[Serialization] Fix source location data loss during decoding.

The delta encoding can produce values up to 33 bits, but the current decoding
logic only preserves the lower 32 bits, potentially causing data loss.

This patch fixes the issue by preserving the lower 33 bits for the
encode.

Author: hokein

clang/include/clang/Serialization/SourceLocationEncoding.h

@@ -48,6 +48,9 @@ class SourceLocationEncoding {
   using UIntTy = SourceLocation::UIntTy;
   constexpr static unsigned UIntBits = CHAR_BIT * sizeof(UIntTy);
 
+  // The maximum number of bits we use for the encoding.
+  constexpr static unsigned EncodingBits = UIntBits + 1;
+
   static UIntTy encodeRaw(UIntTy Raw) {
     return (Raw << 1) | (Raw >> (UIntBits - 1));
   }
@@ -179,20 +182,20 @@ SourceLocationEncoding::encode(SourceLocation Loc, UIntTy BaseOffset,
 
   // 16 bits should be sufficient to store the module file index.
   assert(BaseModuleFileIndex < (1 << 16));
-  Encoded |= (RawLocEncoding)BaseModuleFileIndex << 32;
+  Encoded |= (RawLocEncoding)BaseModuleFileIndex << EncodingBits;
   return Encoded;
 }
 inline std::pair<SourceLocation, unsigned>
 SourceLocationEncoding::decode(RawLocEncoding Encoded,
                                SourceLocationSequence *Seq) {
-  unsigned ModuleFileIndex = Encoded >> 32;
+  unsigned ModuleFileIndex = Encoded >> EncodingBits;
 
   if (!ModuleFileIndex)
     return {Seq ? Seq->decode(Encoded)
                 : SourceLocation::getFromRawEncoding(decodeRaw(Encoded)),
             ModuleFileIndex};
 
-  Encoded &= llvm::maskTrailingOnes<RawLocEncoding>(32);
+  Encoded &= llvm::maskTrailingOnes<RawLocEncoding>(EncodingBits);
   SourceLocation Loc = SourceLocation::getFromRawEncoding(decodeRaw(Encoded));
 
   return {Loc, ModuleFileIndex};

clang/unittests/Serialization/SourceLocationEncodingTest.cpp

@@ -104,6 +104,8 @@ TEST(SourceLocationEncoding, Sequence) {
 
   roundTrip(
       {123 | MacroBit, 1, 9, Biggest, Big, Big + 1, 0, MacroBit | Big, 0});
+
+  roundTrip({1, (1u << 30) + 1});
 }
 
 } // namespace