[clang] [Serialization] Fix source location data loss during decoding. (PR #145529)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/145529.diff


2 Files Affected:

- (modified) clang/include/clang/Serialization/SourceLocationEncoding.h (+6-3) 
- (modified) clang/unittests/Serialization/SourceLocationEncodingTest.cpp (+2) 


``````````diff
diff --git a/clang/include/clang/Serialization/SourceLocationEncoding.h b/clang/include/clang/Serialization/SourceLocationEncoding.h
index 33ca1728fa479..85a7379a5fb55 100644
--- a/clang/include/clang/Serialization/SourceLocationEncoding.h
+++ b/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};
diff --git a/clang/unittests/Serialization/SourceLocationEncodingTest.cpp b/clang/unittests/Serialization/SourceLocationEncodingTest.cpp
index c80a8fd0e52b1..b21035af47cfc 100644
--- a/clang/unittests/Serialization/SourceLocationEncodingTest.cpp
+++ b/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

``````````

</details>


https://github.com/llvm/llvm-project/pull/145529