass(raw_ostream &Str);
+  
+  
+  /// isBitcodeWrapper - Return true fi this is a wrapper for LLVM IR bitcode
+  /// files.
+  static bool inline isBitcodeWrapper(unsigned char *BufPtr,
+                                      unsigned char *BufEnd) {
+    return (BufPtr != BufEnd && BufPtr[0] == 0xDE && BufPtr[1] == 0xC0 && 
+            BufPtr[2] == 0x17 && BufPtr[3] == 0x0B);
+  }
+  
+  /// SkipBitcodeWrapperHeader - Some systems wrap bc files with a special
+  /// header for padding or other reasons.  The format of this header is:
+  ///
+  /// struct bc_header {
+  ///   uint32_t Magic;         // 0x0B17C0DE
+  ///   uint32_t Version;       // Version, currently always 0.
+  ///   uint32_t BitcodeOffset; // Offset to traditional bitcode file.
+  ///   uint32_t BitcodeSize;   // Size of traditional bitcode file.
+  ///   ... potentially other gunk ...
+  /// };
+  /// 
+  /// This function is called when we find a file with a matching magic number.
+  /// In this case, skip down to the subsection of the file that is actually a
+  /// BC file.
+  static inline bool SkipBitcodeWrapperHeader(unsigned char *&BufPtr,
+                                              unsigned char *&BufEnd) {
+    enum {
+      KnownHeaderSize = 4*4,  // Size of header we read.
+      OffsetField = 2*4,      // Offset in bytes to Offset field.
+      SizeField = 3*4         // Offset in bytes to Size field.
+    };
+    
+    // Must contain the header!
+    if (BufEnd-BufPtr < KnownHeaderSize) return true;
+    
+    unsigned Offset = ( BufPtr[OffsetField  ]        |
+                       (BufPtr[OffsetField+1] << 8)  |
+                       (BufPtr[OffsetField+2] << 16) |
+                       (BufPtr[OffsetField+3] << 24));
+    unsigned Size   = ( BufPtr[SizeField    ]        |
+                       (BufPtr[SizeField  +1] << 8)  |
+                       (BufPtr[SizeField  +2] << 16) |
+                       (BufPtr[SizeField  +3] << 24));
+    
+    // Verify that Offset+Size fits in the file.
+    if (Offset+Size > unsigned(BufEnd-BufPtr))
+      return true;
+    BufPtr += Offset;
+    BufEnd = BufPtr+Size;
+    return false;
+  }
 } // End llvm namespace
 
 #endif